OPERATION
MANUAL
E2005 Lennox Industries Inc.
Dallas, Texas, USA
51M37 Merit® Series
5/2 Day Programmable Thermostat
CONTROLS
505,050M
05/05
Supersedes 03/05
Litho U.S.A.
Table of Contents
51M37 Series Thermostat . . . . . . . . . . . . . . . . . . . . . .
Initial Thermostat Power-up . . . . . . . . . . . . . . . . . . . . .
Buttons, Backlight, Timers & Settings . . . . . . . . . . . .
DAY/TIME − Set Day and Time . . . . . . . . . . . . . . . . . .
HEAT − Heat Mode . . . . . . . . . . . . . . . . . . . . . . . . . . . .
COOL − Cool Mode . . . . . . . . . . . . . . . . . . . . . . . . . . . .
HOLD − Temperature Hold Mode . . . . . . . . . . . . . . . .
PROG − Thermostat Programming . . . . . . . . . . . . . . .
FAN − Controlling Fan Operation . . . . . . . . . . . . . . . . .
SETTINGS − Filter and Maintenance Reminders . . .
SETTINGS − Balance Point . . . . . . . . . . . . . . . . . . . . .
Thermostat RESET . . . . . . . . . . . . . . . . . . . . . . . . . . . .
Removing/Installing Thermostat . . . . . . . . . . . . . . . . .
Default Thermostat Settings . . . . . . . . . . . . . . . . . . . .
Technical Specifications . . . . . . . . . . . . . . . . . . . . . . . .
RETAIN THESE INSTRUCTIONS
FOR FUTURE REFERENCE
51M37 Merit® Series Thermostat
®
The Lennox Merit Series 5/2 day programmable
electronic thermostat 51M37 provides excellent
temperature control and a large, easy-to-read display. It
includes a programmable filter change reminder, an
equipment maintenance reminder, and a system check
indicator to notify the user when the equipment requires
service.
The 51M37 is suitable for heat pump, 3-stage heat/2-stage
cool applications using a gas or electric auxiliary heat
source. An optional outdoor temperature sensor provides
auxiliary heat lockout, balance point operation, and
dual−fuel compatibility.
General
Initial Power−Up Display
MAINT
FILTER
MO TU WE TH FR SA SU
These instructions are intended as a general guide and do
not supersede local codes in any way. Consult authorities
having jurisdiction before installation.
SERVICE
AM
PM I8:88
Check equipment for shipping damage. If you find any
damage, immediately contact the last carrier.
FAN
ON
AUTO
PROG A | B | C | D HOLD
Introduction
SYSTEM
COOL
OFF
EMG.
HEAT
ROOM
SETPOINT
°
This document describes the operation of Lennox thermostat
51M37. Refer to the installation manual for instructions re-
garding installation and wiring of the thermostat.
88°
Hi
Lo
88
Initial Thermostat Power-up
Figure 1
When power is initially applied to the thermostat, the dis-
All display segments are momentarily activated. This oc-
curs as a normal part of thermostat initialization.
05/05
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HEAT − Using the Heat Mode
Heat Mode Disabled
SYSTEM
COOL
OFF
ROOM
75°
Normal Heat Mode
SETPOINT
In normal heat mode, both the heat pump and the backup
heat source are used to provide heat. If the thermostat de-
tects that the heat pump is not able to provide enough heat
(as may be the case in very cold weather), then the backup
heat source is activated.
75°
HEAT
Figure 6
Heating Demand
Emergency Heat Mode
The thermostat must be in either normal or emergency
heat mode in order to properly control the heating equip-
ment. In either heat mode, when the actual temperature is
the thermostat detects a heating demand and activates the
heating equipment to satisfy the demand. Heating opera-
tion is indicated by a flame icon in the SYSTEM box.
In emergency heat mode, only the backup heat source
provides heat ꢁthe heat pump is disabled. The backup
heat source is activated only when there is a heat demand.
Enabling Normal Heat Mode
Use the HEAT button to select normal heat mode, emer-
gency heat mode, or to disable heat modes as desired. If
the thermostat is in OFF or COOL mode, pressing the Heat
button enables Heat mode. This is indicated by HEAT in
Heating Demand
SYSTEM
ROOM
SETPOINT
COOL
OFF
Normal Heat Mode Enabled
72°
75°
Figure 7
Heat Cool
HEAT
MO
AM
PM 5:30
FAN
Prog Day/Time
HOLD
AUTO
SYSTEM
When the actual temperature rises above the temperature
setpoint, the flame icon will disappear. This indicates that
the heating demand has been satisfied and that the heat-
ing equipment has been turned off.
ROOM
SETPOINT
66°
Fan Hold
°
HEAT
70
Settings Enter
Heat pump operation is locked out for 5 minutes after a de-
mand has been satisfied. If another heat pump demand
occurs during this 5−minute interval, the flame icon will
flash; however, the heat pump will not run until the 5−min-
ute delay has elapsed.
Figure 4
NOTE − The heat pump is activated for at least 4 minutes if
no buttons are pressed during the demand interval. The
backup heat source is activated for at least 3 minutes if no
buttons are pressed during the demand interval.
Enabling Emergency Heat Mode
If the thermostat is in heat mode, pressing the Heat button
is in normal heat mode when the HEAT button is pressed,
then emergency heat mode is enabled.
If your system supports 3−stage heating (as does the
51M37 thermostat), you may notice various heating levels
being delivered during a demand.
Emergency Heat Mode Enabled
SYSTEM
ROOM
If a small heat pump heating demand is present, "Lo" is dis-
played in the SYSTEM box; if a large heat pump heating
SETPOINT
COOL
OFF
EMG.
HEAT
62°
Disabling Heat Mode
75°
Large Heat Demand
Figure 5
SYSTEM
COOL
OFF
ROOM
65°
SETPOINT
If the thermostat is in emergency heat mode when the
HEAT button is pressed, then heat modes are disabled.
This is indicated by OFF in the SYSTEM box as shown in
75°
Hi
HEAT
Figure 8
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Cooling Demand
COOL − Using the Cool Mode
Enabling and Disabling Cool Mode
Set the thermostat to cool mode to control the cooling
equipment. Then, if the room temperature is higher than
mostat detects a cooling demand and will activate the cool-
ing equipment to satisfy the demand.
Use the COOL button to enable or disable cool mode as
desired. If the thermostat is in HEAT or OFF mode, cool
mode is enabled when the COOL button is pressed. This is
indicated by COOL in the SYSTEM box (see figure 9).
Cooling Demand
MO TU WE TH FR SA SU
AM
PM i0:00
ROOM
FAN
ON
AUTO
Turn Cool ON/OFF
PROG A B C D HOLD
Heat Cool
MO
SYSTEM
COOL
OFF
AM
PM
FAN
I0:30
Prog Day/Time
SETPOINT
75°
HOLD
AUTO
70°
SYSTEM
COOL
ROOM
Hi
SETPOINT
°70°
Fan Hold
HEAT
75
Figure 11
Settings Enter
Cooling operation is indicated by flashing ꢀsnowflake"
icons in the SYSTEM box. When the actual temperature
drops below the temperature setpoint, the snowflake icons
will disappear. This indicates that the cooling demand has
been satisfied and that the cooling equipment has been
turned off.
Figure 9
If your system supports 2−stage cooling (as does the
51M37 thermostat), you may notice various cooling levels
being delivered during a demand. Also, if a small cooling
demand is present, "Lo" will be displayed in the SYSTEM
box. However, if a large cooling demand is present, "Hi" will
If the thermostat is in cool mode, pressing the Cool button
disables COOL mode (indicated by OFF in the SYSTEM
Cool Mode Disabled
NOTE − If no buttons are pressed during a demand for
cooling, the equipment must operate for at least 4 minutes.
After a demand has been satisfied, cooling equipment op-
eration is locked out for 5 minutes. If another cooling de-
mand occurs during this 5−minute interval, ꢀCOOL" and the
snowflakes will flash; however, the cooling equipment will
not operate until the 5−minute delay has elapsed.
SYSTEM
ROOM
SETPOINT
COOL
75°
OFF
75°
HEAT
Figure 10
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PROG − Thermostat Programming
HOLD − Using Temperature Hold Modes
The 51M37 thermostat can be programmed to perform a
set of either heating or cooling events (but not a
combination of heat and cool) for 5 consecutive days using
a set of 4 unique events per day. The remaining 2 days can
then be set for a different set of 4 unique events per day.
Both the consecutive days and the events/temperature are
set by the homeowner.
When HOLD is displayed at the HOME screen, the ther-
mostat is in a temperature hold condition. This means that
the temperature program data is ignored and the thermo-
stat functions much like a non−programmable thermostat.
Adjusting Temperature Setpoint in Hold Mode
The temperature setpoint represents the desired tempera-
ture of the space around the thermostat. The default tem-
perature setpoint in Hold mode is 70°F.
To Change Consecutive Days...
To alter the 5 consecutive days, press and hold the
PROG button for 5 seconds. The 5 consecutive day period
is then displayed (default is MOnday thru FRiday). To
change to a different 5-consecutive days, use the Up/
Down arrow buttons. Any 5 consecutive−day span may be
Wednesday is defined as the 5−day programming (Thurs-
day and Friday would constitute the 2−day programming).
Press the PROG button when finished.
To adjust the setpoint, press the UP or DOWN (YB) arrow
buttons (see figure 12); the existing setpoint is displayed to
the right of the actual room temperature. Each button
press adjusts the setpoint up or down by 1 degree.
Hold Temperature Mode
Heat Cool
MO
AM
PM
FAN
Day/Time
Prog
I0:36
HOLD
AUTO
SYSTEM
COOL
Change Consecutive days
ROOM
SETPOINT
°68°
Fan Hold
MO TU WE TH FR SA SU
72
FILTER
Settings Enter
Figure 14
To Set Program Events and Temperatures...
Figure 15 gives an example of how the two sets of pro-
grams can be set for a normal workweek and weekend.
Figure 12
After the desired setpoint is reached, the HOME screen
will reappear after about 15 seconds.
In the 5−day bar graph, note how programs A and C reflect
the desired warmth while the home IS occupied (72°); B
allows less heating while the home is NOT occupied; D re-
flects a cooler sleeping temperature. The 2−day bar graph
would support day−long occupancy and, because the first
program begins later, a less−demanding time schedule.
Permanent Hold Mode
At any time the program is running, from the HOME
screen, set a permanent hold (program override) by press-
functions much like a non−programmable thermostat. Use
the Up/Down arrow buttons to adjust the hold setpoint. To
return to the program, press HOLD again.
5/2 Program Example
|
5−DAY PROGRAM: | A
B
|
C
|
D
|
72°
68°
64°
6AM
Temporary (2-Hour) Hold
At any time the program is running, from the HOME
screen, set a temporary 2-hour hold by pressing the Up/
Down arrow buttons until the desired setpoint is displayed;
gram for 2 hours from the last button press, then returns to
the program.
NOON
6PM
|
MID
NIGHT
6AM
2−DAY PROGRAM:
| A |
B
C
|
D
|
72°
68°
64°
6AM
NOON
6PM
MID
NIGHT
6AM
Temporary Hold Temperature Mode
Figure 15
Heat Cool
MO
NOTE − Pressing ENTER during the following program-
ming steps, saves and exits to the HOME screen.
AM
PM
FAN
Day/Time
Prog
I0:36
AUTO
PROG
B
HOLD
SYSTEM
COOL
To program events and temperatures, perform the follow-
ing steps, once with Cool selected and once with Heat se-
lected.
ROOM
SETPOINT
°68°
Fan Hold
72
Settings Enter
1. Press and release PROG. ꢀAM 6:00", period ꢀA", and
the 5 consecutive days are displayed; ꢀAM 6" flashes.
2. Use the Up/Down arrow buttons to select the desired
hour; press PROG when the desired hour is reached.
Figure 13
3. Use the Up/Down arrow buttons to select the desired
minute; press PROG.
While in Temporary Hold, press HOLD once to switch to
Permanent Hold (HOLD displays solid; PROG not dis-
played); press HOLD again to return to the program
(PROG displays; HOLD not displayed).
4. Use the Up/Down arrow buttons to select the desired
temperature setpoint; press PROG.
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Reminder Settings Display
For MAINT,
press Settings
button 2 times
MAINT
FILTER
NOTE − This thermostat will NOT automatically switch
from heating to cooling, or vice versa; operator involve-
ment is required. At the change of seasons, or to accomo-
date abnormal seasonal temperature swings, you must
manually select to the opposite conditioning (Heat or Cool)
program.
For FILTER,
press Settings
button 1 time
OFF
Figure 18
FAN − Controlling the Fan Operation
The default setting for the reminders is OFF (disabled).
Press Up/Down arrow buttons to select the desired re-
minder intervals.
Use the FAN button to select either continuous fan mode or
auto fan mode.
To change from continuous to auto fan mode (or vice ver-
sa), press the FAN button. Note whether a fan icon in the
FAN box is present (indicating that the fan is running) or not
(fan not running).
Table 1
Filter and Maintenance Reminders
Buttons to
Use
Available Settings and
How to Use
If continuous fan mode is enabled (ON displayed in FAN
box − see figure 16), the fan will run continuously regard-
less of whether the heating or cooling equipment is run-
ning.
Reminder
FILTER
Settings
(1st press)
then Arrows
to scroll
Total fan run time expressed in
months (Off, 1, 3, 6, 12); for
example, if fan runs 12 hours a
day, 1 month reminder displays
in 2 calendar months.
selections
Using Fan ON
Heat Cool
MO
Settings
(2nd press)
then Arrows
to scroll
Elapsed chronological time in
months (Off, 6, 12). Use this,
for example, to remind yourself
when to perform routine checks
or when to call a technician for
perodic preventive
MAINT
AM
PM I0:I0
FAN
ON
Day/Time
Prog
PROG
B
SYSTEM
HEAT
ROOM
67°
selections
Fan Hold
maintenance.
Settings Enter
Enter
Stores settings.
NOTE − The HOME screen will reappear about 15 seconds
after the final arrow button press. OR, press ENTER at any
time to store any changes and exit to the HOME screen.
Figure 16
If auto fan mode is selected (AUTO displayed in FAN box −
cooling equipment is running.
After either programmed interval has elapsed, the remind-
Using Fan Auto
Heat Cool
Reminders
MO
MAINT
AM
PM
FAN
MO TU WE TH FR SA SU
FILTER
Day/Time
Prog
I0:36
PROG
B
AUTO
SERVICE
SYSTEM
COOL
ROOM
Fan Hold
72°
AM
PM
FAN
ON
AUTO
I0:i2
Settings Enter
PROG A B C D HOLD
ROOM
SYSTEM
COOL
OFF
Figure 17
78°
SETTINGS − Filter/Maintenance Reminders
HEAT
The 51M37 thermostat is designed to remind you when the
filter needs changing or when routine maintenance is re-
quired, as (and if) defined, by you. These optional remind-
ers are not enabled until you activate them. To do so, press
the Settings button (shown below the Fan button in figure
17) once or twice for the desired reminder as shown in fig-
Figure 19
After the filter has been changed or maintenance per-
formed, reset the reminder by pressing the SETTINGS
button for 4 seconds. The screen will blink for a few mo-
ments to indicate that the timer has been reset.
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Service Indicator Flashing
SETTINGS − Balance Point
MO TU WE TH FR SA SU
FILTER
SERVICE
NOTE − The balance point is only available with the out-
door sensor.
AM
PM I0:i4
If the optional outdoor sensor (X2658) is connected to the
thermostat, balance point adjustment is available. The bal-
ance point feature allows the measured outdoor tempera-
ture to govern operation of the heat pump and backup heat
source.
FAN
ON
AUTO
PROG A B C D HOLD
ROOM
SYSTEM
COOL
OFF
79°
Use the SETTINGS button to access the balance point
setting. If the outdoor sensor is attached to the thermostat,
press the SETTINGS button three times to access the Bal-
HEAT
Figure 21
Balance Point Setting
ROOM
Thermostat RESET
SETPOINT
Under some abnormal conditions, it may be necessary to
ꢀreset" the thermostat to its default condition. Such a RE-
SET would delete all programming and settings and there-
fore should only be used on rare occasions when the the
thermostat fails to function as designd and/or as pro-
grammed. Such an instance can occur as a result of a pow-
er surge or similar electrical disturbance (e.g. after an elec-
trical storm or power outage). The RESET button can be
used to recover from this situation.
55°
6p
Figure 20
The balance point can be set at any temperature between
1°F and 55°F (−17°C to 13°C). A balance point setting of
zero disables the balance point function (in this case, the
thermostat behaves as if no outdoor sensor is attached).
The default balance point setting is 55°F.
To adjust the balance point up or down, press the Up/Down
arrow buttons to the right of the display screen. The bal-
ance point setting increases by 1°F each time the Up but-
ton is pressed and decreases by 1°F each time the Down
button is pressed.
CAUTION
When the RESET button is pressed, ALL settings re-
vert back to the defaults, including the default pro-
After the desired balance point is reached, the HOME
screen will reappear after about 15 seconds.
The RESET button is an unlabeled, recessed button lo-
cated behind the door, on the right−hand side of the ther-
paper clip or small pencil to press the RESET button; ALL
thermostat settings will be reset to the defaults listed in the
Default Thermostat Settings section.
NOTE − The balance point feature allows the outdoor tem-
perature to rise or fall 3°F above or below the balance
point. This prevents excessive cycling of the equipment
when the outdoor temperature is near the balance point.
For example, if the balance point is 40°F and the actual
outdoor temperature is 35°F, the outdoor temperature
must rise to 43°F before equipment adjustment occurs.
Conversely, if the balance point is 40°F and the actual out-
door temperature is 45°F, the outdoor temperature must
drop to 37°F before equipment adjustment occurs.
Removing/Installing Thermostat
The thermostat hinges on tabs on the top of the subbase;
no tool is needed to remove the thermostat from the sub-
base. Pivot the bottom of the thermostat outward (releas-
ing the snaps), then lift up to remove.
Service Indicator
When abnormal equipment operation is detected, the
This indicates that the equipment requires service from a
qualified service technician.
To replace it, first position the top tilted toward the wall
bracket and align it until you feel the tabs and slots engage;
then, while the top is in place, pivot the bottom toward the
wall until the thermostat snaps into place.
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Default Thermostat Settings
CAUTION
24VAC is present on the terminals of the thermostat
bracket. If removing the thermostat from the wall,
use caution and avoid touching any of the connec-
tor terminals on the wall bracket.
Default thermostat settings are shown in table 2 and the
default program is shown in table 3.
Table 2
Also, when working with the thermostat dip
switches, use a non−conductive tool and take cau-
tion to avoid making any contact with the circuit
board, its imprinted circuitry and its connector
prongs.
Default Thermostat Settings
Mode
Heat (Permanent Hold
Mode)
Setpoint
70°F (or 21°C)
Fan
Auto
Filter Reminder
Maintenance Reminder
OFF
Temperature Display
Display Scale: Fahrenheit or Celsius user selectable (via
Display range: 35°F (2°C) to 99°F (37°C)
Display resolution: 1°F (1°C)
Display Accuracy: +/−1°F
OFF
Equipment Protection
Timers
Reset Back to Zero
Table 3
Default Program Settings
If the Fahrenheit/Celsius display must be changed, use a
plastic, non−conductive tool to push the dip switch to the
Temperature Temperature
(Heat) (Cool)
Programs
Time
Changing Fahrenheit/Celsius Setting
Weekday − A 6:00am 70_F / 21°C 78_F / 26°C
Weekday − B 8:00am 62_F / 17°C 85_F / 29°C
Weekday − C 6:00pm 70_F / 21°C 78_F / 26°C
Weekday − D 10:00pm 62_F / 17°C 82_F / 28°C
Weekend − A 6:00am 70_F / 21°C 78_F / 26°C
Weekend − B 8:00am 62_F / 17°C 85_F / 29°C
Weekend − C 6:00pm 70_F / 21°C 78_F / 26°C
Weekend − D 10:00pm 62_F / 17°C 82_F / 28°C
1
2
3
4
5
PLUG IN (Use Care To Avoid Bending Prongs.)
