During a call for cooling, the thermostat energizes the R - Y
circuit of the control board. After a 1-second cooling "on" delay,
the control energizes the cooling fan speed. If the fan is already
energized, it remains running and does not de-energize for the
1-second cooling fan "on" delay.
The call for cooling has priority over continuous fan operation
while a call for heating has priority over both a call for cooling or
continuous fan. Ignition lockouts for any reason do not affect
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.
Safety Interlock Switch
When the blower door is removed, the safety interlock switch
breaks the power supply to the burner controls and blower motor.
As cooling demand is met, the thermostat de-energizes the R - Y
circuit of the control board. After a 60-second cooling "off" delay,
the control de-energizes the cooling speed fan. At the end of the
cooling "off" delay period, the control returns to the standby
Ignition Control Board
Disconnect power before servicing.
Replace all parts and panels before operating.
Failure to do so can result in death or electrical shock.
The ignition control board operates the gas control valve,
circulating air blower, combustion blower and any accessories
connected to it. The ignition control contains control logic to
sense proper operating conditions and provides ignition only
when all conditions are properly met.
These models feature user-selectable blower "off" delay times
(60, 90, 120 and 180 seconds) that are factory set to provide a
120-second blower "off" delay on heating. See "Wiring
Gas Control Valve
The gas control valve regulates the manifold gas pressure and
provides gas flow.
Electrical Shock Hazard