Trane UHMB060ACV3VA Installer's Manual page 44

Installer's Guide
SEQUENCE OF OPERATION – 24VAC 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 contacts close signaling the control
board (IFC) to run its self- check routine. After
the control has verified that all safeties are closed
and PS1, PS2, and PS3 pressure switch contacts
are open, the IFC signals the variable speed
inducer drive to start the vent motor at the speed
needed to close pressure switches PS1 and PS2.
Note:
The furnace lights at approximately 65% of capacity .
3. PS1 and PS2 close.
4. The IFC receives a 24 VAC signal from PS1 and
PS2 when they close. This verifies the vent
motor is moving the correct amount of
combustion air through the furnace and the vent
system.
5. IFC starts the hot surface ignitor learning routine
warm-up time cycle.
6. IFC turns on the gas valve. Trial time for ignition
is 5 seconds.
7. The IFC verifies ignition by the flame current
sensing method. 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 communicating comfort control
(ERR 22) and User Interface (RETRY LO) as well
as flash its Red alert LED two times repeatedly.
8. If a flame is detected, the IFC will start the heat
exchanger warm-up time delay for the indoor
blower.
9. "IGNITION" will now be displayed in the
"STATUS" section of the User Interface menu.
10. After 45 seconds, the IFC signals the indoor
blower motor to run at the programmed ignition
sequence speed.
11. The IFC then signals the variable speed inducer
motor drive board to ramp down to the
corresponding speed to keep PS1 closed. The
reduction of the vent motor speed in steps allows
the gas flow through the gas valve to also be
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reduced in steps, decreasing the chance of burner
flame out. The W1 heating capacity is 40%. The
IFC will also decrease the indoor blower motor
speed.
Thermostat call for W2 after W1
12. R and W2 thermostat contacts close signaling a
call for W2 heat. The IFC then signals the
variable speed inducer drive to ramp up the vent
motor allowing flow through the gas valve 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 will also increase the
indoor blower motor speed in appropriate steps.
W2 satisfied, W1 still called for
14. R and W2 thermostat contacts open signaling
that W2 heating requirements have been
satisfied. The IFC will signal the variable speed
vent motor to slow down to its learned W1 speed.
The gas valve will reduce the gas flow to 40%
capacity and the indoor blower motor speed will
be reduced.
W1 satisfied
15. R and W1 thermostat contacts open signaling
that W1 heating requirements have been
satisfied. The gas valve will be de-energized and
gas flow will cease.
The variable speed vent motor will de-energize
approximately after a 5 second post purge.
The indoor blower motor will be de-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. It is factory set at 100 seconds but can be
set to 60, 140, or 180 seconds)
Thermostat call for heat (1 stage heating
thermostat)
Note: The inter-stage delay is field selectable and can
be adjusted through the User interface menu.
It is factory set at 0 minutes but can be adjusted to 5,
10, or 15 minutes. This option can help optimize the
furnace to try to satisfy the heating requirement during
low heating load conditions.
16. W1 and W2 must be jumpered at the IFC. R and
W1 contacts close signaling the control board
(IFC) to run its self- check routine. After the
control has verified that all safeties are closed
and PS1, PS2, and PS3 pressure switch contacts
are open, the IFC signals the variable speed
inducer drive to start the vent motor at the
speed needed to close pressure switches PS1 and
PS2.
Note:
The furnace lights at approximately 65% of capacity .
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