Variable Speed, 2-Stage
Upflow / Horizontal and Downflow / Horizontal, Gas-Fired Furnaces, “Fan Assisted
Combustion System” * - First letter may be “A” or “T”
*UD2B060ACV32A *UD2B080ACV32A *UD2C080ACV42A *UD2B100ACV32A *UD2C100ACV52A *UD2D120ACV52A *UD2D140ACV52A
*DD2B060ACV32A *DD2B080ACV32A *DD2C100ACV52A *DD2D120ACV52A

Furnace Alert Codes

ALERT CODE RECOVERY

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Sequence Of Operation (Communicating

Mode)

Comfort control call for 1st stage heat (A/TCONT900):

The comfort control signals the control board (IFC) to run its self-check routine. After the control has verified that
all safeties are closed and the 1st stage pressure switch con- tacts are open, the draft inducer will be energized on high speed. After the draft inducer comes up to speed, the 1st stage pressure switch contacts will close, the draft inducer switches to low speed, and the igniter warm-up period will begin. The igniter will heat for approximately 20 seconds; the 1st stage gas valve will be energized to permit gas flow to the burners. The flame sensor confirms that ignition has been achieved within the 4 second ignition trial period.
As the flame sensor confirms that ignition has been achieved, the delay to fan ON period begins timing and af- ter approximately 45 seconds the indoor blower motor will be energized at the 1st stage speed and will continue to run during the heating cycle.

Comfort control call for 2nd stage after 1st stage:

The comfort control signals a call for 2nd stage heat. After a
30 second delay, the draft inducer will be energized on high speed. The 2nd stage pressure contacts will close allowing the 2nd stage gas valve to be energized. The indoor blower motor will ramp up to 2nd stage heating airflow.

2nd stage satisfied, 1st stage still called for:

The comfort control signals that 2nd stage heating re- quirements have been satisfied. The draft inducer motor is reduced to low speed allowing the 2nd stage pressure switch to open. The 2nd stage gas valve is de-energized
allowing 1st stage heating operation. After approximately
30 seconds, the indoor blower motor is reduced to 1st stage heating airflow.

1st stage satisfied:

The comfort control signals that the 1st stage heating re- quirements have been satisfied. The 1st stage gas valve will be de-energized and gas flow will cease. The draft inducer motor will be de-energized 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)

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 se- lected but can be adjusted from 25% - 100% through the In- staller 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 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.

Indoor Blower Timing:

The furnace control module (IFC) controls the variable speed motor in the heating mode. The fan “ON” time is set at 45 seconds and is not field selectable. The blower “OFF” delay is factory set at 100 seconds and can be adjusted to
60, 140, or 180 seconds.

Sequence Of Operation (24v Mode) Thermostat call for 1st stage heat (2 stage heating thermostat):

R and W1 thermostats contacts close signaling the control board (IFC) to run its self-check routine. After the control has verified that all safeties are closed and the 1st stage pressure switch contacts are open, the draft inducer will be energized on high speed.
After the draft inducer comes up to speed, the 1st stage pres- sure switch contacts will close, the draft inducer switches to low speed, and the igniter warm-up period will begin. The igniter will heat for approximately 20 seconds; the 1st stage gas valve will be energized to permit gas flow to the burners. The flame sensor confirms that ignition has been achieved within the 4 second ignition trial period.
As the flame sensor confirms that ignition has been achieved, the delay to fan ON period begins timing and after approxi- mately 45 seconds the indoor blower motor will be energized at the 1st stage speed and will continue to run during the heating cycle.

Thermostat call for 2nd stage after 1st stage:

R and W2 thermostat contacts close signaling a call for 2nd stage heat. After a 30 second delay, the draft inducer will be energized on high speed. The 2nd stage pressure contacts will close allowing the 2nd stage gas valve to be energized. The indoor blower motor will ramp up to 2nd stage heating airflow.

2nd stage satisfied, 1st stage still called for:

R and W2 thermostat contacts open signaling that 2nd stage heating requirements have been satisfied. The draft inducer motor is reduced to low speed allowing the 2nd stage pres- sure switch to open. The 2nd stage gas valve is de-energized allowing 1st stage heating operation. After approximately
30 seconds, the indoor blower motor is reduced to 1st stage heating airflow.