6
Weekday
Monday, Tuesday, Wednesday,
Thursday, Friday
F/C (Fahrenheit/Celsius) switch
shown in Celsius position
Weekend
Saturday, Sunday
Figure 22
Temperature Measurement Range
Measurement Scale: Fahrenheit
Measurement Range: 35°F to 99°F
Measurement Resolution: 0.5°F
Measurement Accuracy: +/−1°F
Field Offset: via DIP switches to +/−3°F
Sampling Method: temperature measurements sampled
every 15 seconds. Displayed temperature is the average
of the last four measurements.
Technical Specifications
Thermostat Type
Electronic programmable thermostat for heat pump,
3-stage heat/2-stage cool.
NOTE − 2 stages of heat pump heat and one stage of auxil-
iary (gas or electric) backup heat are supported.
Temperature Setpoint Range
Setting range: 50°F (10°C) to 90°F (32°C)
Setting resolution: 1°F (1°C)
Power Supply Range
18VAC − 30VAC (24VAC nominal), 60Hz
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Smart Setback Recovery (via DIP switch #6)
Smart Setback Recovery (SSR) affects the way the ther-
mostat responds to program events. If SSR is disabled, the
thermostat will react to a program event at the time the
event occurs. However, if SSR is enabled, the thermostat
will react to a program event before the event occurs such
that the desired temperature is reached at the time of the
event, not after.
Equipment Protection Override
Both the minimum compressor OFF timer and the mini-
mum equipment cycle timer can be over−ridden by press-
ing and holding either the HEAT or COOL button down for 4
seconds.
Over−Temperature Protection
Thermal-mechanical switch opens W1, Y1, Y2, and E at
93°F+/−6°F.
Fan Control
AUTO or ON modes.
Filter Reminder
Settings of Off, 1, 3, 6 or 12 (months of fan run time) are
available. When programmed time has elapsed, a FILTER
indicator is displayed.
I/O Relays
All thermostat relays are latching type to minimize power
consumption.
Maintenance Reminder
Table 4
51M37 Terminal Designations
Term. Description
Settings of Off, 6 or 12 (months of chronological time) are
available. When programmed time has elapsed, a mainte-
nance indicator ꢀMAINT" is displayed.
B
Reversing valve, heat active
Reversing valve, cool active
24VAC
Service Reminder
O
R
The SERVICE indicator is displayed only under the follow-
ing conditions:
Y1
First-stage cooling/heating, compressor
energized
S
if the thermostat Y1 terminal has been activated with
24VAC for at least 5 minutes, AND the L terminal is
shorted to the R terminal;
W1*
Y2
Auxiliary heating, furnace energized
Second-stage cooling/heating, compressor
energized
OR
S
if the thermostat Y1 terminal has been activated with
24VAC for at least 5 minutes, AND the L terminal is
shorted to the C terminal.
E*
G
L
Emergency heat
Fan control
Service Indicator
Power Loss/Recovery
C
T
24VAC common
Thermostat memory is retained for a minimum of 24 hours
during a power loss (includes retention of program infor-
mation, HOLD status, programmed temperature setpoint,
heat/cool and fan mode settings, filter reminder status,
maintenance reminder status, and equipment protection
timers). After 24 hours of power loss, programmed settings
will be lost and replaced with default settings.
Outdoor temperature sensor connection 1
Outdoor temperature sensor connection 2
T
* For most applications, E will be jumpered to W1. If sep-
arate wires are not provided for both E and W1, jumper
the E terminal to the W1 terminal on the thermostat sub-
base. For applications involving the use of a balance
point (whereby the outdoor temperature is to be used to
restrict either heat pump operation or backup heat
source operation), the optional outdoor sensor (part
number X2658) MUST be installed.
IMPORTANT
Power must be applied for at least six consecutive
hours prior to a power loss in order for memory to
be retained for the specified time.
Equipment Protection Timers
Minimum Compressor OFF time: 5 minutes
Minimum Compressor ON time: 4 minutes
Minimum Furnace ON time: 3 minutes
LCD Backlight
Activated for 30 seconds when any button is pressed.
Minimum furnace cycle time (elapsed time between any
furnace activation and the next furnace activation): 6 min-
utes.
NOTE − During an electrical storm or similar disturbance,
the backlight may activate for a few seconds. This is nor-
mal and will no longer occur after the electrical disturbance
has passed.
Minimum elapsed time between any compressor activa-
tion and the next compressor activation: 6 minutes.
Thermostat Operating Conditions
NOTE − All protection timers (except the compressor OFF
timer) can be over−ridden if a heating or cooling demand is
initiated or terminated using the UP, DOWN, HEAT, or
COOL buttons.
35°F to 105°F, 5% to 90% RH
Thermostat Storage Conditions
−40°F to 185°F, 5% to 95% RH
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Table 5
Thermostat Output Table
THERMOSTAT OUTPUTS
ious input conditions. The following notes described terms
used in the table.
W1
E
Y1 Y2
G
O
Demand Condition
Cooling Demands
SMALL
X
X
X
X
X
X
NOTES:
LARGE
X
X
S X = output is activated with 24VAC.
S BBP: outdoor temperature is below balance point
S ABP: outdoor temperature is above balance point
No Demand
Heating Demands
w/Electric Backup (no outdoor sensor)
S In all cases, the state of the B terminal is opposite that
SMALL
LARGE
X
X
X
X
X
X
X
of the O terminal
X
X
Very LARGE
Emergency
No Demand
X
S Data are tabulated for AUTO fan setting. If the fan set-
ting is ON, the G output is activated in all cases
X
S Upstage timers:
w/Gas Backup (dual fuel; no outdoor sensor)
30 minutes (when upstaging from small to large
demand)
SMALL
X
X
X
X
LARGE
X
15 minutes (HEAT MODE ONLY − when upstaging
from large to very large demand)
Very LARGE
Emergency
No Demand
X
X
S The temperature ranges expressed in the following
definitions of ꢀSMALL/LARGE/VERY LARGE" de-
mands are for guidance only; actual temperatures
may vary:
w/Electric Backup (outdoor sensor attached)
SMALL (Above Balance Point)
LARGE (ABP)
X
X
X
X
X
X
X
X
X
X
X
X
X
X
X
With a small HEAT demand, temperature is:
below setpoint –0.5F but above setpoint –1.5F
AND 30-minute upstage timer HAS NOT expired.
Very LARGE (ABP)
SMALL (Below Balance Point)
LARGE (BBP)
With a large HEAT demand, temperature is:
less than setpoint –1.5F
OR 30-minute upstage timer HAS expired.
X
X
Very LARGE (BBP)
Emergency
X
X
With a very large heat demand, temperature is:
below setpoint –2.5F
OR 15−minute 2 upstage timer HAS expired.
No Demand
w/Gas Backup (dual fuel; outdoor sensor attached)
nd
SMALL (ABP)
LARGE (ABP)
Very LARGE (ABP)
SMALL (BBP)
LARGE (BBP)
Very LARGE (BBP)
Emergency
X
X
X
X
Emergency heat demand (only if emergency heat is
enabled): temperature is below setpoint –0.5F.
X
X
X
X
X
With a small COOL demand, temperature is:
above setpoint +0.5F but below setpoint +1.5F
AND 30-minute upstage timer HAS NOT expired.
With a large COOL demand, temperature is:
above setpoint +1.5F
OR 30-minute upstage timer HAS expired.
X
No Demand
Page 10
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OPERATION
MANUAL
E2005 Lennox Industries Inc.
Dallas, Texas, USA
51M35 Merit® Series
5/2 Day Programmable Thermostat
CONTROLS
505,049M
05/05
Supersedes 03/05
Litho U.S.A.
Table of Contents
51M35 Series Thermostat . . . . . . . . . . . . . . . . . . . . . .
Initial Thermostat Power-up . . . . . . . . . . . . . . . . . . . . .
Buttons, Backlight, Timers & Settings . . . . . . . . . . . .
DAY/TIME − Set Day and Time . . . . . . . . . . . . . . . . . .
HEAT − Heat Mode . . . . . . . . . . . . . . . . . . . . . . . . . . . .
COOL − Cool Mode . . . . . . . . . . . . . . . . . . . . . . . . . . . .
HOLD − Temperature Hold Mode . . . . . . . . . . . . . . . .
PROG − Thermostat Programming . . . . . . . . . . . . . . .
FAN − Controlling Fan Operation . . . . . . . . . . . . . . . . .
SETTINGS − Filter /Maintenance Reminder . . . . . . .
Thermostat RESET . . . . . . . . . . . . . . . . . . . . . . . . . . . .
Removing/Installing Thermostat . . . . . . . . . . . . . . . . .
Default Thermostat Settings . . . . . . . . . . . . . . . . . . . .
Technical Specifications . . . . . . . . . . . . . . . . . . . . . . . .
Thermostat Output Table . . . . . . . . . . . . . . . . . . . . . . .
RETAIN THESE INSTRUCTIONS
FOR FUTURE REFERENCE
51M35 Merit® Series Thermostat
®
The Lennox Merit Series 5/2 day programmable
electronic thermostat 51M35 provides excellent
temperature control and a large, easy-to-read display. This
product includes a programmable filter change reminder,
an equipment maintenance reminder, and a system check
indicator to notify the user when the equipment requires
service.
Thermostat 51M35 is suitable for non-heat pump, two-
stage heat/two-stage cool applications using a gas or elec-
tric furnace.
General
Initial Power−Up Display
MAINT
FILTER
MO TU WE TH FR SA SU
These instructions are intended as a general guide and do
not supersede local codes in any way. Consult authorities
having jurisdiction before installation.
SERVICE
AM
PM I8:88
Check equipment for shipping damage. If you find any
damage, immediately contact the last carrier.
FAN
ON
AUTO
PROG A | B | C | D HOLD
Introduction
SYSTEM
COOL
OFF
EMG.
HEAT
ROOM
SETPOINT
°
This document describes the operation of Lennox thermostat
51M35. Refer to the installation manual for instructions re-
garding installation and wiring of the thermostat.
88°
Hi
Lo
88
Initial Thermostat Power-up
Figure 1
When power is initially applied to the thermostat, the dis-
All display segments are momentarily activated. This oc-
curs as a normal part of thermostat initialization.
05/05
505,049M
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*P505049M*
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Within a few seconds, the Home Screen appears (see fig-
of initialization time, the actual room temperature will be
displayed.
Thermostat Buttons
Heat Cool
Prog Day/Time
Fan Hold
programming defaults.
MAINT
Home Screen
MO TU WE TH FR SA SU
FILTER
(DOOR
OPEN)
Settings Enter
SERVICE
AM
PM I2:00
FAN
ON
AUTO
PROG A B C D HOLD
Figure 3
ROOM
70°
SYSTEM
COOL
OFF
When an Arrow, HOLD, HEAT, or COOL button is pressed,
SETPOINT and the temperature setting appears for 15
seconds. If desired, start making changes within 15 sec-
onds or the HOME screen will return. The backlight will
turn off 15 seconds after the HOME screen reappears.
HEAT
Figure 2
At this point, the thermostat will be fully functional; its de-
fault temperature setpoint (not shown) is 70°F. At this
point, if the equipment has been fully powered and if a heat
demand were present, the system would begin operating.
DAY/TIME − Setting the Day and Time
Press the DAY/TIME button and set the CURRENT hour,
minute, and day of week as follows:
NOTE − Temperature scale default is Fahrenheit units but
may be reset to show Celsius, if desired. See page 7.
1. ꢀAM12" will flash on the screen. Press the UP/DOWN
arrows to change the hour. (ꢀAM" or ꢀPM" must corre-
spond to time of day.) Press DAY/TIME OR, if adjust-
ing for daylight savings time, pressing ENTER stores
the single change and exits to the HOME screen, by-
passing minutes and day of week.
Buttons, Backlight, Timers & Settings
Buttons are located behind the small door on the right−
hand side of the thermostat (see figure 3).
IMPORTANT
Do NOT begin pressing buttons until after you read
the following section describing each button.
2. Minutes will flash. Use the UP/DOWN arrow button to
display the minutes past the hour. Press DAY/TIME.
3. Day ꢀMO" (Monday) will flash. Use UP or DOWN arrow
buttons to display the current day. Day selections are
abbreviated as ꢀMOꢀ, ꢀTU", ꢀWE", ꢀTH", ꢀFR", ꢀSA",
and ꢀSU". Press DAY/TIME.
A pale blue display backlight illuminates for 30 seconds
each time any button is pressed.
When PROG or DAY/TIME is pressed, a field begins flash-
ing, expecting another input. Start making changes within
15 seconds or the HOME screen will return.
4. The HOME screen reappears; confirm day and time
are correct. This completes day and time setting.
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HEAT − Using the Heat Mode
COOL − Using the Cool Mode
Enabling and Disabling Cool Mode
Enabling and Disabling Heat Mode
Use the COOL button to enable or disable cool mode as
desired. If the thermostat is in HEAT or OFF mode, cool
mode is enabled when the COOL button is pressed. This is
indicated by COOL in the SYSTEM box (see figure 8).
The thermostat must be in the HEAT mode to control the
heating equipment. Press the Heat button to enable or dis-
able HEAT mode. If the thermostat is in COOL or OFF
mode, pressing the Heat button enables the HEAT mode
Turn Cool ON/OFF
Turn Heat ON/OFF
Heat Cool
MO
Heat Cool
AM
PM
FAN
I0:30
Prog Day/Time
MO
AM
PM 5:30
FAN
HOLD
AUTO
SYSTEM
COOL
Prog Day/Time
ROOM
SETPOINT
°70°
HOLD
AUTO
SYSTEM
Fan Hold
ROOM
75
SETPOINT
66°
Fan Hold
°
HEAT
70
Settings Enter
Settings Enter
Figure 7
Figure 4
If the thermostat is in cool mode, pressing the Cool button
disables COOL mode (indicated by OFF in the SYSTEM
box − see figure 5).
If the thermostat is in heat mode, pressing the Heat button
disables HEAT mode (indicated by OFF in the SYSTEM
Cooling Demand
Set the thermostat to cool mode to control the cooling
equipment. Then, if the room temperature is higher than
the temperature setpoint, as shown in figure 8, the thermo-
stat detects a cooling demand and will activate the cooling
equipment to satisfy the demand.
Heat/Cool Mode Disabled
SYSTEM
COOL
OFF
ROOM
75°
Heating Demand
SETPOINT
75°
HEAT
Cooling Demand
Figure 5
SYSTEM
COOL
OFF
ROOM
SETPOINT
75°
Set the thermostat to heat mode to control the heating
equipment. Then, if the room temperature is lower than the
detects a heating demand and will activate the heating
equipment to satisfy the demand.
70°
Hi
HEAT
Figure 8
Cooling operation is indicated by flashing ꢀsnowflake"
icons in the SYSTEM box. When the actual temperature
drops below the temperature setpoint, the snowflake icons
will disappear. This indicates that the cooling demand has
been satisfied and that the cooling equipment has been
turned off.
Heating Demand
SYSTEM
ROOM
SETPOINT
COOL
72°
OFF
75°
If your system supports 2−stage cooling (as does the
51M35 thermostat), you may notice various cooling levels
being delivered during a demand. Also, if a small cooling
demand is present, "Lo" will be displayed in the SYSTEM
box. However, if a large cooling demand is present, "Hi" will
be displayed in the SYSTEM box (shown in figure 8).
HEAT
Figure 6
Heating operation is indicated by a flame icon in the SYS-
TEM box. When the actual temperature rises above the
temperature setpoint, the flame icon will disappear. This
indicates that the heating demand has been satisfied and
that the heating equipment has been turned off.
NOTE − If no buttons are pressed during a demand for
cooling, the equipment must operate for at least 4 minutes.
After a demand has been satisfied, cooling equipment op-
eration is locked out for 5 minutes. If another cooling de-
mand occurs during this 5−minute interval, ꢀCOOL" and the
snowflakes will flash; however, the cooling equipment will
not operate until the 5−minute delay has elapsed.
If your system supports 2−stage heating (as does the
51M35 thermostat), you may notice various heating levels
being delivered during a demand.
NOTE − Heating equipment is activated for at least 3 min-
utes if no buttons are pressed during the demand interval.
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HOLD − Using Temperature Hold Modes
PROG − Thermostat Programming
The 51M35 thermostat can be programmed to perform a
set of either heating or cooling events (but not a
combination of heat and cool) for 5 consecutive days using
a set of 4 unique events per day. The remaining 2 days can
then be set for a different set of 4 unique events per day.
Both the consecutive days and the events/temperature are
set by the homeowner.
When HOLD is displayed at the HOME screen, the ther-
mostat is in a temperature hold condition. This means that
the temperature program data is ignored and the thermo-
stat functions much like a non−programmable thermostat.
Adjusting Temperature Setpoint in Hold Mode
The temperature setpoint represents the desired tempera-
ture of the space around the thermostat. The default tem-
perature setpoint in Hold mode is 70°F.
To Change Consecutive Days...
To alter the 5 consecutive days, press and hold the PROG
button for 5 seconds. The existing 5 consecutive day peri-
od is then displayed (default is MOnday thru FRiday). To
change to a different 5-consecutive days, use the Up/
Down arrow buttons. Any 5 consecutive−day span may be
Wednesday is defined as the 5−day programming (Thurs-
day and Friday would constitute the 2−day programming).
Press the PROG button when finished.
To adjust the setpoint, press the UP or DOWN (YB) arrow
the right of the actual room temperature. Each button
press adjusts the setpoint up or down by 1 degree.
Hold Temperature Mode
Heat Cool
MO
AM
PM
Change Consecutive days
FAN
Day/Time
Prog
I0:36
HOLD
AUTO
MO TU WE TH FR SA SU
SYSTEM
COOL
FILTER
ROOM
SETPOINT
°68°
Fan Hold
72
Figure 11
Settings Enter
To Set Program Events and Temperatures...
Figure 12 gives an example of how the two sets of pro-
grams can be set for a normal workweek and weekend.
In the 5−day bar graph, note how programs A and C reflect
the desired warmth while the home IS occupied (72°); B
allows less heating while the home is NOT occupied; D re-
flects a cool sleeping temperature. The 2−day bar graph
would support day−long occupancy and, because the first
program begins later, a less−demanding time schedule.
Figure 9
After the desired setpoint is reached, the HOME screen
will reappear after about 15 seconds.
Permanent Hold Mode
At any time the program is running, from the HOME
screen, set a permanent hold (program override) by press-
functions much like a non−programmable thermostat. Use
the Up/Down arrow buttons to adjust the hold setpoint. To
return to the program, press HOLD again.
5/2 Program Example
5−DAY PROGRAM: | A
|
B
|
C
|
D
|
72°
68°
64°
6AM
Temporary (2-Hour) Hold
NOON
6PM
|
MID
NIGHT
6AM
At any time the program is running, from the HOME
screen, set a temporary 2-hour hold by pressing the Up/
Down arrow buttons until the desired setpoint is displayed;
gram for 2 hours from the last button press, then returns to
the program.
2−DAY PROGRAM:
| A |
B
C
|
D
|
72°
68°
64°
6AM
NOON
6PM
MID
NIGHT
6AM
Figure 12
Temporary Hold Temperature Mode
NOTE − Pressing ENTER during the following program-
ming steps, saves and exits to the HOME screen.
