24Vac Mode; Thermostat Call For W2 After W1; W2 Satisfied, W1 Still Called For; W1 Satisfied - Trane ACONT900AC43UA Service Manual

Furnace Sequences Of Operation
24 VAC Mode
1. This furnace modulates between 40% and 100% of capacity,
in 3% increments, every 1 minute. The furnace always lights at
approximately 65% and will modulate up or down; depending
on the 24VAC thermostat signal. Requested capacity can be
seen in the "STATUS" section of the User Interface menu.
Note:
Pressure Switch 1 closes at approximately 40% of capacity.
Pressure Switch 2 closes at approximately 65% of capacity.
Pressure Switch 3 closes at approximately 95% of capacity.
Thermostat call for W1 (2 stage heating thermostat)
2. R and W1 2b contacts close signaling the control board
2a
(IFC) to run its self-check routine. After the control has
3
verified that all safeties 4a 4b 4c 4d
and PS3 pressure switch contacts are open, the IFC
5c
5b
signals the variable speed inducer drive
motor 6b at the speed needed to close pressure switches PS1
and PS2.
5a 5b
Note:
The furnace lights at approximately 65% of capacity.
5b
5a
3. PS1 and PS2 close.
4. The IFC 3 receives a 24 VAC signal from PS1
5b when they close. This verifies the vent motor
the correct amount of combustion air through the furnace and
the vent system.
5. IFC 3 starts the hot surface ignitor
warm-up time cycle.
6. IFC 3 turns on the gas valve.
seconds.
7. The IFC 3 verifies ignition by the flame current sensing
If a flame is not detected, the IFC will cycle the furnace three
times to try and verify a flame. If no flame is detected, the IFC
will lockout for one hour. The IFC will send an alert code to the
User Interface 1 as well as flash its Red alert LED two times
repeatedly.
8. If a flame is detected, the IFC
warm-up time delay for the indoor blower.
9. "IGNITION" will now be displayed in the "STATUS" section of
1
the User Interface menu.
GF - A 4
are closed and PS1, 5a PS2,
6a to start the vent
and PS2
5a
is moving
6b
learning routine
7
Trial time for ignition is 5
8
9 method.
3 will start the heat exchanger
10
10. After 45 seconds, the IFC
10
to run at the programmed ignition sequence speed.
3
11. The IFC then signals the variable speed inducer motor
drive board 6a to ramp down to the corresponding speed to
5a
keep PS1 closed. The reduction of the vent motor
in steps allows the gas flow through the gas valve
1
reduced in steps, decreasing the chance of burner flame out.
The W1 2b heating capacity is 40%. The IFC will also decrease
the indoor blower motor

Thermostat call for W2 after W1

12. R and W2
2a 2c
for W2 heat. The IFC
drive to ramp up the vent motor
6a
the gas valve 8 to also be increased in 3% steps.
13. If the call for W2 remains, this 3% increase will be repeated
every 1 minute until the capacity requested is 100%. The IFC
3 will also increase the indoor blower motor
appropriate steps.

W2 satisfied, W1 still called for

14. R 2a and W2 2c
W2 heating requirements have been satisfied. The IFC
signal the variable speed vent motor
learned W1 speed. The gas valve
40% capacity and the indoor blower motor

W1 satisfied

15. R and W1
2a 2b
W1 heating requirements have been satisfied. The gas valve
will be de-energized and gas flow will cease. The variable
8
speed vent motor
6b
second post purge. The indoor blower motor
energized after the fan off delay period has ended. (The indoor
blower heat fan off delay is field selectable and can be adjusted
using the User Interface menu.
seconds but can be set to 60, 140, or 180 seconds
Thermostat call for heat (1 stage heating thermostat)
16. W1 and W2
2b 2c
W1 contacts close signaling the control board (IFC) to run its
2b
self- check routine. After the control has verified that all safeties
are closed and PS1,
contacts are open, the IFC signals the variable speed inducer
6a
drive to start the vent motor
pressure switches PS1
17. PS1 5a and PS2

24VAC Mode

3 signals the indoor blower motor
10 speed.
thermostat contacts close signaling a call
3 then signals the variable speed inducer
allowing flow through
6b
thermostat contacts open signaling that
6b
to slow down to its
will reduce the gas flow to
8
10 speed will be reduced.
thermostat contacts open signaling that
will de-energize approximately after a 5
It is factory set at 100
1
must be jumpered at the IFC.
5a PS2, 5b and PS3 5c pressure switch
6b
at the speed needed to close
5a 5b
and PS2.
5b close.
6b speed
8
to also be
10
speed in
3 will
will be de-
10
R and
3 2a