50A66-743
Integrated Furnace Control
INSTALLATION INSTRUCTIONS
Operator: Save these instructions for future use!
FAILURETO READ AND FOLLOW ALL INSTRUCTIONS CAREFULLY BEFORE
INSTALLING OR OPERATING THIS CONTROL COULD CAUSE PERSONAL
INJURY AND/OR PROPERTY DAMAGE.
DESCRIPTION
50A66-743 Cross Reference Information
The 50A66-743 is an automatic gas interrupted ignition control
that employs a microprocessor to continually monitor, analyze,
and control the proper operation of the gas burner, inducer,
and fan.
Lennox
23W51
White-Rodgers
100925-01
100925-02
100925-03
17W92
69M0801
69M15
50A66-122
50A66-123
50A66-743
Signalsinterpretedduringcontinualsurveillanceofthethermostat
and flame sensing element initiate automatic ignition of the
burner, sensing of the flame, and system shut-off during normal
operation.
23W5101
30W25
69M1501
30W2501
69M08
17W9201
These controls incorporate system fault analysis for quick gas
flow shut-off, coupled with automatic ignition retry upon sensing
a fault correction.
PRECAUTIONS
Installation should be done by a qualified heating and air
conditioning contractor or licensed electrician.
WARNING
Ifindoubtaboutwhetheryourwiringismillivolt,line,orlowvoltage,
have it inspected by a qualified heating and air conditioning
contractor or licensed electrician.
Do not exceed the specification ratings.
All wiring must conform to local and national electrical codes
and ordinances.
Failure to comply with the following
warnings could result in personal injury
or property damage.
This control is a precision instrument, and should be handled
carefully.Rough handling or distorting components could cause
the control to malfunction.
FIRE HAZARD
• Do not exceed the specified voltage.
• Replaceexistingcontrolwithexactmodelanddash
number.
Following installation or replacement, follow manufacturer’s
recommended installation/service instructions to ensure proper
operation.
• Protect the control from direct contact with water
(dripping, spraying, rain, etc.).
• If the control has been in direct contact with water,
replace the control.
• Labelallwiresbeforedisconnectionwhenservicing
controls. Wiring errors can cause improper and
dangerous operation.
CAUTION
Donotshortoutterminalsongasvalveorprimary
control. Short or incorrect wiring may damage
the thermostat.
• Route and secure wiring away from flame.
SHOCK HAZARD
• Disconnect electric power before servicing.
• Ensure proper earth grounding of appliance.
• Ensure proper connection of line neutral and line
hot wires.
CONTENTS
Description ........................................................ 1
Precautions ....................................................... 1
Specifications .................................................... 2
Installation ......................................................... 3
Mounting & Wiring
EXPLOSION HAZARD
• Shut off main gas to appliance until installation is
complete.
Operation ........................................................... 6
System Lockout and Diagnostic Features........... 7
Troubleshooting................................................... 8
PART NO. 37-7473A
1313
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INSTALLATION
WARNING
Donotshortoutterminalsongasvalveorprimary
control. Short or incorrect wiring may damage
the thermostat.
FIRE HAZARD
• Do not exceed the specified voltage.
• Replaceexistingcontrolwithexactmodelanddash
number.
• Protect the control from direct contact with water
(dripping, spraying, rain, etc.).
• Labelallwiresbeforedisconnectionwhenservicing
controls. Wiring errors can cause improper and
dangerous operation.
MOUNTING AND WIRING
All wiring should be installed according to local and na-
tional electrical codes and ordinances.
Thecontrolmustbesecuredtoanareathatwillexperience
a minimum of vibration and remain below the maximum
ambient temperature rating of 175°F. The control is ap-
proved for minimum ambient temperatures of -40°F.
• Route and secure wiring away from flame.
Any orientation is acceptable.
SHOCK HAZARD
Refertothewiringdiagramandwiringtablewhenconnect-
ing the 50A66 control to other components of the system.
• Disconnect electric power before servicing .
• Ensure proper earth grounding of appliance.
• Ensure proper connection of line neutral and line
hot wires.
2
64
UL approved, 105°C rated 18 gauge, stranded, / ” thick
insulation wire is recommended for all low voltage safety
circuit connections.Refer to 50A66 specification sheet for
recommendedterminalstomatewiththoseonthecontrol.
