American Standard UHMB060ACV3VB Series Service Facts page 10

Service Facts
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 mini-
mum capacity change is 1%.
15. The minimum capacity of all furnaces is 40%, with
the exception of the *UHMD120. The minimum ca-
pacity 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 continu-
ous 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 fac-
tory 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 continuous 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.
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 ther-
mostat signal. Requested capacity can be seen in the
"STATUS" section of the User Interface menu.
10
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 mov-
ing 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 and User Interface as well as flash its
Red alert LED two times repeatedly.
8. If a flame is detected, the IFC will start the heat ex-
changer warm-up time delay for the indoor blower.
9. "IGNITION" will now be displayed in the "STATUS" sec-
tion 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 mo-
tor 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 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.
UHM-DHM-SF-1V-EN