Heat Cool
MO
AM
PM
FAN
To program events and temperatures, perform the follow-
ing steps, once with Cool selected and once with Heat se-
lected.
Day/Time
Prog
I0:36
AUTO
PROG
B
HOLD
SYSTEM
COOL
ROOM
SETPOINT
°68°
Fan Hold
72
1. Press and release PROG. ꢀAM 6:00", period ꢀA", and
the 5 consecutive days are displayed; ꢀAM 6" flashes.
2. Use the Up/Down arrow buttons to select the desired
hour; press PROG when the desired hour is reached.
3. Use the Up/Down arrow buttons to select the desired
minute; press PROG.
Settings Enter
Figure 10
4. Use the Up/Down arrow buttons to select the desired
temperature setpoint; press PROG.
While in Temporary Hold, press HOLD once to switch to
Permanent Hold (HOLD displays solid; PROG not dis-
played); press HOLD again to return to the program
(PROG displays; HOLD not displayed).
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NOTE − This thermostat will NOT automatically switch
from heating to cooling, or vice versa; operator involve-
ment is required. At the change of seasons, or to accomo-
date abnormal seasonal temperature swings, you must
manually select to the opposite conditioning (Heat or Cool)
program.
Reminder Settings Display
For MAINT,
press Settings
button 2 times
MAINT
FILTER
For FILTER,
press Settings
button 1 time
OFF
FAN − Controlling the Fan Operation
Use the FAN button to select either continuous fan mode or
auto fan mode.
Figure 15
The default setting for the reminders is OFF (disabled).
Press Up/Down arrow buttons to select the desired re-
minder intervals.
To change from continuous to auto fan mode (or vice ver-
sa), press the FAN button. Note whether a fan icon in the
FAN box is present (indicating that the fan is running) or not
(fan not running).
Table 1
Filter and Maintenance Reminders
If continuous fan mode is enabled (ON displayed in FAN
box − see figure 13), the fan will run continuously regard-
less of whether the heating or cooling equipment is run-
ning.
Buttons to
Use
Available Settings and
How to Use
Reminder
FILTER
Settings
(1st press)
then Arrows
to scroll
Total fan run time expressed in
months (Off, 1, 3, 6, 12); for
example, if fan runs 12 hours a
day, 1 month reminder displays
in 2 calendar months.
Using Fan ON
Heat Cool
MO
AM
PM I0:I0
FAN
ON
selections
Day/Time
Prog
PROG
B
SYSTEM
HEAT
Settings
(2nd press)
then Arrows
to scroll
Elapsed chronological time in
months (Off, 6, 12). Use this,
for example, to remind yourself
when to perform routine checks
or when to call a technician for
perodic preventive
MAINT
ROOM
67°
Fan Hold
Settings Enter
selections
maintenance.
Figure 13
Enter
Stores settings.
If auto fan mode is selected (AUTO displayed in FAN box −
cooling equipment is running.
NOTE − The HOME screen will reappear about 15 seconds
after the final arrow button press. OR, press ENTER at any
time to store any changes and exit to the HOME screen.
After either programmed interval has elapsed, the remind-
Using Fan Auto
Heat Cool
MO
Reminders
MAINT
AM
PM
FAN
Day/Time
Prog
I0:36
MO TU WE TH FR SA SU
FILTER
PROG
B
AUTO
SYSTEM
COOL
ROOM
SERVICE
Fan Hold
72°
AM
PM
FAN
ON
AUTO
I0:i2
Settings Enter
PROG A B C D HOLD
ROOM
SYSTEM
COOL
OFF
Figure 14
78°
SETTINGS − Filter/Maintenance Reminders
HEAT
The 51M35 thermostat is designed to remind you when the
filter needs changing or when routine maintenance is re-
quired, as (and if) defined, by you. These optional remind-
ers are not enabled until you activate them. To do so, press
the Settings button (shown below the Fan button in figure
14) once or twice for the desired reminder as shown in fig-
Figure 16
After the filter has been changed or maintenance per-
formed, reset the reminder by pressing the SETTINGS
button for 4 seconds. The screen will blink for a few mo-
ments to indicate that the timer has been reset.
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Service Indicator
Removing and Installing Thermostat
The thermostat hinges on tabs on the top of the subbase;
no tool is needed to remove the thermostat from the sub-
base. Pivot the bottom of the thermostat outward (releas-
ing the snaps), then lift up to remove.
When abnormal equipment operation is detected, the
This indicates that the equipment requires service from a
qualified service technician.
To install it, first position the top tilted toward the wall brack-
et and align it until you feel the tabs and slots engage; then,
while the top is in place, pivot the bottom toward the wall
until the thermostat snaps into place.
Service Indicator Flashing
MO TU WE TH FR SA SU
FILTER
SERVICE
Default Thermostat Settings
AM
FAN
PM I0:i4
ON
program is shown in table 3.
AUTO
PROG A B C D HOLD
ROOM
Table 2
SYSTEM
COOL
OFF
Default Thermostat Settings
79°
Mode
Heat (Permanent Hold
Mode)
HEAT
Setpoint
70°F (or 21°C)
Figure 17
Fan
Auto
Filter Reminder
Maintenance Reminder
OFF
Thermostat RESET
OFF
Under some abnormal conditions, it may be necessary to
ꢀreset" the thermostat to its default condition. Such a RE-
SET would delete all programming and settings and there-
fore should only be used on rare occasions when the the
thermostat fails to function as designd and/or as pro-
grammed. Such an instance can occur as a result of a pow-
er surge or similar electrical disturbance (e.g. after an elec-
trical storm or power outage). The RESET button can be
used to recover from this situation.
Equipment Protection
Timers
Reset Back to Zero
Table 3
Default Program Settings
Temperature Temperature
(Heat) (Cool)
Programs
Time
Weekday − A 6:00am 70_F / 21°C 78_F / 26°C
Weekday − B 8:00am 62_F / 17°C 85_F / 29°C
Weekday − C 6:00pm 70_F / 21°C 78_F / 26°C
Weekday − D 10:00pm 62_F / 17°C 82_F / 28°C
Weekend − A 6:00am 70_F / 21°C 78_F / 26°C
Weekend − B 8:00am 62_F / 17°C 85_F / 29°C
Weekend − C 6:00pm 70_F / 21°C 78_F / 26°C
Weekend − D 10:00pm 62_F / 17°C 82_F / 28°C
CAUTION
When the RESET button is pressed, ALL settings re-
vert back to the defaults, including the default pro-
The RESET button is an unlabeled, recessed button lo-
cated behind the door on the right−hand side of the thermo-
per clip or small pencil to press the RESET button; ALL
thermostat settings will be reset to the defaults listed in the
Default Thermostat Settings section.
Weekday
Monday, Tuesday, Wednesday,
Thursday, Friday
Weekend
Saturday, Sunday
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Temperature Setpoint Range
Setting range: 50°F (10°C) to 90°F (32°C)
Setting resolution: 1°F (1°C)
Technical Specifications
Thermostat Type
Electronic programmable thermostat for 2−Stage (gas or
electric) Heat/2-Stage Cool, non−heat pump, non−power
robbing applications.
Smart Setback Recovery (via DIP switch #6)
Smart Setback Recovery (SSR) affects the way the ther-
mostat responds to program events. If SSR is disabled, the
thermostat will react to a program event at the time the
event occurs. However, if SSR is enabled, the thermostat
will react to a program event before the event occurs such
that the desired temperature is reached at the time of the
event, not after.
Power Supply Range
18VAC − 30VAC (24VAC nominal), 60Hz
CAUTION
Fan Control
AUTO or ON modes, gas or electric heat compatible via
DIP switches (also see Thermostat Output section).
24VAC is present on the terminals of the thermostat
bracket. If removing the thermostat from the wall,
use caution and avoid touching any of the connec-
tor terminals on the wall bracket.
I/O Relays
All thermostat relays are latching type to minimize power
consumption.
Also, when working with the thermostat dip
switches, use a non−conductive tool and take cau-
tion to avoid making any contact with the circuit
board, its imprinted circuitry and its connector
prongs.
Table 4
51M35 Terminal Designations
Terminal
Description
R
24VAC
Temperature Display
Display Scale: Fahrenheit or Celsius user selectable (via
Display range: 35°F (2°C) to 99°F (37°C)
Display resolution: 1°F (1°C)
Display Accuracy: +/−1°F
Y1
W1
Y2
W2
G
First-stage cooling
First-stage heating
Second-stage cooling
Second-stage heating
Fan control
If the Fahrenheit/Celsius display must be changed, use a
plastic, non−conductive tool to push the dip switch to the
L
Service Indicator
24VAC common
C
Changing Fahrenheit/Celsius Setting
Equipment Protection Timers
Minimum Compressor OFF time: 5 minutes
Minimum Compressor ON time: 4 minutes
Minimum Furnace ON time: 3 minutes
Minimum furnace cycle time (elapsed time between any
furnace activation and the next furnace activation): 6 min-
utes.
1
2
3
4
5
Minimum elapsed time between any compressor activa-
tion and the next compressor activation: 6 minutes.
NOTE − All protection timers (except the compressor OFF
timer) can be over−ridden if a heating or cooling demand is
initiated or terminated using the UP, DOWN, HEAT, or
COOL buttons.
PLUG IN (Use Care To Avoid Bending Prongs.)
6
F/C (Fahrenheit/Celsius) switch
shown in Celsius position
Equipment Protection Override
Both the minimum compressor OFF timer and the mini-
mum equipment cycle timer can be over−ridden by press-
ing and holding either the HEAT or COOL button down for 4
seconds.
Figure 18
Temperature Measurement Range
Measurement Scale: Fahrenheit
Measurement Range: 35°F to 99°F
Measurement Resolution: 0.5°F
Measurement Accuracy: +/−1°F
Over−Temperature Protection
Thermal-mechanical switch opens W1 and W2 at
93°F+/−6°F.
Field Offset: via DIP switches to +/−3°F
Sampling Method: temperature measurements sampled
every 15 seconds. Displayed temperature is the average
of the last four measurements.
Filter Reminder
Settings of Off, 1, 3, 6 or 12 (months of fan run time) are
available. When programmed time has elapsed, a FILTER
indicator is displayed.
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NOTE − The temperature ranges expressed in the follow-
ing definitions of ꢀSMALL/LARGE" demands are for guid-
ance only; actual temperatures may vary.
Maintenance Reminder
Settings of Off, 6 or 12 (months of chronological time) are
available. When programmed time has elapsed, a mainte-
nance indicator ꢀMAINT" is displayed.
With a small HEAT demand, temperature is:
below setpoint –0.5F but above setpoint –1.5F
AND 30-minute upstage timer HAS NOT expired.
Service Reminder
The SERVICE indicator is displayed only under the follow-
ing conditions:
With a large HEAT demand, temperature is:
less than setpoint –1.5F
OR 30-minute upstage timer HAS expired.
S
if the thermostat Y1 terminal has been activated with
24VAC for at least 5 minutes, AND the L terminal is
shorted to the R terminal;
With a small COOL demand, temperature is:
above setpoint +0.5F but below setpoint +1.5F
AND 30-minute upstage timer HAS NOT expired.
OR
S
if the thermostat Y1 terminal has been activated with
24VAC for at least 5 minutes, AND the L terminal is
shorted to the C terminal.
With a large COOL demand, temperature is:
above setpoint +1.5F
OR 30-minute upstage timer HAS expired.
Power Loss/Recovery
Table 5
Thermostat memory is retained for a minimum of 24 hours
during a power loss (includes retention of program infor-
mation, HOLD status, programmed temperature setpoint,
heat/cool and fan mode settings, filter reminder status,
maintenance reminder status, and equipment protection
timers). After 24 hours of power loss, programmed settings
will be lost and replaced with default settings.
Thermostat Outputs
Condition
W1 W2 Y1 Y2
G
Gas Heat, Auto Fan
HEAT Demand, SMALL
HEAT Demand, LARGE
COOL Demand, SMALL
COOL Demand, LARGE
No Demand
X
X
X
X
X
X
X
X
X
X
X
X
X
IMPORTANT
Power must be applied for at least six consecutive
hours prior to a power loss in order for memory to
be retained for the specified time.
Gas Heat, Continuous Fan
HEAT Demand, SMALL
HEAT Demand, LARGE
COOL Demand, SMALL
COOL Demand, LARGE
No Demand
X
X
X
X
X
X
X
LCD Backlight
X
X
Activated for 30 seconds when any button is pressed.
NOTE − During an electrical storm or similar disturbance,
the backlight may activate for a few seconds. This is nor-
mal and will no longer occur after the electrical disturbance
has passed.
Electric Heat, Auto Fan
HEAT Demand, SMALL
HEAT Demand, LARGE
COOL Demand, SMALL
COOL Demand, LARGE
No Demand
X
X
X
X
X
X
Thermostat Operating Conditions
X
X
35°F to 105°F, 5% to 90% RH
Thermostat Storage Conditions
−40°F to 185°F, 5% to 95% RH
Electric Heat, Continuous Fan
Thermostat Output Table
HEAT Demand, SMALL
HEAT Demand, LARGE
COOL Demand, SMALL
COOL Demand, LARGE
No Demand
X
X
X
X
X
X
X
X
ious input conditions. The following notes described terms
used in the table.
X
X
NOTE − X = output is activated with 24VAC.
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OPERATION
MANUAL
E2005 Lennox Industries Inc.
Dallas, Texas, USA
51M34 Merit® Series
5/2 Day Programmable Thermostat
CONTROLS
505,048M
05/05
Supersedes 03/05
Litho U.S.A.
Table of Contents
51M34 Series Thermostat . . . . . . . . . . . . . . . . . . . . . .
Initial Thermostat Power-up . . . . . . . . . . . . . . . . . . . . .
Buttons, Backlight, Timers & Settings . . . . . . . . . . . .
DAY/TIME − Set Day and Time . . . . . . . . . . . . . . . . . .
HEAT − Heat Mode . . . . . . . . . . . . . . . . . . . . . . . . . . . .
COOL − Cool Mode . . . . . . . . . . . . . . . . . . . . . . . . . . . .
HOLD − Temperature Hold Mode . . . . . . . . . . . . . . . .
PROG − Thermostat Programming . . . . . . . . . . . . . . .
FAN − Controlling Fan Operation . . . . . . . . . . . . . . . . .
SETTINGS − Filter /Maintenance Reminder . . . . . . .
Thermostat RESET . . . . . . . . . . . . . . . . . . . . . . . . . . . .
Removing/Installing Thermostat . . . . . . . . . . . . . . . . .
Default Thermostat Settings . . . . . . . . . . . . . . . . . . . .
Technical Specifications . . . . . . . . . . . . . . . . . . . . . . . .
Thermostat Output Table . . . . . . . . . . . . . . . . . . . . . . .
RETAIN THESE INSTRUCTIONS
FOR FUTURE REFERENCE
51M34 Merit® Series Thermostat
®
The Lennox Merit Series 5/2 day programmable
electronic thermostat 51M34 provides excellent
temperature control and a large, easy-to-read display. This
product includes a programmable filter change reminder,
an equipment maintenance reminder, and a system check
indicator to notify the user when the equipment requires
service.
Thermostat 51M34 is suitable for non−heat pump, single-
stage heat/single-stage cool applications with either gas or
electric furnace.
General
Initial Power−Up Display
MAINT
FILTER
MO TU WE TH FR SA SU
These instructions are intended as a general guide and do
not supersede local codes in any way. Consult authorities
having jurisdiction before installation.
SERVICE
AM
PM I8:88
Check equipment for shipping damage. If you find any
damage, immediately contact the last carrier.
FAN
ON
AUTO
PROG A | B | C | D HOLD
Introduction
SYSTEM
COOL
OFF
EMG.
HEAT
ROOM
SETPOINT
°
This document describes the operation of Lennox thermostat
51M34. Refer to the installation manual for instructions re-
garding installation and wiring of the thermostat.
88°
Hi
Lo
88
Initial Thermostat Power-up
Figure 1
When power is initially applied to the thermostat, the dis-
All display segments are momentarily activated. This oc-
curs as a normal part of thermostat initialization.
05/05
505,048M
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*P505048M*
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Within a few seconds, the Home Screen appears (see fig-
of initialization time, the actual room temperature will be
displayed.
Thermostat Buttons
Heat Cool
Prog Day/Time
Fan Hold
MAINT
Home Screen
MO TU WE TH FR SA SU
FILTER
SERVICE
(DOOR
OPEN)
AM
PM I2:00
Settings Enter
FAN
ON
AUTO
PROG A B C D HOLD
ROOM
70°
SYSTEM
COOL
OFF
Figure 3
HEAT
When an Arrow, HOLD, HEAT, or COOL button is pressed,
SETPOINT and the temperature setting appears for 15
seconds. If desired, start making changes within 15 sec-
onds or the HOME screen will return. The backlight will
turn off 15 seconds after the HOME screen reappears.
Figure 2
At this point, the thermostat will be fully functional; its de-
fault temperature setpoint (not shown) is 70°F. At this
point, if the equipment has been fully powered and if a heat
demand were present, the system would begin operating.
DAY/TIME − Setting the Day and Time
NOTE − Temperature scale default is Fahrenheit units but
may be reset to show Celsius, if desired. See page 7.
Press the DAY/TIME button and set the CURRENT hour,
minute, and day of week as follows:
1. ꢀAM12" will flash on the screen. Press the UP/DOWN
arrows to change the hour. (ꢀAM" or ꢀPM" must corre-
spond to time of day.) Press DAY/TIME OR, if adjust-
ing for daylight savings time, pressing ENTER stores
the single change and exits to the HOME screen, by-
passing minutes and day of week.
Buttons, Backlight, Timers & Settings
Buttons are located behind the small door on the right−
hand side of the thermostat (see figure 3).
IMPORTANT
Do NOT begin pressing buttons until after you read
the following section describing each button.
2. Minutes will flash. Use the UP/DOWN arrow button to
display the minutes past the hour. Press DAY/TIME.
3. Day ꢀMO" (Monday) will flash. Use UP or DOWN arrow
buttons to display the current day. Day selections are
abbreviated as ꢀMOꢀ, ꢀTU", ꢀWE", ꢀTH", ꢀFR", ꢀSA",
and ꢀSU". Press DAY/TIME.
A pale blue display backlight illuminates for 30 seconds
each time any button is pressed.
When PROG or DAY/TIME is pressed, a field begins flash-
ing, expecting another input. Start making changes within
15 seconds or the HOME screen will return.
4. The HOME screen reappears; confirm day and time
are correct. This completes day and time setting.
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HEAT − Using the Heat Mode
COOL − Using the Cool Mode
Enabling and Disabling Cool Mode
Enabling and Disabling Heat Mode
The thermostat must be in the HEAT mode to control the
heating equipment. Press the Heat button to enable or dis-
able HEAT mode. If the thermostat is in COOL or OFF
mode, pressing the Heat button enables the HEAT mode
Use the COOL button to enable or disable cool mode as
desired. If the thermostat is in HEAT or OFF mode, cool
mode is enabled when the COOL button is pressed. This is
indicated by COOL in the SYSTEM box (see figure 7).
Turn Heat ON/OFF
Turn Cool ON/OFF
Heat Cool
Heat Cool
MO
MO
AM
PM 5:30
FAN
AM
PM
FAN
I0:30
Prog Day/Time
Prog Day/Time
Fan Hold
HOLD
AUTO
HOLD
AUTO
SYSTEM
SYSTEM
COOL
ROOM
ROOM
SETPOINT
°70°
SETPOINT
66°
Fan Hold
°
HEAT
75
70
Settings Enter
Settings Enter
Figure 4
Figure 7
If the thermostat is in heat mode, heat mode is disabled
when the HEAT button is pressed. This is indicated by OFF
If the thermostat is in cool mode, pressing the Cool button
disables COOL mode (indicated by OFF in the SYSTEM
box − see figure 5).