4
EXPLOSION HAZARD
• Shut off main gas to appliance until installation is
complete.
64
UL approved 105°C rated 16 gauge min., stranded, / ”
thick insulation wire is recommended for all line voltage
connections. Refer to 50A66 specification sheet for rec-
ommended terminals to mate with those on the control.
After installation or replacement, follow appliance manu-
facturer’srecommendedinstallationorserviceinstructions
to ensure proper operation.
3
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WIRING
TYPICAL SYSTEM WIRING DIAGRAM
HOT
(LINE)
NEUTRAL
(LINE)
120 VAC
24 VAC CLASS II
TRANSFORMER
TH
24 VAC
50A66
TR
FAN
COOL
HEAT
PARK
PARK
LINE
CIRCULATOR
BLOWER
XFMR
EAC
HUM
PARK
INDUCER
IGNITOR
IND
IGN
IGN N
IND N
HUMIDIFIER
ELECTRONIC
AIR CLEANER
Y
W
G
R
Y
W
G
R
COMPRESSOR
CONTACTOR
24 V
HUMIDIFIER
THERMOSTAT
C
(E2)
24 V HUM
FLAME
SENSOR
PROBE
(E33)
FP
PRESSURE SWITCH (N. O.)
PSI
PSO
HLI
HLO
HIGH LIMIT
AUX. HIGH
LIMIT (N. C.)
RO OUT
GND
(N. C.)
ROLLOUT
(N. C.)
RO IN
2 STAGE
GAS
VALVE
MV LO
MV HI
MV COM
TR
TH
[12-Pin Connector]
LEGEND
PSO
HLI
PSI
HLO
Low Voltage (24 VAC)
Line Voltage (120 VAC)
MV
LO
RO
RO
GND
OUT
OUT
MV
IND
IGN
N
TR
MV
IND
TH
IGN
COM
N
N. C. = Normally closed switch
N. O. = Normally open switch
4-Pin Connector
12-Pin Connector
4
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WIRING
TYPICAL SYSTEM WIRING TABLE
50A66
TERMINAL
TERMINAL
SYSTEM COMPONENT
CONNECTION
TYPE
W
G
R
Y
low voltage thermostat W terminal (or equivalent)
low voltage thermostat G terminal (or equivalent)
low voltage thermostat R terminal (or equivalent)
low voltage thermostat Y terminal (or equivalent)
Terminal
block with
captive
screws
(2nd wire from Y terminal goes to 24 VAC HOT side of
compressor contactor coil)
C
24 VAC COMMON side of compressor contactor coil
high limit OUTPUT
HLO (Pin 1)
Not Used (Pin 2)
TH (Pin 3)
24 VAC transformer (low voltage HIGH side)
Not Used (Pin 4)
RO1 (Pin 5)
TR (Pin 6)
PSO
HLI
PSI
HLO
rollout switch OUTPUT
MV
LO
RO
RO
GND
OUT
OUT
24 VAC transformer (low voltage COMMON side)
high limit INPUT
MV
TR
MV
HLI (Pin 7)
TH
COM
GND (Pin 8)
MV COM (Pin 9)
PSI (Pin 10)
RO2 (Pin 11)
MV (Pin 12)
MUST BE RELIABLY GROUNDED TO CHASSIS
gas valve COMMON
pressure switch INPUT
rollout switch INPUT
gas valve
IND
IGN
inducer HOT side
ignitor HOT side
IND
N
IGN
N
IND
IGN
IND N
IGN N
inducer NEUTRAL side
ignitor NEUTRAL side
COOL 1/4” spade terminal
HEAT 1/4” spade terminal
PARK (3 terminals) 1/4” spade terminal
LINE 1/4” spade terminal
circulator blower COOL SPEED terminal
circulator blower HEAT SPEED terminal
unused circulator blower terminals
input voltage (120 VAC) HOT side
24 VAC transformer line voltage HOT side
electronic air cleaner HOT side
XFMR 1/4” spade terminal
EAC (optional) 1/4” spade terminal
HUM (optional) 1/4” spade terminal
humidifier HOT side
Terminals for 120 Volt NEUTRAL for line, transformer,
circulator, electronic air cleaner and humidifier
120 VOLT NEUTRAL
1/4” spade terminal
(5 terminals)
3/16” spade terminal
flame sensor probe*
FP (E33)
1/4” spade terminal
FAN
Circulator blower fan speed terminal
24 VAC humidifier
1/4” receptacle
24 V HUM (E2)
* maximum recommended flame probe wire length is 36 inches.
