Sequence Of Operation - American Standard UHMB080ACV3VB Series Installer's Manual

SEQUENCE OF OPERATION
SEQUENCE OF OPERATION – COMMUNICATING
MODE
1. This furnace is fully modulating between 40% and
100% of capacity in 1% increments. The furnace
always lights at approximately 65% and will modulate
up or down; depending on the communicating
comfort control demand. 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.
2. The communicating comfort control signals the furnace
IFC for heat.
3. The IFC then checks all safeties, thermostats, and
pressure switches PS1, PS2, and PS3.
4. The IFC signals the variable speed inducer drive to
start the vent motor at the speed needed to close
pressure switches PS1 and PS2.
5. PS1 and PS2 close.
6. 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.
7. IFC starts the hot surface ignitor learning routine
warm-up time cycle.
8. IFC turns on the gas valve. Trial time for ignition is 5
seconds.
Note:
The furnace lights at approximately 65% of capacity.
9. 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). The Red alert LED two times repeatedly.
10. If a flame is detected, the IFC will start the heat
exchanger warm-up time delay for the indoor blower.
11. "IGNITION" will now be displayed in the "STATUS"
section of the User Interface menu.
12. After 45 seconds, the IFC signals the indoor blower
motor and run at the programmed ignition sequence
speed.
13. Depending on the communicating comfort control
demand, the IFC will then signal the variable speed
inducer motor drive board and the indoor blower
motor to ramp up or down.
14. With a heat demand less than 65% from the
communicating comfort control, the IFC signals the
variable speed inducer motor drive board to ramp
down to that corresponding demand speed. The
reduction of the vent motor speed in steps allows the
gas flow through the gas valve to also be reduced
in steps, decreasing the chance of burner flame out.
The minimum capacity change is 1%.
15. The minimum capacity of all furnaces is 40%, with the
exception of the *UHMC120. The minimum capacity
for this furnace is 45%.
16. The communicating comfort control will continue
to call for the requested capacity demand until the
indoor temperature is back at the set point OR,
17. If the communicating comfort control does not detect
that the indoor temperature is moving back towards
its set point or the indoor temperature is still moving
away from the set point, it will signal the IFC to go to
a higher capacity of heat.
18. The IFC then signals the variable speed inducer drive
to ramp up the vent motor in steps. As the vent motor
speed increases the amount of gas coming through
the gas valve will increase. The IFC will also increase
the indoor blower motor speed.
19. The communicating comfort control will continue to
monitor the indoor temperature and send signals to
the IFC to modulate the heating capacity or turn off to
maintain the homeowner's set point.
20. Once the communicating comfort control senses
that the 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.
All pressure switches will open and the indoor blower
motor will then run the heat off delay that is selected
in the User Interface menu.
Indoor Blower motor operation thermostat fan switch "ON"
(Communicating Mode)
The communicating comfort control signals a continuous
fan call. The factory setting is 50% of the cooling cfm
selected but can be adjusted from 25% - 100% through
the Installer Setup menu on the communicating comfort
control. If the outdoor unit is a 2 stage system, the factory
setting is 50% of the 2nd stage cooling cfm. If the outdoor
unit is a 24 volt single stage cooling system, the factory
setting is 50% of the cooling cfm.
NOTE: If the actual relative humidity (RH) is at or
below the set-point, the fan will run until the continu-
ous fan call is removed or the actual RH exceeds the
set-point. If the actual RH is above the set-point, the
fan will not turn on.
See the communicating comfort control Installer's Guide
for additional information.
18-CD30D1-20-EN
Installer's Guide
43