Heat/Cool Mode Disabled
Cooling Demand
SYSTEM
COOL
OFF
ROOM
75°
Heating Demand
Set the thermostat to cool mode to control the cooling
equipment. Then, if the room temperature is higher than
the temperature setpoint, as shown in figure 8, the thermo-
stat detects a cooling demand and will activate the cooling
equipment to satisfy the demand.
SETPOINT
75°
HEAT
Figure 5
Cooling Demand
Set the thermostat to heat mode to control the heating
equipment. Then, if the room temperature is lower than the
detects a heating demand and will activate the heating
equipment to satisfy the demand.
SYSTEM
COOL
OFF
ROOM
75°
SETPOINT
70°
HEAT
Figure 8
Heating Demand
SYSTEM
ROOM
Cooling operation is indicated by flashing ꢀsnowflake"
icons in the SYSTEM box. When the actual temperature
drops below the temperature setpoint, the snowflake icons
will disappear. This indicates that the cooling demand has
been satisfied and that the cooling equipment has been
turned off.
SETPOINT
COOL
72°
OFF
75°
HEAT
Figure 6
NOTE − If no buttons are pressed during a demand for
cooling, the equipment must operate for at least 4 minutes.
After a demand has been satisfied, cooling equipment op-
eration is locked out for 5 minutes. If another cooling de-
mand occurs during this 5−minute interval, ꢀCOOL" and the
snowflakes will flash; however, the cooling equipment will
not operate until the 5−minute delay has elapsed.
Heating operation is indicated by a flame icon in the SYS-
TEM box. When the actual temperature rises above the
temperature setpoint, the flame icon will disappear. This
indicates that the heating demand has been satisfied and
that the heating equipment has been turned off.
NOTE − Heating equipment is activated for at least 3 min-
utes if no buttons are pressed during the demand interval.
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HOLD − Using Temperature Hold Modes
PROG − Thermostat Programming
The 51M34 thermostat can be programmed to perform a
set of either heating or cooling events (but not a
combination of heat and cool) for 5 consecutive days using
a set of 4 unique events per day. The remaining 2 days can
then be set for a different set of 4 unique events per day.
Both the consecutive days and the events/temperature are
set by the homeowner.
When HOLD is displayed at the HOME screen, the ther-
mostat is in a temperature hold condition. This means that
the temperature program data is ignored and the thermo-
stat functions much like a non−programmable thermostat.
Adjusting Temperature Setpoint in Hold Mode
The temperature setpoint represents the desired tempera-
ture of the space around the thermostat. The default tem-
perature setpoint in Hold mode is 70°F.
To Change Consecutive Days...
To alter the 5 consecutive days, press and hold the PROG
button for 5 seconds. The existing 5 consecutive day peri-
od is then displayed (default is MOnday thru FRiday). To
change to a different 5-consecutive days, use the Up/
Down arrow buttons. Any 5 consecutive−day span may be
Wednesday is defined as the 5−day programming (Thurs-
day and Friday would constitute the 2−day programming).
Press the PROG button when finished.
To adjust the setpoint, press the UP or DOWN (YB) arrow
the right of the actual room temperature. Each button
press adjusts the setpoint up or down by 1 degree.
Hold Temperature Mode
Heat Cool
MO
AM
PM
Change Consecutive days
FAN
Day/Time
Prog
I0:36
HOLD
AUTO
MO TU WE TH FR SA SU
SYSTEM
COOL
FILTER
ROOM
SETPOINT
°68°
Fan Hold
72
Figure 11
Settings Enter
To Set Program Events and Temperatures...
Figure 12 gives an example of how the two sets of pro-
grams can be set for a normal workweek and weekend.
In the 5−day bar graph, note how programs A and C reflect
the desired warmth while the home IS occupied (72°); B
allows less heating while the home is NOT occupied; D re-
flects a cool sleeping temperature. The 2−day bar graph
would support day−long occupancy and, because the first
program begins later, a less−demanding time schedule.
Figure 9
After the desired setpoint is reached, the HOME screen
will reappear after about 15 seconds.
Permanent Hold Mode
At any time the program is running, from the HOME
screen, set a permanent hold (program override) by press-
functions much like a non−programmable thermostat. Use
the Up/Down arrow buttons to adjust the hold setpoint. To
return to the program, press HOLD again.
5/2 Program Example
5−DAY PROGRAM: | A
|
B
|
C
|
D
|
72°
68°
64°
6AM
Temporary (2-Hour) Hold
NOON
6PM
|
MID
NIGHT
6AM
At any time the program is running, from the HOME
screen, set a temporary 2-hour hold by pressing the Up/
Down arrow buttons until the desired setpoint is displayed;
gram for 2 hours from the last button press, then returns to
the program.
2−DAY PROGRAM:
| A |
B
C
|
D
|
72°
68°
64°
6AM
NOON
6PM
MID
NIGHT
6AM
Figure 12
Temporary Hold Temperature Mode
NOTE − Pressing ENTER during the following program-
ming steps, saves and exits to the HOME screen.
Heat Cool
MO
AM
PM
FAN
To program events and temperatures, perform the follow-
ing steps, once with Cool selected and once with Heat se-
lected.
Day/Time
Prog
I0:36
AUTO
PROG
B
HOLD
SYSTEM
COOL
ROOM
SETPOINT
°68°
Fan Hold
72
1. Press and release PROG. ꢀAM 6:00", period ꢀA", and
the 5 consecutive days are displayed; ꢀAM 6" flashes.
2. Use the Up/Down arrow buttons to select the desired
hour; press PROG when the desired hour is reached.
3. Use the Up/Down arrow buttons to select the desired
minute; press PROG.
Settings Enter
Figure 10
4. Use the Up/Down arrow buttons to select the desired
temperature setpoint; press PROG.
While in Temporary Hold, press HOLD once to switch to
Permanent Hold (HOLD displays solid; PROG not dis-
played); press HOLD again to return to the program
(PROG displays; HOLD not displayed).
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NOTE − This thermostat will NOT automatically switch
from heating to cooling, or vice versa; operator involve-
ment is required. At the change of seasons, or to accomo-
date abnormal seasonal temperature swings, you must
manually select to the opposite conditioning (Heat or Cool)
program.
Reminder Settings Display
For MAINT,
press Settings
button 2 times
MAINT
FILTER
For FILTER,
press Settings
button 1 time
OFF
FAN − Controlling the Fan Operation
Figure 15
Use the FAN button to select either continuous fan mode or
auto fan mode.
The default setting for the reminders is OFF (disabled).
Press Up/Down arrow buttons to select the desired re-
minder intervals.
To change from continuous to auto fan mode (or vice ver-
sa), press the FAN button. Note whether a fan icon in the
FAN box is present (indicating that the fan is running) or not
(fan not running).
Table 1
Filter and Maintenance Reminders
If continuous fan mode is enabled (ON displayed in FAN
box − see figure 13), the fan will run continuously regard-
less of whether the heating or cooling equipment is run-
ning.
Buttons to
Use
Available Settings and
How to Use
Reminder
FILTER
Settings
(1st press)
then Arrows
to scroll
Total fan run time expressed in
months (Off, 1, 3, 6, 12); for
example, if fan runs 12 hours a
day, 1 month reminder displays
in 2 calendar months.
Using Fan ON
Heat Cool
MO
AM
PM I0:I0
FAN
ON
selections
Day/Time
Prog
PROG
B
Settings
(2nd press)
then Arrows
to scroll
Elapsed chronological time in
months (Off, 6, 12). Use this,
for example, to remind yourself
when to perform routine checks
or when to call a technician for
perodic preventive
MAINT
SYSTEM
HEAT
ROOM
67°
Fan Hold
selections
Settings Enter
maintenance.
Enter
Stores settings.
Figure 13
NOTE − The HOME screen will reappear about 15 seconds
after the final arrow button press. OR, press ENTER at any
time to store any changes and exit to the HOME screen.
If auto fan mode is selected (AUTO displayed in FAN box −
cooling equipment is running.
After either programmed interval has elapsed, the remind-
Using Fan Auto
Heat Cool
MO
Reminders
MAINT
AM
PM
FAN
Day/Time
Prog
MO TU WE TH FR SA SU
I0:36
FILTER
PROG
B
AUTO
SYSTEM
COOL
ROOM
SERVICE
Fan Hold
72°
AM
PM
FAN
ON
AUTO
I0:i2
Settings Enter
PROG A B C D HOLD
ROOM
SYSTEM
COOL
OFF
Figure 14
78°
SETTINGS − Filter/Maintenance Reminders
HEAT
The 51M34 thermostat is designed to remind you when the
filter needs changing or when routine maintenance is re-
quired, as (and if) defined, by you. These optional remind-
ers are not enabled until you activate them. To do so, press
the Settings button (shown below the Fan button in figure
14) once or twice for the desired reminder as shown in fig-
Figure 16
After the filter has been changed, reset the filter reminder
by pressing the SETTINGS button for 4 seconds. The
screen will blink for a few moments to indicate that the tim-
er has been reset.
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Service Indicator
Removing/Installing Thermostat
The thermostat hinges on tabs on the top of the subbase;
no tool is needed to remove the thermostat from the sub-
base. Pivot the bottom of the thermostat outward (releas-
ing the snaps), then lift up to remove.
When abnormal equipment operation is detected, the
This indicates that the equipment requires service from a
qualified service technician.
To replace it, first position the top tilted toward the wall
bracket and align it until you feel the tabs and slots engage;
then, while the top is in place, pivot the bottom toward the
wall until the thermostat snaps into place.
Service Indicator Flashing
MO TU WE TH FR SA SU
FILTER
SERVICE
Default Thermostat Settings
AM
FAN
PM I0:i4
ON
program is shown in table 3.
AUTO
PROG A B C D HOLD
ROOM
Table 2
SYSTEM
COOL
OFF
Default Thermostat Settings
79°
Mode
Heat (Permanent Hold
Mode)
HEAT
Setpoint
70°F (or 21°C)
Figure 17
Fan
Auto
Filter Reminder
Maintenance Reminder
OFF
Thermostat RESET
OFF
Under some abnormal conditions, it may be necessary to
ꢀreset" the thermostat to its default condition. Such a RE-
SET would delete all programming and settings and there-
fore should only be used on rare occasions when the the
thermostat fails to function as designd and/or as pro-
grammed. Such an instance can occur as a result of a pow-
er surge or similar electrical disturbance (e.g. after an elec-
trical storm or power outage). The RESET button can be
used to recover from this situation.
Equipment Protection
Timers
Reset Back to Zero
Table 3
Default Program Settings
Temperature Temperature
(Heat) (Cool)
Programs
Time
Weekday − A 6:00am 70_F / 21°C 78_F / 26°C
Weekday − B 8:00am 62_F / 17°C 85_F / 29°C
Weekday − C 6:00pm 70_F / 21°C 78_F / 26°C
Weekday − D 10:00pm 62_F / 17°C 82_F / 28°C
Weekend − A 6:00am 70_F / 21°C 78_F / 26°C
Weekend − B 8:00am 62_F / 17°C 85_F / 29°C
Weekend − C 6:00pm 70_F / 21°C 78_F / 26°C
Weekend − D 10:00pm 62_F / 17°C 82_F / 28°C
CAUTION
When the RESET button is pressed, ALL settings re-
vert back to the defaults, including the default pro-
The RESET button is an unlabeled, recessed button lo-
cated behind the door on the right−hand side of the thermo-
per clip or small pencil to press the RESET button; ALL
thermostat settings will be reset to the defaults listed in the
Default Thermostat Settings section.
Weekday
Monday, Tuesday, Wednesday,
Thursday, Friday
Weekend
Saturday, Sunday
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Field Offset: via DIP switches to +/−3°F
Technical Specifications
Thermostat Type
Sampling Method: temperature measurements sampled
every 15 seconds. Displayed temperature is the average
of the last four measurements.
Electronic programmable thermostat for 1-Stage-
Heat/1-Stage Cool, gas or electric heat, non−heat pump,
non−power robbing applications.
Temperature Setpoint Range
Setting range: 50°F (10°C) to 90°F (32°C)
Setting resolution: 1°F (1°C)
Power Supply Range
Smart Setback Recovery (via DIP switch #6)
Smart Setback Recovery (SSR) affects the way the ther-
mostat responds to program events. If SSR is disabled, the
thermostat will react to a program event at the time the
event occurs. However, if SSR is enabled, the thermostat
will react to a program event before the event occurs such
that the desired temperature is reached at the time of the
event, not after.
18VAC − 30VAC (24VAC nominal), 60Hz
CAUTION
24VAC is present on the terminals of the thermostat
bracket. If removing the thermostat from the wall,
use caution and avoid touching any of the connec-
tor terminals on the wall bracket.
Fan Control
AUTO or ON modes, gas or electric heat compatible via
DIP switches (also see Thermostat Output section).
Also, when working with the thermostat dip
switches, use a non−conductive tool and take cau-
tion to avoid making any contact with the circuit
board, its imprinted circuitry and its connector
prongs.
I/O Relays
All thermostat relays are latching type to minimize power
consumption.
Table 4
51M34 Terminal Designations
Temperature Display
Terminal
Description
Display Scale: Fahrenheit or Celsius user selectable (via
Display range: 35°F (2°C) to 99°F (37°C)
Display resolution: 1°F (1°C)
Display Accuracy: +/−1°F
R
24VAC
Y1
W1
G
First stage cooling
First stage heating
Fan control
L
Service Indicator
24VAC common
If the Fahrenheit/Celsius display must be changed, use a
plastic, non−conductive tool to push the dip switch to the
C
Equipment Protection Timers
Minimum Compressor OFF time: 5 minutes
Minimum Compressor ON time: 4 minutes
Minimum Furnace ON time: 3 minutes
Changing Fahrenheit/Celsius Setting
Minimum furnace cycle time (elapsed time between any
furnace activation and the next furnace activation): 6 min-
utes.
Minimum elapsed time between any compressor activa-
tion and the next compressor activation: 6 minutes.
1
2
3
4
5
NOTE − All protection timers (except the compressor OFF
timer) can be over−ridden if a heating or cooling demand is
initiated or terminated using the UP, DOWN, HEAT, or
COOL buttons.
PLUG IN (Use Care To Avoid Bending Prongs.)
6
Equipment Protection Override
Both the minimum compressor OFF timer and the mini-
mum equipment cycle timer can be over−ridden by press-
ing and holding either the HEAT or COOL button down for 4
seconds.
F/C (Fahrenheit/Celsius) switch
shown in Celsius position
Figure 18
Over−Temperature Protection
Thermal-mechanical switch opens W1 at 93°F+/−6°F.
Temperature Measurement Range
Measurement Scale: Fahrenheit
Measurement Range: 35°F to 99°F
Measurement Resolution: 0.5°F
Measurement Accuracy: +/−1°F
Filter Reminder
Settings of Off, 1, 3, 6 or 12 (months of fan run time) are
available. When programmed time has elapsed, a FILTER
indicator is displayed.
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Maintenance Reminder
Thermostat Operating Conditions
Settings of Off, 6 or 12 (months of chronological time) are
available. When programmed time has elapsed, a mainte-
nance indicator ꢀMAINT" is displayed.
35°F to 105°F, 5% to 90% RH
Thermostat Storage Conditions
Service Reminder
The SERVICE indicator is displayed only under the follow-
ing conditions:
−40°F to 185°F, 5% to 95% RH
Thermostat Output Table
S
if the thermostat Y1 terminal has been activated with
24VAC for at least 5 minutes, AND the L terminal is
shorted to the R terminal;
ious input conditions.
OR
Table 5
S
if the thermostat Y1 terminal has been activated with
24VAC for at least 5 minutes, AND the L terminal is
shorted to the C terminal.
Thermostat Outputs
Condition
W1
Y1
G
Power Loss/Recovery
Gas Heat, Auto Fan
HEAT Demand
Thermostat memory is retained for a minimum of 24 hours
during a power loss (includes retention of program infor-
mation, HOLD status, programmed temperature setpoint,
heat/cool and fan mode settings, filter reminder status,
maintenance reminder status, and equipment protection
timers). After 24 hours of power loss, programmed settings
will be lost and replaced with default settings.
X
COOL Demand
No Demand
X
X
Gas Heat, Continuous Fan
HEAT Demand
X
X
X
X
X
COOL Demand
No Demand
X
X
X
IMPORTANT
Power must be applied for at least six consecutive
hours prior to a power loss in order for memory to
be retained for the specified time.
Electric Heat, Auto Fan
HEAT Demand
X
X
COOL Demand
No Demand
LCD Backlight
Activated for 30 seconds when any button is pressed.
Electric Heat, Continuous Fan
HEAT Demand
COOL Demand
No Demand
X
X
X
X
NOTE − During an electrical storm or similar disturbance,
the backlight may activate for a few seconds. This is nor-
mal and will no longer occur after the electrical disturbance
has passed.
NOTE − X = output is activated with 24VAC.
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OPERATION
MANUAL
E2005 Lennox Industries Inc.
Dallas, Texas, USA
51M39 Merit® Series
7 Day Programmable Thermostat
CONTROLS
505,068M
07/05
Litho U.S.A.
Table of Contents
51M39 Series Thermostat . . . . . . . . . . . . . . . . . . . . . .
Initial Thermostat Power-up . . . . . . . . . . . . . . . . . . . . .
Buttons, Backlight, Timers & Settings . . . . . . . . . . . .
DAY/TIME − Set Day and Time . . . . . . . . . . . . . . . . . .
HEAT − Heat Mode . . . . . . . . . . . . . . . . . . . . . . . . . . . .
COOL − Cool Mode . . . . . . . . . . . . . . . . . . . . . . . . . . . .
AUTO − Using the Autochangeover Mode . . . . . . . . .
HOLD − Temperature Hold Mode . . . . . . . . . . . . . . . .
PROG − Thermostat Programming . . . . . . . . . . . . . . .
COPY − Copy a Program . . . . . . . . . . . . . . . . . . . . . . .
FAN − Controlling Fan Operation . . . . . . . . . . . . . . . . .
Displaying Outside Temperature . . . . . . . . . . . . . . . . .
SETTINGS − Filter /Maintenance Reminder . . . . . . .
Thermostat RESET . . . . . . . . . . . . . . . . . . . . . . . . . . . .
Removing/Installing Thermostat . . . . . . . . . . . . . . . . .
Default Thermostat Settings . . . . . . . . . . . . . . . . . . . .
Thermostat Output Table . . . . . . . . . . . . . . . . . . . . . . .
Technical Specifications . . . . . . . . . . . . . . . . . . . . . . . .
RETAIN THESE INSTRUCTIONS
FOR FUTURE REFERENCE
51M39 Merit® Series Thermostat
®
The Lennox Merit Series 7 day programmable electronic
thermostat 51M39 provides excellent temperature control
and a large, easy-to-read display. This product includes a
programmable filter change reminder, an equipment
maintenance reminder, and a system check indicator to
notify the user when the equipment requires service.
Thermostat 51M39 is suitable for non-heat pump, two-
stage heat/two-stage cool applications using a gas or elec-
tric furnace. Also, the thermostat provides autochange-
over capability.
General
Initial Power−Up Display
Copy tofrom MO TU WE TH FR SA SU
These instructions are intended as a general guide and do
not supersede local codes in any way. Consult authorities
having jurisdiction before installation.
PROG
A B C D
AM
PM
I8:88
HOLD
Check equipment for shipping damage. If you find any
damage, immediately contact the last carrier.