5
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OPERATION
OPTION SWITCHES
The option switches on the 50A66-743 control are used
to determine the length of the heat delay-to-fan-off period.
The following table shows the time periods that will result
from the various switch positions.
satisfied, the gas valve is de-energized. After proof of
flameloss, theinducerblowerremainsenergizedtopurge
the system for 5 seconds and the delay-to-fan-off period
begins. When the purge is complete, the inducer blower
andhumidifierarede-energized.Afterthedelay-to-fan-off
period ends, the circulator fan and electronic air cleaner
are de-energized.
OPTION SWITCH POSITIONS
If flame is not detected, both valves are de-energized,
the ignitor is turned off, and the 50A66 control goes into
the “retry” sequence. The “retry” sequence provides a
15-secondwaitfollowinganunsuccessfulignitionattempt
(flame not detected).After this wait, the ignition sequence
is restarted. If this ignition attempt is unsuccessful, three
more retries will be made before the control goes into
system lockout.
HEAT delay-
to-fan-off:
Set switch
#1
#2
60 sec.
90 sec.*
120 sec.
180 sec.
Off
Off
On
On
Off
On
Off
On
COOL delay-
to-fan-off:
Set switch
#3
Ifflameisdetected,thenlost,the50A66controlwillrepeat
the initial ignition sequence for a total of three “recycles”.
After three unsuccessful “recycle” attempts, the control
will go into system lockout.
2 sec.
45 sec.*
Off
On
* Factory Setting
If flame is established for more than 10 seconds after
ignition, the50A66controllerwillcleartheignitionattempt
(or retry) counter. If flame is lost after 10 seconds, it will
restart the ignition sequence.This can occur a maximum
of four times before system lockout.
HEAT MODE
In a typical system, a call for heat is initiated by closing
the thermostat contacts. This starts the 50A66 control’s
heating sequence.The inducer blower and humidifier are
energized. After a 15-second prepurge period, the 768A
Silicon Nitride ignitor is powered.
A momentary loss of gas supply, flame blowout, or a
shorted or open condition in the flame probe circuit will be
sensed within 2 seconds. The gas valve will de-energize
andthecontrolwillrestarttheignitionsequence.Recycles
will begin and the burner will operate normally if the gas
supply returns, or the fault condition is corrected, before
the last ignition attempt. Otherwise, the control will go
into system lockout.
This controller has an adaptive algorithm that reduces the
ignitor temperature to slightly greater than the minimum
temperature required to ignite gas in each particular
application. The control measures the line voltage and
determines an initial ignitor temperature setting based
on the measurement. After each successful ignition, the
control lowers the ignitor temperature slightly for the next
ignition attempt.The control continues to lower the ignitor
temperature until ignition does not occur, and the control
goes into retry mode. For the second attempt to ignite
gas within the same call for heat, the control increases
the ignitor temperature to the value it was on the third
previous successful ignition. After ignition is successful,
the control sets the ignition temperature at this value for
the next 255 calls for heat, after which the control repeats
the adaptive algorithm. The control is constantly making
adjustments to the ignitor temperature to compensate for
changes in the line voltage.
Ifthecontrolhasgoneintosystemlockout,itmaybepossi-
ble to reset the control by a momentary power interruption
of ten seconds or longer. Refer to PRECAUTIONARY,
SYSTEM LOCKOUT, AND DIAGNOSTIC FEATURES.
COOL MODE
In a typical system, a call for cool is initiated by closing
the thermostat contacts. This starts the 50A66 control’s
cooling sequence. The compressor is energized and the
delay-to-fan-on period begins. After the delay-to-fan-on
period ends, the circulator fan is energized at cool speed.
Theelectronicaircleaner(optional)isalsoenergized.After
thethermostatissatisfied,thecompressorisde-energized
and the delay-to-fan-off period begins. After the delay-to-
fan-off period ends, the circulator fan and electronic air
cleaner are de-energized.
The 80 VAC Silicon Nitride ignitor manufactured by
White-Rodgersmustbeused.Theseignitorsarespecial-
ly designed to operate with the 50A66’s adaptive ignition
routine to ensure the most efficient ignitor temperature.
