GF - A 12
Furnace Sequences Of Operation
Three-Stage Gas Furnace
The comfort control thermostat signals (17) the Furnace IFC
for First Stage Heat. (16)
IFC checks all limits (10) & Pressure Switches
PS-1, 2, 3. (9)
IFC signals the Variable Speed Inducer Drive, IMDB (8) to
start the Vent Motor (8) and go to its second stage speed.
Furnace always lights off in second stage. (9)
IFC sees 24 Volt AC signal from Pressure Switches PS-1 &
PS-2 when they close. (9)
This proves Vent Motor is moving the correct amount of
combustion air through the furnace and vent system.
IFC starts the Hot Surface Igniter learning routine warm-
up time cycle. (6)
IFC turns on the Gas Valve (11). Trial for Ignition is five
seconds.
The IFC proves Ignition by the flame current sensing method
(7). If flame is not detected, the IFC will cycle the furnace three
times to try and prove flame and then the IFC locks out for one
hour. The IFC will send an Alarm Code. It will also flash its Red
Fault LED two times repeatedly.
If flame is detected, the IFC will start the heat exchanger
warm-up time delay for the indoor blower.
45 Seconds later the IFC signals the Indoor Blower (4) to
come on and run at its second stage heat speed.
The IFC then signals the Inducer Motor Drive Board, IMDB,
(8) to ramp down the vent motor in steps to the first stage
vent motor speed. This reduction in steps of the Vent Motor
Speed will cause the Gas Flow through the Gas Valve to go
down in steps which will reduce the chances of a burner flame
out. When the IFC sees the 24 Volt AC signal from Pressure
Switch # 2 (PS-2) (9) go away, it will then signal the Indoor
Blower (4) to go to the First Stage Heat speed.
The comfort control thermostat (17) will continue to call for
first stage heat operation until the indoor temperature is back at
the comfort control thermostat set point or, if the comfort control
thermostat (17) sees the indoor temperature is not moving
back towards its set point in time or the indoor temperature is
still moving away from comfort control thermostat set point, it
will Signal the IFC (17) to go to second stage heat. The
comfort control thermostat (17) then signals the Furnace IFC
for second Stage Heat.
The IFC (8) signals the Inducer Motor Drive Board, IMDB,
(8) to ramp the Vent Motor up in steps to its Learned
Second Stage RPM speed.
As the Vent Motor (8) speed increases, the amount of gas
coming through the gas valve will increase.
When the IFC sees the 24 Volt AC Signal from Pressure
Switch #2, PS-2, (9) closing, the IFC will then signal the
Indoor Blower (4) to go to its Second Stage Heat Speed.
The comfort control thermostat (17) will continue to call for
second stage heat operation until the indoor temperature moves
back toward its set point. Before the indoor temperature reaches
the comfort control thermostat (17) set point, the thermostat
will signal the Furnace IFC to go back to first stage operation.
Or, if the comfort control thermostat (17) sees the indoor
temperature is not moving back towards its set point in time, or the
indoor temperature is still moving away from thermostat’s set point,
it will signal the furnace IFC to go to Third Stage Heat.
The IFC signals the Inducer Motor Drive Board, IMDB (8)
to ramp the vent motor in steps to its learned third stage RPM
speed.
As the Vent Motor Speed increases, the amount of gas
coming through the gas valve will increase.
When the IFC sees the 24 Volt AC Signal from Pressure
Switch 3, PS-3 (9) closing, the IFC will then signal the indoor
blower (4) to go to its Third Stage Heat speed.
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Three-Stage Gas Furnace
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