FAN ON
AUTO
COOL SETPOINT
FILTER
SERVICE
°
COOL
OFF
MAINTENANCE
Introduction
AUTO
EMG.
HEAT
OUTROOM
i88°
HEAT SETPOINT
This document describes the operation of Lennox thermo-
stat 51M39. Refer to the installation manual for instructions
regarding installation and wiring of the thermostat.
°
Hi
Lo
Initial Thermostat Power-up
Figure 1
When power is initially applied to the thermostat, the dis-
All display segments are momentarily activated. This oc-
curs as a normal part of thermostat initialization.
07/05
505,068M
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Within a few seconds, the HOME screen appears (see fig-
of initialization time, the actual room temperature will be
displayed.
Thermostat Buttons
Heat Cool
Prog Auto
Copy Day/Time
Fan Hold
programming defaults.
HOME Screen
(DOOR
OPEN)
MO
PROG
A B C D
HOLD
Settings Enter
(Outside)
AM
PM
I0:26
COOL
OFF
FAN ON
AUTO
Figure 3
ROOM
AUTO
EMG.
HEAT
70°
When an Arrow, HOLD, HEAT, or COOL button is pressed,
SETPOINT and the temperature setting appears for 15
seconds. If desired, start making changes within 15 sec-
onds or the HOME screen will return. The backlight will
turn off 15 seconds after the HOME screen reappears.
Figure 2
At this point, the thermostat will be fully functional; its de-
fault temperature setpoint (not shown) is 70°F. At this
point, if the equipment has been fully powered and if a heat
demand were present, the system would begin operating.
DAY/TIME − Setting the Day and Time
NOTE − Temperature scale default is Fahrenheit units but
may be reset to show Celsius, if desired. See page 8.
Press the DAY/TIME button and set the CURRENT hour,
minute, and day of week as follows:
Buttons, Backlight, Timers & Settings
1. ꢀAM12" will flash on the screen. Press the UP/DOWN
arrows to change the hour. (ꢀAM" or ꢀPM" must corre-
spond to time of day.) Press DAY/TIME OR, if adjust-
ing for daylight savings time, pressing the ENTER but-
ton stores the single change and exits to the HOME
screen, bypassing minutes and day of week.
Buttons are located behind the small door on the right−
hand side of the thermostat (see figure 3).
IMPORTANT
Do NOT begin pressing buttons until after you read
the following section describing each button.
2. Minutes will flash. Use the UP/DOWN arrow button to
display the minutes past the hour. Press DAY/TIME.
A pale blue display backlight illuminates for 30 seconds
each time any button is pressed.
3. Day ꢀMO" (Monday) will flash. Use UP or DOWN arrow
buttons to display the current day. Day selections are
abbreviated as ꢀMOꢀ, ꢀTU", ꢀWE", ꢀTH", ꢀFR", ꢀSA",
and ꢀSU". Press DAY/TIME.
When PROG or DAY/TIME button is pressed, a field be-
gins flashing, expecting another input. Start making
changes within 15 seconds or the HOME screen will re-
turn.
4. The HOME screen reappears; confirm day and time
are correct. This completes day and time setting.
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HEAT − Using the Heat Mode
COOL − Using the Cool Mode
Enabling and Disabling Heat Mode
Enabling and Disabling Cool Mode
Use the COOL button to enable or disable cool mode as
desired. If the thermostat is in heat or off mode, cool mode
is enabled when the COOL button is pressed. This is indi-
cated by COOL on the right side of the display (see figure
The thermostat must be in the heat mode to control the
heating equipment. Press the HEAT button to enable or
disable HEAT mode. If the thermostat is in cool or off
mode, pressing the HEAT button enables the heat mode
Turn Heat ON/OFF
Turn Cool ON/OFF
Heat Cool
Heat Cool
Prog Auto
Prog Auto
5:30
i0:30
Copy Day/Time
Copy Day/Time
82
70
Fan Hold
(DOOR
OPEN)
Fan Hold
70
66
(DOOR
OPEN)
75
66
Settings Enter
(Outside)
Settings Enter
(Outside)
Figure 4
Figure 7
If the thermostat is in heat mode, heat mode is disabled
when the HEAT button is pressed. This is indicated by OFF
If the thermostat is in cool mode, pressing the COOL but-
ton disables cool mode (indicated by OFF in the SYSTEM
box − see figure 5).
Heat/Cool Mode Disabled
ROOM
Cooling Demand
OFF
Set the thermostat to cool mode to control the cooling
equipment. Then, if the room temperature is higher than
the temperature setpoint, as shown in figure 7, the thermo-
stat detects a cooling demand and will activate the cooling
equipment to satisfy the demand.
HEAT SETPOINT
75°
75°
Figure 5
Cooling operation is indicated by flashing ꢀsnowflake"
icons on the right side of the display. When the actual tem-
perature drops below the temperature setpoint, the snow-
flake icons will disappear. This indicates that the cooling
demand has been satisfied and that the cooling equipment
has been turned off.
Heating Demand
Set the thermostat to heat mode to control the heating
equipment. Then, if the room temperature is lower than the
detects a heating demand and will activate the heating
equipment to satisfy the demand.
If your system supports 2−stage cooling (as does the
51M39 thermostat), you may notice various cooling levels
being delivered during a demand. Also, if a small cooling
demand is present, "Lo" will be displayed in the SYSTEM
box. However, if a large cooling demand is present, "Hi" will
be displayed in the lower right corner (shown in figure 8).
Heating Demand
ROOM
HEAT SETPOINT
72°
HEAT
75°
Figure 6
Cooling Demand
Heating operation is indicated by a flame icon in the lower
right corner. When the actual temperature rises above the
temperature setpoint, the flame icon will disappear. This
indicates that the heating demand has been satisfied and
that the heating equipment has been turned off.
COOL SETPOINT
FAN
AUTO
76°
COOL
ROOM
80°
If your system supports 2−stage heating (as does the
51M39 thermostat), you may notice various heating levels
being delivered during a demand.
HEAT SETPOINT
68°
Hi
Lo
NOTE − Heating equipment is activated for at least 3 min-
utes if no buttons are pressed during the demand interval.
Figure 8
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NOTE − If no buttons are pressed during a demand for
cooling, the equipment must operate for at least 4 minutes.
After a demand has been satisfied, cooling equipment op-
eration is locked out for 5 minutes. If another cooling de-
mand occurs during this 5−minute interval, ꢀCOOL" and the
snowflakes will flash; however, the cooling equipment will
not operate until the 5−minute delay has elapsed.
After the desired setpoint is reached, the HOME screen
will reappear after about 15 seconds.
Permanent Hold Mode
At any time the program is running, from the HOME
screen, set a permanent hold (program override) by press-
functions much like a non−programmable thermostat. Use
the Up/Down arrow buttons to adjust the hold setpoint. To
return to the program, press the HOLD button again.
AUTO − Using the Autochangeover Mode
When in either heat mode or cool mode, autochangeover
can be enabled or disabled by pressing the AUTO button. If
enabled, AUTO appears at the lower right corner of the dis-
play. The thermostat will then automatically change over
from heating to cooling and vice versa, to keep the room
temperature in between the heating and cooling setpoints.
Autochangeover can be used in either of the thermostat
hold modes, or when the thermostat program is running.
These modes are described later.
Temporary (2-Hour) Hold
At any time the program is running, from the HOME
screen, set a temporary 2-hour hold by pressing the Up/
Down arrow buttons until the desired setpoint is displayed;
gram for 2 hours from the last button press, then returns to
the program.
While in Temporary Hold, press the HOLD button once to
switch to Permanent Hold (HOLD displays solid; PROG
not displayed); press the HOLD button again to return to
the program (PROG displays; HOLD not displayed).
HOLD − Using Temperature Hold Modes
When HOLD is displayed at the HOME screen, the ther-
mostat is in a temperature hold condition. This means that
the temperature program data is ignored and the thermo-
stat functions much like a non−programmable thermostat.
Temporary Hold Temperature Mode
Heat Cool
Adjusting Temperature Setpoint in Hold Mode
Prog Auto
The temperature setpoint represents the desired tempera-
ture of the space around the thermostat. The default heat
setpoint in hold mode is 70°F; the default cool setpoint in
hold mode is 78°F.
i0:30
Copy Day/Time
70
Fan Hold
(DOOR
OPEN)
72
68
Hold Temperature Mode
Settings Enter
(Outside)
Heat Cool
Prog Auto
i0:30
Figure 10
Copy Day/Time
78
PROG − Thermostat Programming
Fan Hold
(DOOR
OPEN)
80
70
The 51M39 thermostat can be programmed to perform a
set of both heating and cooling events for each day of the
week. Each day can be programmed for 4 unique events
per day, and each day can be different from any other day.
Settings Enter
(Outside)
To Set Program Events and Temperatures
grams can be set. Programs A and C reflect the desired
warmth while the home IS occupied (72°); B allows less
heating while the home is NOT occupied; D reflects a cool
sleeping temperature.
Figure 9
To adjust the setpoint, press the UP or DOWN (YB) arrow
buttons; the existing setpoint is displayed to the right of the
actual room temperature. Each button press adjusts the
setpoint up or down by 1 degree.
NOTE − This example shows heat setpoints; a similar pro-
gram may be set for cool setpoints.
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C
want the same program.
Program Example
MONDAY PROGRAM: 72°
A
C
68°
64°
D
Copy a Programmed Day to Another Day
B
Heat Cool
TUESDAY PROGRAM: 72°
68°
64°
A
C
D
B
Prog Auto
Copy Day/Time
Fan Hold
Copy from MO
WEDNESDAY PROGRAM: 72°
68°
64°
A
C
D
B
(DOOR
OPEN)
THURSDAY PROGRAM: 72°
A
A
C
68°
64°
D
B
B
Settings Enter
(outside)
FRIDAY PROGRAM: 72°
68°
64°
C
D
Figure 12
SATURDAY PROGRAM: 72°
C
A
68°
64°
B
peratures for days not yet programmed.
D
SUNDAY PROGRAM: 72°
FAN − Controlling the Fan Operation
A
C
68°
64°
B
D
Use the FAN button to select either continuous fan mode or
auto fan mode.
6AM
NOON
6PM
MID
NIGHT
6AM
To change from continuous to auto fan mode (or vice ver-
sa), press the FAN button. Note whether a fan icon in the
FAN box is present (indicating that the fan is running) or not
(fan not running).
Figure 11
NOTE − Pressing the ENTER button during the following
programming steps, saves and exits to the HOME screen.
If continuous fan mode is enabled (ON displayed in FAN
box − see figure 13), the fan will run continuously regard-
less of whether the heating or cooling equipment is run-
ning.
To program events and temperatures, perform the follow-
ing steps.
1. Press and release the PROG button. "MO" (Monday),
"AM 6:00", period "A", "COOL SETPOINT 78", and
"HEAT SETPOINT 70" are displayed. "AM 6" flashes.
Heat Cool
Using Fan ON
2. Use the Up/Down arrow buttons to select the desired
hour; press the PROG button when the desired hour
is reached; use the Up/Down arrow buttons again to
select desired minute. The selected hour/minute is
when the program will start. Press the PROG button.
Prog Auto
i0:30
Copy Day/Time
Fan Hold
72
3. Use the Up/Down arrow buttons to select the desired
heat setpoint; press the PROG button.
4. Use the Up/Down arrow buttons to select the desired
cool setpoint; press the PROG button.
Figure 13
If auto fan mode is selected (AUTO displayed in FAN box −
cooling equipment is running.
6. COPY A PROGRAM − If the program entered for
Monday is satisfactory for other days of the week, do
the following to repeat the same program for other
days:
Heat Cool
Using Fan AUTO
A
Press the PROG button, then press the COPY
button. The display clears the screen except for
ꢀCopy from MOꢀ with ꢀMO" flashing. Press the
COPY button again to copy Monday’s program.
The display changes to ꢀCopy to MO". Use the ar-
row buttons to scroll to the desired day where you
want the same program as Monday. Press the
COPY button.
Prog Auto
i0:30
Copy Day/Time
B
Fan Hold
72
Figure 14
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After either programmed interval has elapsed, the remind-
Displaying Outside Temperature
Pressing the ENTER (Outside) button at the HOME
several seconds, then returns to room temperature.
Reminders
MO
PROG
A B C D
HOLD
AM
PM
Check Outside Temperature
I0:26
COOL
OFF
FAN ON
AUTO
Filter &
Maintenance
Indicators
Fan Hold
FILTER SERVICE
MAINTENANCE
ROOM
88
AUTO
EMG.
HEAT
Settings Enter
(Outside)
76°
Figure 15
FAN ON
AUTO
FILTER SERVICE
MAINTENANCE
ROOM
Service
Indicator
Flashing
COOL
OFF
SETTINGS − Filter/Maintenance Reminders
AUTO
EMG.
HEAT
The 51M39 thermostat is designed to remind you when the
filter needs changing or when routine maintenance is re-
quired, as (and if) defined, by you. These optional remind-
ers are not enabled until you activate them. To do so, press
the SETTINGS button once or twice for the desired re-
76°
Figure 17
After the filter has been changed or maintenance per-
formed, reset the reminder by pressing the SETTINGS
button for 4 seconds at the corresponding reminder set-
tings screen. The screen will blink for a few moments to in-
dicate that the timer has been reset.
Reminder Settings Display
OFF
Service Indicator
For FILTER, press SETTINGS
button 1 time
When abnormal equipment operation is detected, the
This indicates that the equipment requires service from a
qualified service technician.
FILTER
For MAINTENANCE, press
SETTINGS button 2 times
MAINTENANCE
Figure 16
Thermostat RESET
The default setting for the reminders is OFF (disabled).
Press Up/Down arrow buttons to select the desired re-
minder intervals.
Under some abnormal conditions, it may be necessary to
ꢀreset" the thermostat to its default condition. Such a RE-
SET would delete all programming and settings and there-
fore should only be used on rare occasions when the the
thermostat fails to function as designed and/or as pro-
grammed. Such an instance can occur as a result of a pow-
er surge or similar electrical disturbance (e.g. after an elec-
trical storm or power outage). The RESET button can be
used to recover from this situation.
Table 1
Filter and Maintenance Reminders
Buttons to
Use
Available Settings and
How to Use
Reminder
FILTER
Settings
(1st press)
then Arrows
to scroll
Total fan run time expressed in
months (Off, 1, 3, 6, 12); for
example, if fan runs 12 hours a day,
1 month reminder displays in 2
calendar months.
selections
CAUTION
When the RESET button is pressed, ALL settings re-
vert back to the defaults, including the default pro-
Settings
(2nd press)
then Arrows
to scroll
Elapsed chronological time in
months (Off, 6, 12). Use this, for
example, to remind yourself when to
perform routine checks or when to
call a technician for perodic
MAINTE-
NANCE
selections
preventive maintenance.
The RESET button is an unlabeled, recessed button lo-
cated behind the door on the right−hand side of the thermo-
per clip or small pencil to press the RESET button; ALL
thermostat settings will be reset to the defaults listed in the
Default Thermostat Settings section.
Enter
Stores settings.
NOTE − The HOME screen will reappear about 15 seconds
after the final arrow button press. OR, press the ENTER
button at any time to store any changes and exit to the
HOME screen.
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Table 4
Removing and Installing Thermostat
Thermostat Outputs
Condition
W1 W2 Y1 Y2
G
The thermostat hinges on tabs on the top of the subbase;
no tool is needed to remove the thermostat from the sub-
base. Pivot the bottom of the thermostat outward (releas-
ing the snaps), then lift up to remove.
Gas Heat, Auto Fan
Heat Demand, SMALL
Heat Demand, LARGE
Cool Demand, SMALL
Cool Demand, LARGE
No Demand
X
X
X
X
X
X
X
X
X
To install it, first position the top tilted toward the wall brack-
et and align it until you feel the tabs and slots engage; then,
while the top is in place, pivot the bottom toward the wall
until the thermostat snaps into place.
X
X
X
X
X
Gas Heat, Continuous Fan
Heat Demand, SMALL
Heat Demand, LARGE
Cool Demand, SMALL
Cool Demand, LARGE
No Demand
X
X
X
X
X
X
X
Default Thermostat Settings
X
X
program is shown in table 3.
Table 2
Electric Heat, Auto Fan
Heat Demand, SMALL
Heat Demand, LARGE
Cool Demand, SMALL
Cool Demand, LARGE
No Demand
Default Thermostat Settings
X
X
X
X
X
X
Mode
Heat (Perm. Hold)
70°F (or 21°C)
78°F (or 26°C)
Auto
Heat Setpoint (Perm Hold)
Cool Setpoint (Perm Hold)
Fan
X
X
Filter Reminder
Maintenance Reminder
OFF
Electric Heat, Continuous Fan
OFF
Heat Demand, SMALL
Heat Demand, LARGE
Cool Demand, SMALL
Cool Demand, LARGE
No Demand
X
X
X
X
X
X
X
Equipment Protection Timers Reset Back to Zero
Autochangeover OFF
X
X
Table 3
Default Program Settings (Time & Temp.)
NOTE − X = output is activated with 24VAC.
NOTE − The temperature ranges expressed in the follow-
ing definitions of ꢀSMALL/LARGE" demands are for guid-
ance only; actual temperatures may vary.
Programs
Time
Temp.− Heat Temp.− Cool
All days − A 6:00am 70_F / 21°C
All days − B 8:00am 62_F / 17°C
All days − C 6:00pm 70_F / 21°C
All days − D 10:00pm 62_F / 17°C
78_F / 26°C
85_F / 29°C
78_F / 26°C
82_F / 28°C
With a SMALL heat demand, temperature is:
below setpoint –0.5F but above setpoint –1.5F
AND 30-minute upstage timer HAS NOT expired.
With a LARGE heat demand, temperature is:
less than setpoint –1.5F
OR 30-minute upstage timer HAS expired.
With a SMALL cool demand, temperature is:
above setpoint +0.5F but below setpoint +1.5F
AND 30-minute upstage timer HAS NOT expired.
Thermostat Output Table
ious input conditions. The following notes described terms
used in the table.
With a LARGE cool demand, temperature is:
above setpoint +1.5F
OR 30-minute upstage timer HAS expired.
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Display range: 35°F (2°C) to 99°F (37°C)
Display resolution: 1°F (1°C)
Technical Specifications
Display Accuracy: +/−1°F
Thermostat Type
Electronic programmable thermostat for 2−Stage (gas or
electric) Heat/2-Stage Cool, non−heat pump, non−power
robbing applications.
If the Fahrenheit/Celsius display must be changed, use a
plastic, non−conductive tool to push the dip switch to the
Table 5
51M39 Terminal Designations
Terminal Description
Indoor Temperature Measurement Range
Measurement Scale: Fahrenheit
Measurement Range: 35°F to 99°F
Measurement Resolution: 0.5°F
Measurement Accuracy: +/−1°F
Field Offset: via DIP switches to +/−3°F
Sampling Method: temperature measurements sampled
every 15 seconds. Displayed temperature is the average
of the last four measurements.
R
24VAC
Y1
W1
Y2
W2
G
First-stage cooling
First-stage heating
Second-stage cooling
Second-stage heating
Fan control
Outdoor Temperature Measurement Range
Measurement Scale: Fahrenheit
Measurement Range: −22°F to 122°F
Measurement Resolution: 1°F
Measurement Accuracy: +/−2°F
L
Service Indicator
C
24VAC common
T
Outdoor temperature sensor connection 1
Outdoor temperature sensor connection 2
T
Temperature Setpoint Range
Setting range: 50°F (10°C) to 90°F (32°C)
Setting resolution: 1°F (1°C)
Power Supply Range
18VAC − 30VAC (24VAC nominal), 60Hz
Smart Setback Recovery (via DIP switch #6)
Smart Setback Recovery (SSR) affects the way the ther-
mostat responds to program events. If SSR is disabled, the
thermostat will react to a program event at the time the
event occurs. However, if SSR is enabled, the thermostat
will react to a program event before the event occurs such
that the desired temperature is reached at the time of the
event, not after.