At the end of the ignitor warm-up time, both valves in
the 36E manifold gas valve are opened. Flame must be
detected within 4 seconds.
MANUAL FAN ON MODE
If the thermostat fan switch is moved to the ON position,
the circulator fan (heat speed) and optional electronic air
cleaner are energized. When the fan switch is returned
to the AUTO position, the circulator fan and electronic air
cleaner (optional) are de-energized.
If flame is detected, the delay-to-fan-on period begins.
After the delay-to-fan-on period ends, the circulator fan is
energized at heat speed. If there is an optional electronic
air cleaner on the system, the electronic air cleaner and
the humidifier are energized. When the thermostat is
6
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SYSTEM LOCKOUT AND DIAGNOSTIC FEATURES
2. Interrupt the 24 VAC power at the control for at least
SYSTEM LOCKOUT FEATURES
ten seconds. You may also need to reset the flame
rollout sensor switch.
Whensystemlockoutoccurs,thegasvalveisde-energized.
The circulator blower may be energized at heat speed for
some faults, and if flame is sensed, the inducer blower is
energized.The diagnostic indicator light will flash or glow
continuously to indicate system status. (System lockout
will never override the precautionary features.)
3. Afteronehourinlockout,thecontrolwillautomatically
reset itself.
DIAGNOSTIC FEATURES
To reset the control after system lockout, do one of
The 50A66-743 control continuously monitors its own
operation and the operation of the system. If a failure oc-
curs, the DS 1 (red) and DS 2 (green) LEDs will indicate
a failure code as shown below.
the following:
1. Interrupt the call for heat at the thermostat for at least
one second, but for less than 20 seconds (if flame is
sensedwiththegasvalvede-energized,interruptingthe
call for heat at the thermostat will not reset the control).
7
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TROUBLESHOOTING
DIAGNOSTIC TABLE
DS 1 (Red) DS 2 (Green) Error/Condition
Comments/Troubleshooting
Simultaneous Simultaneous
slow flash slow flash
Normal operation
No fault
Simultaneous Simultaneous
Normal operation with Normal operation
call for heat
fast flash
Slow flash
Off
fast flash
On
Open limit switch
Verify continuity through rollout switch circuit
Slow flash
Pressure switch stuck
closed/open
Pressure switch stuck closed. Check switch function, verify
inducer is turning off.
Pressure switch stuck open. Check pressure switch function
and tubing. Verify inducer is turning on and pulling sufficient
vacuum to engage switch.
Alternate
slow flash
Alternate slow
flash
External lockout
(retries)
Failure to sense flame is often caused by carbon deposits on
the flame sensor, a disconnected or shorted flame sensor
lead or a poorly grounded furnace. Carbon deposits can be
cleaned with emery cloth. Verify sensor is not contacting the
burner and is located in a good position to sense flame.
Ignitor must be positioned to light the gas immediately when
the valve opens. If the ignitor has been replaced, verify hot
spot position has not changed.
Check sensor lead for shorting and verify furnace is
grounded properly.
Slow flash
Off
Flame sensed with gas Verify the gas is operating and shutting down properly. Flame
valve de-energized
in burner assembly should extinguish promptly at the end of
the cycle. Check orifices and gas pressure.
On
On
Slow flash
Open rollout switch
Verify continuity through rollout switch circuit.
On
Module – internal fault Module contacts for gas valve not operating or processor
condition
fault. Reset control, if condition persists, replace module.
Slow flash
Fast flash
Low flame sense
current
Low flame sense current is often caused by carbon deposits
on the flame sensor, a poorly grounded furnace or a mis-
aligned flame sense probe. Carbon deposits can be cleaned
with emery cloth.
Check or improve furnace and module ground. Verify sensor
is located in or very near flame as specified by the appliance
manufacturer.
Fast flash
Slow flash
Grounding or
reversed polarity
Verify the control and furnace are properly grounded. Check
and reverse polarity (primary) if incorrect.
Alternate fast
flash
Alternate fast
flash
Module ignitor
contact failure
Fault code indicates the module ignitor contacts are not
functioning properly. Replace module.
HOMEOWNER HELP LINE: 1-800-284-2925
White-Rodgers is a business
of Emerson Electric Co.
The Emerson logo is a
trademark and service mark
of Emerson Electric Co.
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