Trane ACONT900AC43UA Service Manual page 130

Furnace Sequences Of Operation
Three-Stage Gas Furnace
The comfort control thermostat
3
for First Stage Heat. (16)
IFC 3 checks all limits
4a 4b
5a 5b 5c
PS-1, 2, 3. (9)
IFC 3 signals the Variable Speed Inducer Drive, IMDB
start the Vent Motor (8) and go to its second stage speed.
6b
Furnace always lights off in second stage. (9)
IFC 3 sees 24 Volt AC signal from Pressure Switches PS-1
PS-2 5b when they close. (9)
This proves Vent Motor 6b
combustion air through the furnace and vent system.
3
IFC starts the Hot Surface Igniter
up time cycle. (6)
3
IFC turns on the Gas Valve
seconds.
The IFC proves Ignition by the flame current sensing
3
(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 11 two times repeatedly.
If flame is detected, the IFC
warm-up time delay for the indoor blower.
45 Seconds later the IFC
3
come on and run at its second stage heat speed.
3
The IFC then signals the Inducer Motor Drive Board, IMDB,
(8) to ramp down the vent motor
6a
vent motor speed. This reduction in steps of the Vent Motor
Speed will cause the Gas Flow through the Gas Valve
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
5b
Blower (4) to go to the First Stage Heat speed.
10
GF - A 12
2 signals (17) the Furnace IFC
(10) & Pressure Switches
4c 4d 4e
is moving the correct amount of
7
learning routine warm-
8 (11). Trial for Ignition is five
9
will start the heat exchanger
3
10
signals the Indoor Blower
6b
in steps to the first stage
Three-Stage Gas Furnace
The comfort control thermostat
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
2
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
comfort control thermostat
3
for second Stage Heat.
(8) to
6a
The IFC 3 (8) signals the Inducer Motor Drive Board, IMDB,
(8) to ramp the Vent Motor
6a
Second Stage RPM speed.
5a
&
As the Vent Motor
coming through the gas valve
When the IFC
Switch #2, PS-2,
Indoor Blower
10
The comfort control thermostat
second stage heat operation until the indoor temperature moves
back toward its set point. Before the indoor temperature reaches
the comfort control thermostat
method
will signal the Furnace IFC
Or, if the comfort control thermostat
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
The IFC 3 signals the Inducer Motor Drive Board, IMDB
to ramp the vent motor
(4) to
10
speed.
As the Vent Motor
coming through the gas valve
to go
8
When the IFC
Switch 3, PS-3
blower (4) to go to its Third Stage Heat speed.
10
2
(17) will continue to call for
3 (17) to go to second stage heat. The
(17) then signals the Furnace IFC
2
6b
up in steps to its Learned
(8) speed increases, the amount of gas
6b
8
will increase.
sees the 24 Volt AC Signal from Pressure
3
5b (9) closing, the IFC will then signal the
(4) to go to its Second Stage Heat Speed.
(17) will continue to call for
2
(17) set point, the thermostat
2
3
to go back to first stage operation.
(17) sees the indoor
2
3 to go to Third Stage Heat.
6b
in steps to its learned third stage RPM
Speed increases, the amount of gas
6b
8
will increase.
3
sees the 24 Volt AC Signal from Pressure
(9) closing, the IFC will then signal the indoor
5c
(8)
6a