CAUTION
24VAC is present on the terminals of the thermostat
bracket. If removing the thermostat from the wall,
use caution and avoid touching any of the connec-
tor terminals on the wall bracket.
Also, when working with the thermostat dip
switches, use a non−conductive tool and take cau-
tion to avoid making any contact with the circuit
board, its imprinted circuitry and its connector
prongs.
Autochangeover Deadband Selection (via DIP
switch #7)
Autochangeover deadband can be set to 4 or 6 degrees.
When autochangeover is enabled (via the AUTO button),
the thermostat will automatically change over from heating
to cooling and vice versa, to keep the room temperature in
between the heating and cooling setpoints. The deadband
is the minimum difference between the heating and cooling
setpoints.
Temperature Display
Display Scale: Fahrenheit or Celsius user selectable (via
Fan Control
AUTO or ON modes, gas or electric heat compatible via
DIP switches (also see Thermostat Output section).
Changing Fahrenheit/Celsius Setting
I/O Relays
All thermostat relays are latching type to minimize power
consumption.
Equipment Protection Timers
1
2
3
4
5
Minimum Compressor OFF time: 5 minutes
Minimum Compressor ON time: 4 minutes
Minimum Furnace ON time: 3 minutes
PLUG IN (Use Care To Avoid Bending Prongs.)
6
7
Minimum furnace cycle time (elapsed time between any
furnace activation and the next furnace activation): 6 min-
utes.
F/C (Fahrenheit/Celsius) switch
shown in Celsius position
Minimum elapsed time between any compressor activa-
tion and the next compressor activation: 6 minutes.
Figure 18
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NOTE − All protection timers (except the compressor OFF
timer) can be over−ridden if a heating or cooling demand is
initiated or terminated using the UP, DOWN, HEAT, or
COOL buttons.
S
if the thermostat Y1 terminal has been activated with
24VAC for at least 5 minutes, AND the L terminal is
shorted to the C terminal.
Power Loss/Recovery
Equipment Protection Override
Thermostat memory is retained for a minimum of 24 hours
during a power loss (includes retention of clock setting,
program information, HOLD status, programmed temper-
ature setpoint, heat/cool and fan mode settings, filter re-
minder status, maintenance reminder status, and equip-
ment protection timers). After 24 hours of power loss,
programmed settings will be lost and replaced with default
settings.
Both the minimum compressor OFF timer and the mini-
mum equipment cycle timer can be over−ridden by press-
ing and holding either the HEAT or COOL button down for 4
seconds.
Over−Temperature Protection
Thermal-mechanical switch opens W1 and W2 at
93°F+/−6°F.
Filter Reminder
IMPORTANT
Power must be applied for at least six consecutive
hours prior to a power loss in order for memory to
be retained for the specified time.
Settings of Off, 1, 3, 6 or 12 (months of fan run time) are
available. When programmed time has elapsed, a FILTER
indicator is displayed.
Maintenance Reminder
LCD Backlight
Activated for 30 seconds when any button is pressed.
Settings of Off, 6 or 12 (months of chronological time) are
available. When programmed time has elapsed, a mainte-
nance indicator ꢀMAINTENANCE" is displayed.
NOTE − During an electrical storm or similar disturbance,
the backlight may activate for a few seconds. This is nor-
mal and will no longer occur after the electrical disturbance
has passed.
Service Reminder
The SERVICE indicator is displayed only under the follow-
ing conditions:
Thermostat Operating Conditions
35°F to 105°F, 5% to 90% RH
S
if the thermostat Y1 terminal has been activated with
24VAC for at least 5 minutes, AND the L terminal is
shorted to the R terminal;
Thermostat Storage Conditions
−40°F to 185°F, 5% to 95% RH
OR
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51M39 SERIES 5/2
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OPERATION
MANUAL
E2005 Lennox Industries Inc.
Dallas, Texas, USA
51M42 Merit® Series
7−Day Programmable Thermostat
CONTROLS
505,069M
07/05
Litho U.S.A.
Table of Contents
51M42 Series Thermostat . . . . . . . . . . . . . . . . . . . . . .
Initial Thermostat Power-up . . . . . . . . . . . . . . . . . . . . .
Buttons, Backlight, Timers & Settings . . . . . . . . . . . .
DAY/TIME − Set Day and Time . . . . . . . . . . . . . . . . . .
HEAT − Heat Mode . . . . . . . . . . . . . . . . . . . . . . . . . . . .
COOL − Cool Mode . . . . . . . . . . . . . . . . . . . . . . . . . . . .
AUTO − Using the Autochangeover Mode . . . . . . . . .
HOLD − Temperature Hold Mode . . . . . . . . . . . . . . . .
PROG − Thermostat Programming . . . . . . . . . . . . . . .
COPY − Copy a Program . . . . . . . . . . . . . . . . . . . . . . .
FAN − Controlling Fan Operation . . . . . . . . . . . . . . . . .
Displaying Outside Temperature . . . . . . . . . . . . . . . . .
SETTINGS − Filter and Maintenance Reminders . . .
SETTINGS − Balance Point . . . . . . . . . . . . . . . . . . . . .
Thermostat RESET . . . . . . . . . . . . . . . . . . . . . . . . . . . .
Removing/Installing Thermostat . . . . . . . . . . . . . . . . .
Default Thermostat Settings . . . . . . . . . . . . . . . . . . . .
Technical Specifications . . . . . . . . . . . . . . . . . . . . . . . .
Thermostat Output Table . . . . . . . . . . . . . . . . . . . . . . .
RETAIN THESE INSTRUCTIONS
FOR FUTURE REFERENCE
51M42 Merit® Series Thermostat
®
The Lennox Merit Series 7−day programmable electronic
thermostat 51M42 provides excellent temperature control
and a large, easy-to-read display. It includes a
programmable filter change reminder, an equipment
maintenance reminder, and a system check indicator to
notify the user when the equipment requires service.
Thermostat 51M42 is suitable for heat pump, three-stage
heat/two-stage cool applications using a gas or electric
auxiliary heat source. In addition to displaying outdoor
temperature, the outdoor temperature sensor provides
auxiliary heat lockout, balance point operation, and dual−
fuel compatibility. Also, the thermostat provides auto-
changeover capability.
General
Initial Power−Up Display
Copy tofrom MO TU WE TH FR SA SU
These instructions are intended as a general guide and do
not supersede local codes in any way. Consult authorities
having jurisdiction before installation.
PROG
A B C D
AM
PM
I8:88
HOLD
Check equipment for shipping damage. If you find any
damage, immediately contact the last carrier.
FAN ON
AUTO
COOL SETPOINT
FILTER
SERVICE
°
COOL
OFF
MAINTENANCE
Introduction
AUTO
EMG.
HEAT
OUTROOM
i88°
HEAT SETPOINT
This document describes the operation of Lennox thermostat
51M42. Refer to the installation manual for instructions re-
garding installation and wiring of the thermostat.
°
Hi
Lo
Initial Thermostat Power-up
Figure 1
When power is initially applied to the thermostat, the dis-
All display segments are momentarily activated. This oc-
curs as a normal part of thermostat initialization.
07/05
505,069M
Page 1
*2P0705*
*P505069M*
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Within a few seconds, the HOME screen appears (see fig-
of initialization time, the actual room temperature will be
displayed.
Thermostat Buttons
Heat Cool
Prog Auto
Copy Day/Time
Fan Hold
programming defaults.
HOME Screen
(DOOR
OPEN)
MO
PROG
A B C D
HOLD
Settings Enter
(Outside)
AM
PM
I0:26
COOL
OFF
FAN ON
AUTO
Figure 3
ROOM
AUTO
EMG.
HEAT
70°
When an Arrow, HOLD, HEAT, or COOL button is pressed,
SETPOINT and the temperature setting appears for 15
seconds. If desired, start making changes within 15 sec-
onds or the HOME screen will return. The backlight will
turn off 15 seconds after the HOME screen reappears.
Figure 2
At this point, the thermostat will be fully functional; its de-
fault temperature setpoint (not shown) is 70°F. At this
point, if the equipment has been fully powered and if a heat
demand were present, the system would begin operating.
DAY/TIME − Setting the Day and Time
NOTE − Temperature scale default is Fahrenheit units but
may be reset to show Celsius, if desired. See page 9.
Press the DAY/TIME button and set the CURRENT hour,
minute, and day of week as follows:
Buttons, Backlight, Timers & Settings
1. ꢀAM12" will flash on the screen. Press the Up/Down ar-
rows buttons to change the hour. (ꢀAM" or ꢀPM" must
correspond to time of day.) Press DAY/TIME OR, if ad-
justing for daylight savings time, pressing the ENTER
button stores the single change and exits to the HOME
screen, bypassing minutes and day of week.
Buttons are located behind the small door on the right−
hand side of the thermostat (see figure 3).
IMPORTANT
Do NOT begin pressing buttons until after you read
the following section describing each button.
2. Minutes will flash. Use the Up/Down arrow buttons to
display the minutes past the hour. Press DAY/TIME.
3. Day ꢀMO" (Monday) will flash. Use Up/Down arrow
buttons to display the current day. Day selections are
abbreviated as ꢀMOꢀ, ꢀTU", ꢀWE", ꢀTH", ꢀFR", ꢀSA",
and ꢀSU". Press DAY/TIME.
A pale blue display backlight illuminates for 30 seconds
each time any button is pressed.
When PROG or DAY/TIME button is pressed, a field be-
gins flashing, expecting another input. Start making
changes within 15 seconds or the HOME screen will re-
turn.
4. The HOME screen reappears; confirm day and time
are correct. This completes day and time setting.
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HEAT − Using the Heat Mode
Heat Mode Disabled
ROOM
OFF
Normal Heat Mode
HEAT SETPOINT
75°
In normal heat mode, both the heat pump and the backup
heat source are used to provide heat. If the thermostat de-
tects that the heat pump is not able to provide enough heat
(as may be the case in very cold weather), then the backup
heat source is activated.
75°
Figure 6
Heating Demand
Emergency Heat Mode
The thermostat must be in either normal or emergency
heat mode in order to properly control the heating equip-
ment. In either heat mode, when the actual temperature is
the thermostat detects a heating demand and activates the
heating equipment to satisfy the demand. Heating opera-
tion is indicated by a flame icon in bottom right−hand corner of
the display.
In emergency heat mode, only the backup heat source
provides heat ꢁthe heat pump is disabled. The backup
heat source is activated only when there is a heat demand.
Enabling Normal Heat Mode
Use the HEAT button to select normal heat mode, emer-
gency heat mode, or to disable heat modes as desired. If
the thermostat is in off or cool mode, pressing the HEAT
button enables heat mode. This is indicated by HEAT in the
lower right corner (see figure 4).
Heating Demand
ROOM
Normal Heat Mode Enabled
HEAT SETPOINT
72°
HEAT
75°
Heat Cool
Prog Auto
Figure 7
5:30
When the actual temperature rises above the temperature
setpoint, the flame icon will disappear. This indicates that
the heating demand has been satisfied and that the heat-
ing equipment has been turned off.
Copy Day/Time
82
Fan Hold
(DOOR
OPEN)
70
70
Settings Enter
(Outside)
Heat pump operation is locked out for 5 minutes after a de-
mand has been satisfied. If another heat pump demand
occurs during this 5−minute interval, the flame icon will
flash; however, the heat pump will not run until the 5−min-
ute delay has elapsed.
Figure 4
Enabling Emergency Heat Mode
NOTE − The heat pump is activated for at least 4 minutes if
no buttons are pressed during the demand interval. The
backup heat source is activated for at least 3 minutes if no
buttons are pressed during the demand interval.
If the thermostat is in heat mode, pressing the HEAT button
mostat is in normal heat mode when the HEAT button is
pressed, then emergency heat mode is enabled.
If your system supports 3−stage heating (as does the
51M42 thermostat), you may notice various heating levels
being delivered during a demand.
Emergency Heat Mode Enabled
ROOM
If a small heat pump heating demand is present, "Lo" is dis-
played in the SYSTEM box; if a large heat pump heating
HEAT SETPOINT
EMG.
HEAT
62°
75°
Large Heat Heating Demand
Figure 5
ROOM
65°
Disabling Heat Mode
HEAT SETPOINT
HEAT
If the thermostat is in emergency heat mode when the
HEAT button is pressed, then heat modes are disabled.
This is indicated by OFF near the right side of the display
75°
Hi
Lo
Figure 8
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NOTE − If no buttons are pressed during a demand for
cooling, the equipment must operate for at least 4 minutes.
After a demand has been satisfied, cooling equipment op-
eration is locked out for 5 minutes. If another cooling de-
mand occurs during this 5−minute interval, ꢀCOOL" and the
snowflakes will flash; however, the cooling equipment will
not operate until the 5−minute delay has elapsed.
COOL − Using the Cool Mode
Enabling and Disabling Cool Mode
Use the COOL button to enable or disable cool mode as
desired. If the thermostat is in heat or off mode, cool mode
is enabled when the COOL button is pressed. This is indi-
cated by COOL on the right side of the display (see figure
AUTO − Using the Autochangeover Mode
When in either heat mode or cool mode, autochangeover
can be enabled or disabled by pressing the AUTO button. If
enabled, AUTO appears at the lower right corner of the dis-
play. The thermostat will then automatically change over
from heating to cooling and vice versa, to keep the room
temperature in between the heating and cooling setpoints.
Autochangeover can be used in either of the thermostat
hold modes, or when the thermostat program is running.
These modes are described later.
Turn Cool ON/OFF
Heat Cool
Prog Auto
i0:30
Copy Day/Time
70
Fan Hold
(DOOR
OPEN)
75
66
HOLD − Using Temperature Hold Modes
Settings Enter
(Outside)
When the HOLD button is displayed at the HOME screen,
the thermostat is in a temperature hold condition. This
means that the temperature program data is ignored and
the thermostat functions much like a non−programmable
thermostat.
Figure 9
If the thermostat is in cool mode, pressing the COOL but-
ton disables cool mode (indicated by OFF on the right side
of the display).
Adjusting Temperature Setpoint in Hold Mode
The temperature setpoint represents the desired tempera-
ture of the space around the thermostat. The default heat
setpoint in hold mode is 70°F; the default cool setpoint in
hold mode is 78°F.
Cooling Demand
Set the thermostat to cool mode to control the cooling
equipment. Then, if the room temperature is higher than
mostat detects a cooling demand and will activate the cool-
ing equipment to satisfy the demand.
To adjust the setpoint, press the UP or DOWN (YB) arrow
buttons (see figure 11); the existing setpoint is displayed to
the right of the actual room temperature. Each button
press adjusts the setpoint up or down by 1 degree.
Hold Temperature Mode
Cooling Demand
COOL SETPOINT
Heat Cool
FAN
AUTO
76°
Prog Auto
COOL
i0:30
Copy Day/Time
ROOM
80°
78
HEAT SETPOINT
Fan Hold
(DOOR
OPEN)
68°
80
70
Hi
Lo
Settings Enter
(Outside)
Figure 10
Cooling operation is indicated by flashing ꢀsnowflake"
icons. When the actual temperature drops below the tem-
perature setpoint, the snowflake icons will disappear. This
indicates that the cooling demand has been satisfied and
that the cooling equipment has been turned off.
Figure 11
After the desired setpoint is reached, the HOME screen
will reappear after about 15 seconds.
Permanent Hold Mode
If your system supports 2−stage cooling (as does the
51M42 thermostat), you may notice various cooling levels
being delivered during a demand. Also, if a small cooling
demand is present, "Lo" will be displayed in the SYSTEM
box. However, if a large cooling demand is present, "Hi" will
be displayed in the lower right side of the display (shown in
At any time the program is running, from the HOME
screen, set a permanent hold (program override) by press-
functions much like a non−programmable thermostat. Use
the Up/Down arrow buttons to adjust the hold setpoint. To
return to the program, press the HOLD button again.
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Temporary (2-Hour) Hold
Program Example
At any time the program is running, from the HOME
screen, set a temporary 2-hour hold by pressing the Up/
Down arrow buttons until the desired setpoint is displayed;
gram for 2 hours from the last button press, then returns to
the program.
MONDAY PROGRAM: 72°
A
C
68°
64°
D
B
TUESDAY PROGRAM: 72°
68°
64°
A
C
D
B
WEDNESDAY PROGRAM: 72°
68°
64°
A
C
Temporary Hold Temperature Mode
D
B
Heat Cool
THURSDAY PROGRAM: 72°
A
A
C
68°
64°
Prog Auto
D
B
B
i0:30
FRIDAY PROGRAM: 72°
Copy Day/Time
70
68°
64°
C
D
Fan Hold
(DOOR
OPEN)
72
68
SATURDAY PROGRAM: 72°
C
A
68°
64°
B
Settings Enter
(Outside)
D
SUNDAY PROGRAM: 72°
A
C
68°
64°
B
D
Figure 12
6AM
NOON
6PM
MID
NIGHT
6AM
While in Temporary Hold, press the HOLD button once to
switch to Permanent Hold (HOLD displays solid; PROG
not displayed); press the HOLD button again to return to
the program (PROG displays; HOLD not displayed).
Figure 13
6. COPY A PROGRAM − If the program entered for
Monday is satisfactory for other days of the week, do
the following to repeat the same program for other
days:
PROG − Thermostat Programming
The 51M42 thermostat can be programmed to perform a
set of both heating and cooling events for each day of the
week. Each day can be programmed for 4 unique events
per day, and each day can be different from any other day.
A
B
C
Press the PROG button, then press the COPY
button. The display clears the screen except for
ꢀCopy from MOꢀ with ꢀMO" flashing. Press the
COPY button again to copy Monday’s program.
The display changes to ꢀCopy to MO". Use the ar-
row buttons to scroll to the desired day where you
want the same program as Monday. Press the
COPY button.
To Set Program Events and Temperatures
Figure 13 gives an example of how up to 7 different pro-
grams can be set. Programs A and C reflect the desired
warmth while the home IS occupied (72°); B allows less
heating while the home is NOT occupied; D reflects a cool
sleeping temperature.
want the same program.
similar program may be set for cool setpoints.
Copy a Programmed Day to Another Day
1. Press and release the PROG button. "MO" (Monday),
"AM 6:00", period "A", "COOL SETPOINT 78", and
"HEAT SETPOINT 70" are displayed. "AM 6" flashes.
Heat Cool
Copy from MO
Prog Auto
2. Use the Up/Down arrow buttons to select the desired
hour; press the PROG button when the desired hour
is reached; use the Up/Down arrow buttons again to
select desired minute. The selected hour/minute is
when the program will start. Press the PROG button.
Copy Day/Time
Fan Hold
(DOOR
OPEN)
3. Use the Up/Down arrow buttons to select the desired
heat setpoint; press the PROG button.
Settings Enter
4. Use the Up/Down arrow buttons to select the desired
cool setpoint; press the PROG button.
(Outside)
NOTE − Pressing the ENTER button during the following
programming steps, saves and exits to the HOME screen.
Figure 14
To program events and temperatures, perform the follow-
ing steps.
peratures for days not yet programmed.
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FAN − Controlling the Fan Operation
SETTINGS − Filter/Maintenance Reminders
Use the FAN button to select either continuous fan mode or
auto fan mode.
The 51M42 thermostat is designed to remind you when the
filter needs changing or when routine maintenance is re-
quired, as (and if) defined, by you. These optional remind-
ers are not enabled until you activate them. To do so, press
the SETTINGS button (shown below the FAN button in fig-
To change from continuous to auto fan mode (or vice ver-
sa), press the FAN button. Note whether a fan icon in the
FAN box is present (indicating that the fan is running) or not
(fan not running).
