Sequence of Operation
A call for heat from the thermostat closes R to W on the blower
control board, which begins the ignition sequence. The induced
draft blower output energizes. The pressure switch senses
normal combustion air flow and closes. After a 15-second pre-
purge, the control energizes the hot surface igniter. After an
igniter warm-up period, the main gas valve energizes. Upon main
burner ignition, the circulating air blower energizes following a
When the call for heat is satisfied, R to W opens and the gas to
the burners shuts off. The control turns off the inducer after a 5-
second post-purge delay and the heating speed blower output
turns off following the blower "off" delay.
A call for fan from the thermostat closes R to G on the control
board. The control waits for a 1-second thermostat debounce
delay before responding by energizing the heat speed blower.
When the call for fan is turned off, the control de-energizes the
heating speed blower after a 60-second delay.
A call for cooling from the thermostat closes R to Y and R to G on
the blower control board. The control waits for a 1-second
thermostat debounce delay before energizing the cool speed
blower. When the call for cooling is satisfied, the control
de-energizes the cooling speed blower after a 60-second delay.
The pressure switch is a normally open switch that monitors
combustion airflow. Inadequate airflow resulting from excessive
venting system restriction or a failed combustion blower will
cause the switch to remain open.
The rollout switch(es) are normally closed switch(es) that open
when abnormal temperatures exist in the burner area. This can
be caused by a restricted heat exchanger causing main burner
flame to "roll out" into the vestibule area or burner box.
The rollout switch(es) must be manually reset by pushing the
button on top to restore furnace operation.
Primary Limit Control
This is a normally closed control that opens if abnormally high
circulating air temperatures occur. It is an automatic reset control.
Auxiliary Limit Control (on some models)
This is a normally closed control, located on the circulating air
blower housing, that opens under abnormal "reverse air flow"
conditions that could occur in a counterflow or horizontal
installation if the circulating air blower fails. It is an automatic
Safety Interlock Switch
When the blower door is removed, the safety interlock switch
breaks the power supply to the burner controls and blower motor.
Blower Control Board
Electrical Shock Hazard
Disconnect power before servicing.
Replace all parts and panels before operating.
Failure to do so can result in death or electrical shock.
The blower control board operates the circulating air blower, the
combustion blower and any accessories connected to it.
WGFDC and WGFDU models
Feature user-selectable blower "off" delay times (60, 90, 120, and
150 seconds) that are factory set to provide a 120-second blower
"off" delay on heating.
WFCU and WFCC models
Feature a fixed blower "off" delay of 90 seconds which is one-
time adjustable to 150 seconds. See the "Wiring Connection"
In the event the limit control senses an abnormally high
temperature and opens, the ignition control de-energizes, and the
combustion blower and circulating air blower heating speed
continue to energize.
Gas Control Valve
The gas control valve contains control logic to sense proper
operating conditions and provides gas flow and ignition only
when all conditions are properly met. The gas control valve also
regulates the manifold gas pressure.