506305-01 Page 27 of 36Issue 0938
OPERATION
Sequence of Operation
Heating
During a call for heat the thermostat closes the R-W circuit of
the control board. The control board verifies limit switches are
closed and pressure switch is open. The induced draft blower
relay closes causing the blower to run. As vent pressure is
developed by the induced draft blower, the pressure switch
closes. After a 15-second pre-purge, the control energizes the
hot surface ignitor. After the 7-second warmup time, the control
energizes the gas valve causing the burners to ignite. The hot
surface ignitor is de-energized 3 seconds after the valve opens.
If flame is sensed during this time the valve remains energized
and the control starts the 30-second heat blower “on” delay.
As heating demand is met, the thermostat de-energizes the
R-W circuit. The control de-energizes the gas valve causing
the burners to shut off. The induced draft blower shuts off
after a 15-second post-purge delay. The circulating air blower
will continue to operate until the user-selectable heat blower
“off” delay expires. The control return to standby mode once
the heat blower “off” delay expires.
Fan “On”
During a fan “on” call, the thermostat energizes the R-G
circuit of the control board, immediately causing the fan to
energize the heat speed. The fan remains energized as long
as the thermostat calls for fan “on” operation.
If a call for heat is energized during a fan “on” call, the fan
continues to operate at the heat speed. If a call for cooling is
energized during a fan “on” call, the fan switches to cooling
speed.
At the end of the fan “on” call the thermostat de-energizes
the R-G circuit of the control, causing the fan to be de-
energized immediately.
Cooling
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 cooling operation.
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 mode.
Controls
Following is a description of the operation of some of the
controls used in this furnace. All models use one of each
control, except as noted.
Pressure Switch
The pressure switch is a normally open switch that monitors
combustion air flow. Inadequate air flow resulting from
excessive venting system restriction or a failed combustion
blower will cause the switch to remain open.
Rollout Switch
The rollout switch is a normally closed switch that opens when
abnormal temperatures exist in the burner area. This can be
caused by a restricted heat exchanger causing burner flame
to “roll out” into the vestibule area or burner box.
This switch 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
This is a normally closed control that opens under abnormal
“reverse air flow” conditions that could occur in a counterflow
or horizontal installation if the circulating blower fails. It is an
automatic reset control.
G1D91BT and G1D93BT models have two auxiliary limit
controls.
G1D91BU and G1D93BU upflow models do not include an
auxiliary limit control.
Interlock (Blower Door) Switch
When the blower door is removed, the interlock switch
breaks the power supply to the burner controls and blower
motor. The switch operation must be checked to confirm it
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