If continuous fan mode is enabled (ON displayed in FAN
box − see figure 15), the fan will run continuously regard-
less of whether the heating or cooling equipment is run-
ning.
Reminder Settings Display
Heat Cool
Using Fan ON
OFF
Prog Auto
i0:30
For FILTER, press SETTINGS
button 1 time
Copy Day/Time
FILTER
For MAINTENANCE, press
SETTINGS button 2 times
Fan Hold
72
MAINTENANCE
Figure 18
Figure 15
The default setting for the reminders is OFF (disabled).
Press Up/Down arrow buttons to select the desired re-
minder intervals.
If auto fan mode is selected (AUTO displayed in FAN box −
cooling equipment is running.
Table 1
Heat Cool
Using Fan AUTO
Filter and Maintenance Reminders
Prog Auto
i0:30
Buttons to
Use
Available Settings and
How to Use
Reminder
FILTER
Copy Day/Time
Settings
(1st press)
then Arrows
to scroll
Total fan run time expressed in
months (Off, 1, 3, 6, 12); for
example, if fan runs 12 hours a
day, 1 month reminder displays
in 2 calendar months.
Fan Hold
72
selections
Figure 16
Settings
(2nd press)
then Arrows
to scroll
Elapsed chronological time in
months (Off, 6, 12). Use this,
for example, to remind yourself
when to perform routine checks
or when to call a technician for
perodic preventive
MAINTE-
NANCE
Displaying Outside Temperature
Pressing the ENTER (Outside) button at the HOME
several seconds, then returns to room temperature.
selections
maintenance.
Check Outside Temperature
Enter
Stores settings.
Fan Hold
88
NOTE − The HOME screen will reappear about 15 seconds
after the final arrow button press. OR, press the ENTER
button at any time to store any changes and exit to the
HOME screen.
Settings Enter
(Outside)
After either programmed interval has elapsed, the remind-
Figure 17
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To adjust the balance point up or down, press the Up/Down
arrow buttons to the right of the display screen. The bal-
ance point setting increases by 1°F each time the Up but-
ton is pressed and decreases by 1°F each time the Down
button is pressed. After the desired balance point setting is
reached, the HOME screen will reappear after about 15
seconds.
Reminders
MO
PROG
A B C D
HOLD
AM
PM
I0:26
COOL
OFF
FAN ON
AUTO
Filter &
Maintenance
Indicators
FILTER SERVICE
MAINTENANCE
ROOM
NOTE − The balance point feature allows the outdoor tem-
perature to rise or fall 3°F above or below the balance
point. This prevents excessive cycling of the equipment
when the outdoor temperature is near the balance point.
For example, if the balance point is 40°F and the actual
outdoor temperature is 35°F, the outdoor temperature
must rise to 43°F before equipment adjustment occurs.
Conversely, if the balance point is 40°F and the actual out-
door temperature is 45°F, the outdoor temperature must
drop to 37°F before equipment adjustment occurs.
AUTO
EMG.
HEAT
76°
FILTER SERVICE
MAINTENANCE
ROOM
FAN ON
AUTO
Service
Indicator
Flashing
COOL
OFF
AUTO
EMG.
HEAT
76°
Service Indicator
When abnormal equipment operation is detected, the
service from a qualified service technician.
Figure 19
After the filter has been changed or maintenance per-
formed, reset the reminder by pressing the SETTINGS
button for 4 seconds at the corresponding reminder set-
tings screen. The screen will blink for a few moments to in-
dicate that the timer has been reset.
Thermostat RESET
Under some abnormal conditions, it may be necessary to
ꢀreset" the thermostat to its default condition. Such a RE-
SET would delete all programming and settings and there-
fore should only be used on rare occasions when the the
thermostat fails to function as designed and/or as pro-
grammed. Such an instance can occur as a result of a pow-
er surge or similar electrical disturbance (e.g. after an elec-
trical storm or power outage). The RESET button can be
used to recover from this situation.
SETTINGS − Balance Point
NOTE − The balance point is only available with the out-
door sensor.
When the outdoor sensor (X2658) is connected to the
51M42 thermostat, balance point adjustment is available.
The balance point feature allows the measured outdoor
temperature to govern operation of the heat pump and
backup heat source.
CAUTION
When the RESET button is pressed, ALL settings re-
vert back to the defaults, including the default pro-
Press the SETTINGS button three times to access the Bal-
The RESET button is an unlabeled, recessed button lo-
cated behind the door, on the right−hand side of the ther-
paper clip or small pencil to press the RESET button; ALL
thermostat settings will be reset to the defaults listed in the
Default Thermostat Settings section.
Balance Point Setting
SETPOINT
Removing/Installing Thermostat
°
6p
Figure 20
55
The thermostat hinges on tabs on the top of the subbase;
no tool is needed to remove the thermostat from the sub-
base. Pivot the bottom of the thermostat outward (releas-
ing the snaps), then lift up to remove.
The balance point can be set at any temperature between
1°F and 55°F (−17°C to 13°C). A balance point setting of
zero disables the balance point function (in this case, the
thermostat behaves as if no outdoor sensor is attached).
The default balance point setting is 55°F.
To replace it, first position the top tilted toward the wall
bracket and align it until you feel the tabs and slots engage;
then, while the top is in place, pivot the bottom toward the
wall until the thermostat snaps into place.
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With a VERY LARGE heat demand, temperature is:
below setpoint –2.5F
OR 15−minute 2 upstage timer HAS expired.
Default Thermostat Settings
nd
Default thermostat settings are shown in table 2 and the
default program is shown in table 3.
Emergency heat demand (only if emergency heat is
enabled): temperature is below setpoint –0.5F.
Table 2
Default Thermostat Settings
With a SMALL cool demand, temperature is:
above setpoint +0.5F but below setpoint +1.5F
AND 30-minute upstage timer HAS NOT expired.
Mode
Heat (Perm. Hold)
70°F (or 21°C)
78°F (or 26°C)
Auto
Heat Setpoint (Perm Hold)
Cool Setpoint (Perm Hold)
Fan
With a LARGE cool demand, temperature is:
above setpoint +1.5F
OR 30-minute upstage timer HAS expired.
Filter Reminder
Maintenance Reminder
OFF
Table 4
OFF
THERMOSTAT OUTPUTS
Equipment Protection Timers Reset Back to Zero
Autochangeover OFF
W1
E
Y1 Y2
G
O
Demand Condition
Cooling Demands
Table 3
Default Program Settings − Time & Temp.
SMALL
X
X
X
X
X
X
LARGE
X
X
No Demand
Programs
Time
Temp. − Heat Temp. − Cool
All days − A 6:00am 70_F / 21°C
All days − B 8:00am 62_F / 17°C
All days − C 6:00pm 70_F / 21°C
All days − D 10:00pm 62_F / 17°C
78_F / 26°C
85_F / 29°C
78_F / 26°C
82_F / 28°C
Heating Demands
With Electric Backup (no outdoor sensor)
SMALL
X
X
X
X
X
X
X
LARGE
X
X
VERY LARGE
Emergency
No Demand
X
Thermostat Output Table
X
ious input conditions. The following notes described terms
used in the table.
With Gas Backup (dual fuel; no outdoor sensor)
SMALL
X
X
X
X
NOTES:
LARGE
X
S X = output is activated with 24VAC.
S BBP: outdoor temperature is below balance point
S ABP: outdoor temperature is above balance point
VERY LARGE
Emergency
No Demand
X
X
With Electric Backup (outdoor sensor attached)
S In all cases, the state of the B terminal is opposite that
SMALL (Above Balance Point)
LARGE (ABP)
X
X
X
X
X
X
X
X
X
X
X
X
X
of the O terminal
X
X
S Data are tabulated for AUTO fan setting. If the fan set-
VERY LARGE (ABP)
SMALL (Below Balance Point)
LARGE (BBP)
ting is ON, the G output is activated in all cases
S Upstage timers:
X
X
30 minutes (when upstaging from small to large
demand)
VERY LARGE (BBP)
Emergency
X
X
15 minutes (HEAT MODE ONLY − when upstaging
from large to very large demand)
No Demand
With Gas Backup (dual fuel; outdoor sensor attached)
S The temperature ranges expressed in the following
definitions of ꢀSMALL/LARGE/VERY LARGE" de-
mands are for guidance only; actual temperatures
may vary:
SMALL (ABP)
LARGE (ABP)
VERY LARGE (ABP)
SMALL (BBP)
LARGE (BBP)
VERY LARGE (BBP)
Emergency
X
X
X
X
X
X
X
X
X
With a SMALL heat demand, temperature is:
below setpoint –0.5F but above setpoint –1.5F
AND 30-minute upstage timer HAS NOT expired.
With a LARGE heat demand, temperature is:
less than setpoint –1.5F
OR 30-minute upstage timer HAS expired.
X
No Demand
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Indoor Temperature Measurement Range
Technical Specifications
Measurement Scale: Fahrenheit
Measurement Range: 35°F to 99°F
Thermostat Type
Electronic programmable thermostat for heat pump,
3-stage heat/2-stage cool.
Measurement Resolution: 0.5°F
Measurement Accuracy: +/−1°F
NOTE − 2 stages of heat pump heat and one stage of auxil-
iary (gas or electric) backup heat are supported.
Field Offset: via DIP switches to +/−3°F
Sampling Method: temperature measurements sampled
every 15 seconds. Displayed temperature is the average
of the last four measurements.
Power Supply Range
18VAC − 30VAC (24VAC nominal), 60Hz
Outdoor Temperature Measurement Range
CAUTION
24VAC is present on the terminals of the thermostat
bracket. If removing the thermostat from the wall,
use caution and avoid touching any of the connec-
tor terminals on the wall bracket.
Measurement Scale: Fahrenheit
Measurement Range: −22°F to 122°F
Measurement Resolution: 1°F
Measurement Accuracy: +/−2°F
Also, when working with the thermostat dip
switches, use a non−conductive tool and take cau-
tion to avoid making any contact with the circuit
board, its imprinted circuitry and its connector
prongs.
Temperature Setpoint Range
Setting range: 50°F (10°C) to 90°F (32°C)
Setting resolution: 1°F (1°C)
Smart Setback Recovery (via DIP switch #6)
Smart Setback Recovery (SSR) affects the way the ther-
mostat responds to program events. If SSR is disabled, the
thermostat will react to a program event at the time the
event occurs. However, if SSR is enabled, the thermostat
will react to a program event before the event occurs such
that the desired temperature is reached at the time of the
event, not after.
Temperature Display
Display Scale: Fahrenheit or Celsius user selectable (via
Display range: 35°F (2°C) to 99°F (37°C)
Display resolution: 1°F (1°C)
Display Accuracy: +/−1°F
If the Fahrenheit/Celsius display must be changed, use a
plastic, non−conductive tool to push the dip switch to the
Autochangeover Deadband Selection (via DIP
switch #7)
Changing Fahrenheit/Celsius Setting
Autochangeover deadband can be set to 4 or 6 degrees.
When autochangeover is enabled (via the AUTO button),
the thermostat will automatically change over from heating
to cooling and vice versa, to keep the room temperature in
between the heating and cooling setpoints. The deadband
is the minimum difference between the heating and cooling
setpoints.
1
2
3
4
5
Fan Control
PLUG IN (Use Care To Avoid Bending Prongs.)
6
7
AUTO or ON modes.
I/O Relays
F/C (Fahrenheit/Celsius) switch
shown in Celsius position
All thermostat relays are latching type to minimize power
consumption.
Figure 21
Page 9
51M42 SERIES 5/2
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Table 5
Over−Temperature Protection
Thermal-mechanical switch opens W1, Y1, Y2, and E at
93°F+/−6°F.
51M42 Terminal Designations
Term. Description
Filter Reminder
B
Reversing valve, heat active
Reversing valve, cool active
24VAC
Settings of Off, 1, 3, 6 or 12 (months of fan run time) are
available. When programmed time has elapsed, a FILTER
indicator is displayed.
O
R
Y1
First-stage cooling/heating, compressor
energized
Maintenance Reminder
Settings of Off, 6 or 12 (months of chronological time) are
available. When programmed time has elapsed, a mainte-
nance indicator ꢀMAINTENANCE" is displayed.
W1*
Y2
Auxiliary heating, furnace energized
Second-stage cooling/heating, compressor
energized
Service Reminder
The SERVICE indicator is displayed only under the follow-
ing conditions:
E*
G
L
Emergency heat
Fan control
Service Indicator
S
if the thermostat Y1 terminal has been activated with
24VAC for at least 5 minutes, AND the L terminal is
shorted to the R terminal;
C
T
24VAC common
Outdoor temperature sensor connection 1
Outdoor temperature sensor connection 2
OR
T
S
if the thermostat Y1 terminal has been activated with
24VAC for at least 5 minutes, AND the L terminal is
shorted to the C terminal.
* For most applications, E will be jumpered to W1. If sep-
arate wires are not provided for both E and W1, jumper
the E terminal to the W1 terminal on the thermostat sub-
base. For applications involving the use of a balance
point (whereby the outdoor temperature is to be used to
restrict either heat pump operation or backup heat
source operation), the outdoor sensor (part number
X2658) MUST be installed.
Power Loss/Recovery
Thermostat memory is retained for a minimum of 24 hours
during a power loss (includes retention of clock setting,
program information, HOLD status, programmed temper-
ature setpoint, heat/cool and fan mode settings, filter re-
minder status, maintenance reminder status, and equip-
ment protection timers). After 24 hours of power loss,
programmed settings will be lost and replaced with default
settings.
Equipment Protection Timers
Minimum Compressor OFF time: 5 minutes
Minimum Compressor ON time: 4 minutes
Minimum Furnace ON time: 3 minutes
IMPORTANT
Power must be applied for at least six consecutive
hours prior to a power loss in order for memory to
be retained for the specified time.
Minimum furnace cycle time (elapsed time between any
furnace activation and the next furnace activation): 6 min-
utes.
Minimum elapsed time between any compressor activa-
tion and the next compressor activation: 6 minutes.
LCD Backlight
Activated for 30 seconds when any button is pressed.
NOTE − All protection timers (except the compressor OFF
timer) can be over−ridden if a heating or cooling demand is
initiated or terminated using the UP, DOWN, HEAT, or
COOL buttons.
NOTE − During an electrical storm or similar disturbance,
the backlight may activate for a few seconds. This is nor-
mal and will no longer occur after the electrical disturbance
has passed.
Equipment Protection Override
Thermostat Operating Conditions
35°F to 105°F, 5% to 90% RH
Both the minimum compressor OFF timer and the mini-
mum equipment cycle timer can be over−ridden by press-
ing and holding either the HEAT or COOL button down for 4
seconds.
Thermostat Storage Conditions
−40°F to 185°F, 5% to 95% RH
Page 10
505069M 07/05
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OPERATION
MANUAL
E2005 Lennox Industries Inc.
Dallas, Texas, USA
51M38 Merit® Series
7 Day Programmable Thermostat
CONTROLS
505,067M
07/05
Litho U.S.A.
Table of Contents
51M38 Series Thermostat . . . . . . . . . . . . . . . . . . . . . .
Initial Thermostat Power-up . . . . . . . . . . . . . . . . . . . . .
Buttons, Backlight, Timers & Settings . . . . . . . . . . . .
DAY/TIME − Set Day and Time . . . . . . . . . . . . . . . . . .
HEAT − Heat Mode . . . . . . . . . . . . . . . . . . . . . . . . . . . .
COOL − Cool Mode . . . . . . . . . . . . . . . . . . . . . . . . . . . .
AUTO − Using the Autochangeover Mode . . . . . . . . .
HOLD − Temperature Hold Mode . . . . . . . . . . . . . . . .
PROG − Thermostat Programming . . . . . . . . . . . . . . .
COPY − Copy a Program . . . . . . . . . . . . . . . . . . . . . . .
FAN − Controlling Fan Operation . . . . . . . . . . . . . . . . .
Displaying Outside Temperature . . . . . . . . . . . . . . . . .
SETTINGS − Filter /Maintenance Reminder . . . . . . .
Thermostat RESET . . . . . . . . . . . . . . . . . . . . . . . . . . . .
Removing/Installing Thermostat . . . . . . . . . . . . . . . . .
Default Thermostat Settings . . . . . . . . . . . . . . . . . . . .
Thermostat Output Table . . . . . . . . . . . . . . . . . . . . . . .
Technical Specifications . . . . . . . . . . . . . . . . . . . . . . . .
RETAIN THESE INSTRUCTIONS
FOR FUTURE REFERENCE
51M38 Merit® Series Thermostat
®
The Lennox Merit Series 7 day programmable electronic
thermostat 51M38 provides excellent temperature control
and a large, easy-to-read display. This product includes a
programmable filter change reminder, an equipment
maintenance reminder, and a system check indicator to
notify the user when the equipment requires service.
Thermostat 51M38 is suitable for non−heat pump, single-
stage heat/single-stage cool applications with either gas or
electric furnace. Also, the thermostat provides auto-
changeover capability.
General
Initial Power−Up Display
Copy tofrom MO TU WE TH FR SA SU
These instructions are intended as a general guide and do
not supersede local codes in any way. Consult authorities
having jurisdiction before installation.
PROG
A B C D
AM
PM
I8:88
HOLD
Check equipment for shipping damage. If you find any
damage, immediately contact the last carrier.
FAN ON
AUTO
COOL SETPOINT
FILTER
SERVICE
°
COOL
OFF
MAINTENANCE
Introduction
AUTO
EMG.
HEAT
OUTROOM
i88°
HEAT SETPOINT
This document describes the operation of Lennox thermo-
stat 51M38. Refer to the installation manual for instructions
regarding installation and wiring of the thermostat.
°
Hi
Lo
Initial Thermostat Power-up
Figure 1
When power is initially applied to the thermostat, the dis-
All display segments are momentarily activated. This oc-
curs as a normal part of thermostat initialization.
07/05
505,067M
Page 1
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*P505067M*
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Within a few seconds, the HOME screen appears (see fig-
of initialization time, the actual room temperature will be
displayed.
Thermostat Buttons
Heat Cool
Prog Auto
Copy Day/Time
Fan Hold
HOME Screen
MO
PROG
A B C D
HOLD
(DOOR
OPEN)
AM
PM
I0:26
COOL
OFF
Settings Enter
(Outside)
FAN ON
AUTO
ROOM
AUTO
EMG.
HEAT
Figure 3
70°
When an Arrow, HOLD, HEAT, or COOL button is pressed,
SETPOINT and the temperature setting appears for 15
seconds. If desired, start making changes within 15 sec-
onds or the HOME screen will return. The backlight will
turn off 15 seconds after the HOME screen reappears.
Figure 2
At this point, the thermostat will be fully functional; its de-
fault temperature setpoint (not shown) is 70°F. At this
point, if the equipment has been fully powered and if a heat
demand were present, the system would begin operating.
DAY/TIME − Setting the Day and Time
NOTE − Temperature scale default is Fahrenheit units but
may be reset to show Celsius, if desired. See page 7.
Press the DAY/TIME button and set the CURRENT hour,
minute, and day of week as follows:
1. ꢀAM12" will flash on the screen. Press the UP/DOWN
arrows to change the hour. (ꢀAM" or ꢀPM" must corre-
spond to time of day.) Press the DAY/TIME button OR,
if adjusting for daylight savings time, pressing the EN-
TER button stores the single change and exits to the
HOME screen, bypassing minutes and day of week.
Buttons, Backlight, Timers & Settings
Buttons are located behind the small door on the right−
hand side of the thermostat (see figure 3).