1st stage satisfied:

R and W1 thermostat contacts open signaling that the 1st stage heating requirements have been satisfied. The 1st stage gas valve will be de-energized and gas flow will cease. The draft inducer motor will be de-energized after approxi- mately 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) W1 and W2 must be jumpered at the control board.
R and W thermostat contacts close signaling a call for heat. The control board (IFC) runs it self check routine. After the control has verified that all safeties are closed and the 1st stage pressure switch contacts are open, the draft inducer will be energized on high speed. After the draft inducer
comes up to speed, the 1st stage pressure switch contacts will close, the draft inducer switches to low speed, and the igniter warm-up period will begin. The igniter will heat for approxi- mately 20 seconds; the 1st stage gas valve will be energized
to permit gas flow to the burners. The flame sensor confirms that ignition has been achieved within the 4 second ignition trial period.
As the flame sensor confirms that ignition has been achieved, the delay to fan ON period begins timing and after approxi- mately 45 seconds the indoor blower motor will be energized at the 2nd stage speed heating airflow. At the same time,
the 2nd stage draft inducer is energized and the 2nd stage pressure switch then closes. The 2nd stage gas valve is then energized. (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.

Thermostat satisfied:

R and W thermostat contacts open signaling the heating requirements have been satisfied. The gas valve is de- energized and after a 5 second draft inducer post purge, the draft inducer is de-energized.
If the inter-stage delay was met and the furnace was oper- ating on 2nd stage heat, the indoor motor will operate at
2nd stage heating airflow for the duration of the 2nd stage heating fan off delay.
If the furnace was operating on 1st stage heat, the indoor motor will operate at 1st stage heating airflow for the dura- tion of the 1st stage heating fan off delay.

Indoor Blower motor operation thermostat fan switch “ON” (24V Mode)

R and G comfort control contacts close signaling a continu- ous fan call. The continuous fan cfm is field selectable and can be adjusted through the User Interface menu. The fac- tory setting is 50% of the cooling cfm selected but can be set at 25%, 50%, 75%, or 100%. If the system has a 2 stage out- door unit, the setting is 50% of the 2nd stage cooling cfm. Indoor Blower Timing:
The furnace control module (IFC) controls the variable speed motor in the heating mode. The fan “ON” time is set at 45 seconds and is not field selectable. The blower “OFF” delay is factory set at 100 seconds and can be adjusted.

PERSONALITY MODULE

The Personality Module is a removable memory device, on

which is stored model specific data required for proper furnace operation. The Personality Module is tethered to the unit and must remain with the furnace at all times. The Personality module must remain plugged into the furnace IFC.

UNIT TEST MODE

Unit Test Mode

This furnace is equipped with a Unit Test mode that can be accessed through the User Interface. See the User Interface menu on how to enter this option. The test mode can be performed in communicating or 24vac modes.
In communicating or 24vac modes, the unit test can only be entered when there is not a heat, cool, continuous fan, or circulation demand. If a heat demand occurs when the unit test is running, the test mode will be aborted and the
normal heating sequence will be started. If a cooling, circu- lator, or continuous fan demand occurs when the unit test
is running, the test mode will be completed before the other demand is started.
All safety functions will be operable during the Unit Test function. Any safety device that is triggered during the test mode will terminate the test and the fault will be reported in the Indoor Alerts and the Last Four Faults sections of
the user interface.
When the Unit Test option is activated, the unit will start the indoor blower and energize the Y1 output. After ap- proximately 10 seconds, the indoor blower and Y1 will turn off. Next, the furnace will begin the ignition sequence.
After the flame has been sensed, the 45 second fan on delay will be initiated. After the 45 second period, the indoor blower is energized for 10 seconds in 1st stage heating airflow. The furnace will then transition to 2nd stage gas valve and blower operation for 10 seconds. The furnace will then transition to off, running the normal post-purge and the normal heat off delays.