IMPORTANT
Do NOT begin pressing buttons until after you read
the following section describing each button.
2. Minutes will flash. Use the UP/DOWN arrow button to
display the minutes past the hour. Press the DAY/
TIME button.
3. Day ꢀMO" (Monday) will flash. Use UP or DOWN arrow
buttons to display the current day. Day selections are
abbreviated as ꢀMOꢀ, ꢀTU", ꢀWE", ꢀTH", ꢀFR", ꢀSA",
and ꢀSU". Press the DAY/TIME button.
A pale blue display backlight illuminates for 30 seconds
each time any button is pressed.
When the PROG or DAY/TIME button is pressed, a field
begins flashing, expecting another input. Start making
changes within 15 seconds or the HOME screen will re-
turn.
4. The HOME screen reappears; confirm day and time
are correct. This completes day and time setting.
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HEAT − Using the Heat Mode
COOL − Using the Cool Mode
Enabling and Disabling Cool Mode
Use the COOL button to enable or disable cool mode as
desired. If the thermostat is in heat or off mode, cool mode
is enabled when the COOL button is pressed. This is indi-
cated by COOL on the right side of the display (see figure
Enabling and Disabling Heat Mode
The thermostat must be in the heat mode to control the
heating equipment. Press the HEAT button to enable or
disable heat mode. If the thermostat is in cool or off mode,
pressing the HEAT button enables the heat mode (indi-
Turn Cool ON/OFF
Heat Cool
Turn Heat ON/OFF
Heat Cool
Prog Auto
i0:30
Prog Auto
Copy Day/Time
5:30
70
Copy Day/Time
Fan Hold
(DOOR
OPEN)
75
82
66
Fan Hold
(DOOR
OPEN)
70
Settings Enter
66
Settings Enter
(Outside)
(Outside)
Figure 7
If the thermostat is in cool mode, pressing the COOL but-
ton disables cool mode (indicated by OFF, similar to figure
Figure 4
If the thermostat is in heat mode, heat mode is disabled
when the HEAT button is pressed. This is indicated by OFF
Cooling Demand
Set the thermostat to cool mode to control the cooling
equipment. Then, if the room temperature is higher than
the temperature setpoint, as shown in figure 7, the thermo-
stat detects a cooling demand and will activate the cooling
equipment to satisfy the demand.
Heat/Cool Mode Disabled
OFF
ROOM
75°
HEAT SETPOINT
Cooling operation is indicated by flashing ꢀsnowflake"
icons in the display. When the actual temperature drops
below the temperature setpoint, the snowflake icons will
disappear. This indicates that the cooling demand has
been satisfied and that the cooling equipment has been
turned off.
75°
Figure 5
Heating Demand
NOTE − If no buttons are pressed during a demand for
cooling, the equipment must operate for at least 4 minutes.
After a demand has been satisfied, cooling equipment op-
eration is locked out for 5 minutes. If another cooling de-
mand occurs during this 5−minute interval, ꢀCOOL" and the
snowflakes will flash; however, the cooling equipment will
not operate until the 5−minute delay has elapsed.
Set the thermostat to heat mode to control the heating
equipment. Then, if the room temperature is lower than the
detects a heating demand and will activate the heating
equipment to satisfy the demand.
Heating Demand
NOTE − The cooling setpoint cannot be lower than the
heating setpoint. Also, the difference between the cooling
and heating setpoint must be at least 4 degrees.
ROOM
72°
HEAT SETPOINT
HEAT
75°
AUTO − Using the Autochangeover Mode
When in either heat mode or cool mode, autochangeover
can be enabled or disabled by pressing the AUTO button. If
enabled, AUTO appears at the lower right corner of the dis-
play. The thermostat will then automatically change over
from heating to cooling and vice versa, to keep the room
temperature in between the heating and cooling setpoints.
Autochangeover can be used in either of the thermostat
hold modes, or when the thermostat program is running.
These modes are described later.
Figure 6
Heating operation is indicated by a flame icon in the lower
right corner. When the actual temperature rises above the
temperature setpoint, the flame icon will disappear. This
indicates that the heating demand has been satisfied and
that the heating equipment has been turned off.
NOTE − Heating equipment is activated for at least 3 min-
utes if no buttons are pressed during the demand interval.
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Temporary Hold Temperature Mode
HOLD − Using Temperature Hold Modes
Heat Cool
When HOLD is displayed at the HOME screen, the ther-
mostat is in a temperature hold condition. This means that
the temperature program data is ignored and the thermo-
stat functions much like a non−programmable thermostat.
Prog Auto
i0:30
Copy Day/Time
70
Adjusting Temperature Setpoint in Hold Mode
Fan Hold
(DOOR
OPEN)
72
68
The temperature setpoint represents the desired tempera-
ture of the space around the thermostat. The default heat
setpoint in hold mode is 70°F; the default cool setpoint in
hold mode is 78°F.
Settings Enter
(Outside)
To adjust the setpoint, press the UP or DOWN (YB) arrow
the right of the actual room temperature. Each button
press adjusts the setpoint up or down by 1 degree.
Figure 9
PROG − Thermostat Programming
The 51M38 thermostat can be programmed to perform a
set of both heating and cooling events for each day of the
week. Each day can be programmed for 4 unique events
per day, and each day can be different from any other day.
Hold Temperature Mode
To Set Program Events and Temperatures
Heat Cool
Figure 10 gives an example of how up to 7 different pro-
grams can be set. Programs A and C reflect the desired
warmth while the home IS occupied (72°); B allows less
heating while the home is NOT occupied; D reflects a cool
sleeping temperature.
Prog Auto
i0:30
Copy Day/Time
78
similar program may be set for cool setpoints.
Fan Hold
(DOOR
OPEN)
80
70
Settings Enter
(Outside)
Program Example
MONDAY PROGRAM: 72°
A
C
68°
64°
D
B
Figure 8
TUESDAY PROGRAM: 72°
68°
64°
A
C
D
B
After the desired setpoint is reached, the HOME screen
will reappear after about 15 seconds.
WEDNESDAY PROGRAM: 72°
68°
64°
A
C
D
B
Permanent Hold Mode
THURSDAY PROGRAM: 72°
A
At any time the program is running, from the HOME
screen, set a permanent hold (program override) by press-
functions much like a non−programmable thermostat. Use
the Up/Down arrow buttons to adjust the hold setpoint. To
return to the program, press HOLD again.
C
68°
64°
D
B
B
FRIDAY PROGRAM: 72°
A
68°
64°
C
D
SATURDAY PROGRAM: 72°
C
A
68°
64°
B
D
Temporary (2-Hour) Hold
At any time the program is running, from the HOME
screen, set a temporary 2-hour hold by pressing the Up/
Down arrow buttons until the desired setpoint is displayed;
for 2 hours from the last button press, then returns to the
program.
SUNDAY PROGRAM: 72°
A
C
68°
64°
B
D
6AM
NOON
6PM
MID
NIGHT
6AM
Figure 10
While in Temporary Hold, press HOLD once to switch to
Permanent Hold (HOLD displays solid; PROG not dis-
played); press HOLD again to return to the program
(PROG displays; HOLD not displayed).
NOTE − Pressing the ENTER button during the following
programming steps, saves and exits to the HOME screen.
To program events and temperatures, perform the follow-
ing steps.
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1. Press and release the PROG button. "MO" (Monday),
"AM 6:00", period "A", "COOL SETPOINT 78", and
"HEAT SETPOINT 70" are displayed. "AM 6" flashes.
Heat Cool
Prog Auto
Using Fan ON
2. Use the Up/Down arrow buttons to select the desired
hour; press the PROG button when the desired hour
is reached; use the Up/Down arrow buttons again to
select desired minute. The selected hour/minute is
when the program will start. Press the PROG button.
i0:30
Copy Day/Time
Fan Hold
72
3. Use the Up/Down arrow buttons to select the desired
heat setpoint; press the PROG button.
Figure 12
4. Use the Up/Down arrow buttons to select the desired
cool setpoint; press the PROG button.
If auto fan mode is selected (AUTO displayed in FAN box −
cooling equipment is running.
6. COPY A PROGRAM − If the program entered for
Monday is satisfactory for other days of the week, do
the following to repeat the same program for other
days:
Heat Cool
Using Fan Auto
Prog Auto
i0:30
A
B
C
Press the PROG button, then press the COPY
button. The display clears the screen except for
ꢀCopy from MOꢀ with ꢀMO" flashing. Press the
COPY button again to copy Monday’s program.
The display changes to ꢀCopy to MO". Use the ar-
row buttons to scroll to the desired day where you
want the same program as Monday. Press the
COPY button.
Copy Day/Time
Fan Hold
72
Figure 13
Displaying Outside Temperature
want the same program.
Pressing the ENTER (Outside) button at the HOME
several seconds, then returns to room temperature.
Copy a Programmed Day to Another Day
Check Outside Temperature
Heat Cool
Prog Auto
Copy Day/Time
Fan Hold
Copy from MO
Fan Hold
88
Settings Enter
(Outside)
(DOOR
OPEN)
Figure 14
Settings Enter
(Outside)
SETTINGS − Filter/Maintenance Reminders
The 51M38 thermostat is designed to remind you when the
filter needs changing or when routine maintenance is re-
quired, as (and if) defined, by you. These optional remind-
ers are not enabled until you activate them. To do so, press
the SETTINGS button once or twice for the desired re-
Figure 11
peratures for days not yet programmed.
FAN − Controlling the Fan Operation
Reminder Settings Display
Use the FAN button to select either continuous fan mode or
auto fan mode.
To change from continuous to auto fan mode (or vice ver-
sa), press the FAN button. Note whether a fan icon in the
FAN box is present (indicating that the fan is running) or not
(fan not running).
OFF
For FILTER, press SETTINGS
button 1 time
If continuous fan mode is enabled (ON displayed in FAN
box − see figure 12), the fan will run continuously regard-
less of whether the heating or cooling equipment is run-
ning.
FILTER
For MAINTENANCE, press
SETTINGS button 2 times
MAINTENANCE
Figure 15
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The default setting for the reminders is OFF (disabled).
Press Up/Down arrow buttons to select the desired re-
minder intervals.
Service Indicator
When abnormal equipment operation is detected, the
This indicates that the equipment requires service from a
qualified service technician.
Table 1
Filter and Maintenance Reminders
Buttons to
Use
Available Settings and
How to Use
Thermostat RESET
Reminder
FILTER
Under some abnormal conditions, it may be necessary to
ꢀreset" the thermostat to its default condition. Such a RE-
SET would delete all programming and settings and there-
fore should only be used on rare occasions when the the
thermostat fails to function as designed and/or as pro-
grammed. Such an instance can occur as a result of a pow-
er surge or similar electrical disturbance (e.g. after an elec-
trical storm or power outage). The RESET button can be
used to recover from this situation.
SETTINGS
(1st press)
then Arrows
to scroll
Total fan run time expressed in
months (Off, 1, 3, 6, 12); for
example, if fan runs 12 hours a
day, 1 month reminder displays
in 2 calendar months.
selections
SETTINGS
(2nd press)
then Arrows
to scroll
Elapsed chronological time in
months (Off, 6, 12). Use this,
for example, to remind yourself
when to perform routine checks
or when to call a technician for
perodic preventive
MAINTE-
NANCE
selections
CAUTION
When the RESET button is pressed, ALL settings re-
vert back to the defaults, including the default pro-
maintenance.
Enter
Stores settings.
NOTE − The HOME screen will reappear about 15 seconds
after the final arrow button press. OR, press the ENTER
button at any time to store any changes and exit to the
HOME screen.
The RESET button is an unlabeled, recessed button lo-
cated behind the door on the right−hand side of the thermo-
per clip or small pencil to press the RESET button; ALL
thermostat settings will be reset to the defaults listed in the
Default Thermostat Settings section.
After either programmed interval has elapsed, the remind-
Reminders
Removing/Installing Thermostat
MO
PROG
A B C D
HOLD
AM
PM
The thermostat hinges on tabs on the top of the subbase;
no tool is needed to remove the thermostat from the sub-
base. Pivot the bottom of the thermostat outward (releas-
ing the snaps), then lift up to remove.
I0:26
COOL
OFF
FAN ON
AUTO
FILTER SERVICE
MAINTENANCE
ROOM
Filter &
Maintenance
Indicators
To replace it, first position the top tilted toward the wall
bracket and align it until you feel the tabs and slots engage;
then, while the top is in place, pivot the bottom toward the
wall until the thermostat snaps into place.
AUTO
EMG.
HEAT
76°
Default Thermostat Settings
FAN ON
AUTO
program is shown in table 3.
FILTER SERVICE
MAINTENANCE
ROOM
Service
Indicator
Flashing
COOL
OFF
Table 2
AUTO
EMG.
HEAT
Default Thermostat Settings
76°
Mode
Heat (Perm. Hold)
70°F (or 21°C)
78°F (or 26°C)
Auto
Heat Setpoint (Perm Hold)
Cool Setpoint (Perm Hold)
Fan
Figure 16
Filter Reminder
Maintenance Reminder
OFF
After the filter has been changed or maintenance per-
formed, reset the reminder by pressing the SETTINGS
button for 4 seconds at the corresponding reminder set-
tings screen. The screen will blink for a few moments to in-
dicate that the timer has been reset.
OFF
Equipment Protection Timers Reset Back to Zero
Autochangeover
OFF
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Table 3
CAUTION
24VAC is present on the terminals of the thermostat
bracket. If removing the thermostat from the wall,
use caution and avoid touching any of the connec-
tor terminals on the wall bracket.
Default Program Settings (Time & Temp.)
Programs
Time
Temp. − Heat Temp. Cool
All days − A 6:00am 70_F / 21°C
All days − B 8:00am 62_F / 17°C
All days − C 6:00pm 70_F / 21°C
All days − D 10:00pm 62_F / 17°C
78_F / 26°C
85_F / 29°C
78_F / 26°C
82_F / 28°C
Also, when working with the thermostat dip
switches, use a non−conductive tool and take cau-
tion to avoid making any contact with the circuit
board, its imprinted circuitry and its connector
prongs.
Temperature Display
Display Scale: Fahrenheit or Celsius user selectable (via
Thermostat Output Table
Display range: 35°F (2°C) to 99°F (37°C)
Display resolution: 1°F (1°C)
Display Accuracy: +/−1°F
ious input conditions.
If the Fahrenheit/Celsius display must be changed, use a
plastic, non−conductive tool to push the dip switch to the
Table 4
Thermostat Outputs
Condition
W1
Y1
G
Changing Fahrenheit/Celsius Setting
Gas Heat, Auto Fan
Heat Demand
X
Cool Demand
X
X
No Demand
Gas Heat, Continuous Fan
Heat Demand
X
X
X
X
X
1
2
3
4
5
Cool Demand
X
X
X
No Demand
PLUG IN (Use Care To Avoid Bending Prongs.)
6
7
Electric Heat, Auto Fan
Heat Demand
X
X
F/C (Fahrenheit/Celsius) switch
shown in Celsius position
Cool Demand
No Demand
Figure 17
Electric Heat, Continuous Fan
Indoor Temperature Measurement Range
Measurement Scale: Fahrenheit
Measurement Range: 35°F to 99°F
Measurement Resolution: 0.5°F
Measurement Accuracy: +/−1°F
Heat Demand
Cool Demand
No Demand
X
X
X
X
NOTE − X = output is activated with 24VAC.
Field Offset: via DIP switches to +/−3°F
Sampling Method: temperature measurements sampled
every 15 seconds. Displayed temperature is the average
of the last four measurements.
Technical Specifications
Thermostat Type
Outdoor Temperature Measurement Range
Measurement Scale: Fahrenheit
Measurement Range: −22°F to 122°F
Measurement Resolution: 1°F
Measurement Accuracy: +/−2°F
Electronic programmable thermostat for 1-Stage
Heat/1-Stage Cool, gas or electric heat, non−heat pump,
non−power robbing applications.
Temperature Setpoint Range
Setting range: 50°F (10°C) to 90°F (32°C)
Setting resolution: 1°F (1°C)
Power Supply Range
18VAC − 30VAC (24VAC nominal), 60Hz
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51M38 SERIES 5/2
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Smart Setback Recovery (via DIP switch #6)
Equipment Protection Override
Both the minimum compressor OFF timer and the mini-
mum equipment cycle timer can be over−ridden by press-
ing and holding either the HEAT or COOL button down for 4
seconds.
Smart Setback Recovery (SSR) affects the way the ther-
mostat responds to program events. If SSR is disabled, the
thermostat will react to a program event at the time the
event occurs. However, if SSR is enabled, the thermostat
will react to a program event before the event occurs such
that the desired temperature is reached at the time of the
event, not after.
Over−Temperature Protection
Thermal-mechanical switch opens W1 at 93°F+/−6°F.
Filter Reminder
Autochangeover Deadband Selection (via DIP
switch #7)
Settings of Off, 1, 3, 6 or 12 (months of fan run time) are
available. When programmed time has elapsed, a FILTER
indicator is displayed.
Autochangeover deadband can be set to 4 or 6 degrees.
When autochangeover is enabled (via the AUTO button),
the thermostat will automatically change over from heating
to cooling and vice versa, to keep the room temperature in
between the heating and cooling setpoints. The deadband
is the minimum difference between the heating and cooling
setpoints.
Maintenance Reminder
Settings of Off, 6 or 12 (months of chronological time) are
available. When programmed time has elapsed, a MAIN-
TENANCE indicator is displayed.
Service Reminder
The SERVICE indicator is displayed only under the follow-
ing conditions:
Fan Control
AUTO or ON modes, gas or electric heat compatible via
DIP switches (also see Thermostat Output section).
S
if the thermostat Y1 terminal has been activated with
24VAC for at least 5 minutes, AND the L terminal is
shorted to the R terminal;
I/O Relays
All thermostat relays are latching type to minimize power
consumption.
OR
S
if the thermostat Y1 terminal has been activated with
24VAC for at least 5 minutes, AND the L terminal is
shorted to the C terminal.
Table 5
51M38 Terminal Designations
Terminal Description
Power Loss/Recovery
Thermostat memory is retained for a minimum of 24 hours
during a power loss (includes retention of clock setting,
program information, HOLD status, programmed temper-
ature setpoint, heat/cool and fan mode settings, filter re-
minder status, maintenance reminder status, and equip-
ment protection timers). After 24 hours of power loss,
programmed settings will be lost and replaced with default
settings.
R
24VAC
Y1
W1
G
First stage cooling
First stage heating
Fan control
L
Service Indicator
C
24VAC common
T
Outdoor temperature sensor connection 1
IMPORTANT
T
Outdoor temperature sensor connection 2
Power must be applied for at least six consecutive
hours prior to a power loss in order for memory to
be retained for the specified time.
Equipment Protection Timers
Minimum Compressor OFF time: 5 minutes
Minimum Compressor ON time: 4 minutes
Minimum Furnace ON time: 3 minutes
LCD Backlight
Activated for 30 seconds when any button is pressed.
Minimum furnace cycle time (elapsed time between any
furnace activation and the next furnace activation): 6 min-
utes.
NOTE − During an electrical storm or similar disturbance,
the backlight may activate for a few seconds. This is nor-
mal and will no longer occur after the electrical disturbance
has passed.
Minimum elapsed time between any compressor activa-
tion and the next compressor activation: 6 minutes.
Thermostat Operating Conditions
35°F to 105°F, 5% to 90% RH
NOTE − All protection timers (except the compressor OFF
timer) can be over−ridden if a heating or cooling demand is
initiated or terminated using the UP, DOWN, HEAT, or
COOL buttons.
Thermostat Storage Conditions
−40°F to 185°F, 5% to 95% RH
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505067M 7/13/05
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