Damper Failure

Damper Failure is an indication of underventilation. This is determined by comparing the supply temperature to the outside air temperature when the Outside Air damper is commanded 100% open.


If the Outside Air Damper is commanded 100% open and no heating or cooling has been energized for 40 minutes, and the Outside Air and Supply Air temperatures are greater than 10°F apart, an alarm is generated.  This alarm essentially indicates excessive Return AIr when the Return Damper is supposed to be closed


The alarm is reset automatically if the Outside Air Damper is commanded to 100% and the Outside Air and the Supply Air temperatures are within 10°F of each other. The alarm can also be reset by setting the Service Switch to Econ 100%.


Possible causes

Mechanical Failure

Check for proper heating signal to unit.  Using a volt meter, check for voltage between the heating signal (W1 or O terminal or white wire from green jacketed 8 conductor cable) and common (C or B terminal or black wire from green jacketed 8 conductor cable).  Voltage should be 24VAC.  If the unit has two stages of heating, repeat the test between the heating stage two signal (W2 terminal or brown wire from green jacketed 8 conductor cable) and common.  Voltage should be 24VAC.

Other Unit Faults

A Heating Failure Alarm can be triggered by other unit faults.  If any of these other faults are present, it is likely that the Heating Failure Alarm is related to these.

Fan Faults

Any fan fault that might disrupt airflow across the heating coil is likely to cause a Heating Failure Alarm.  These include Drive Communication Fault, Drive Fault, Drive Run Fault and Fan Belt Fault.  See these sections for troubleshooting.

Damper Failure

A damper failure resulting in the Outside Air Damper being stuck in the open position can trigger a heating failure.  On a cold day, too much outside air will result in the unit not being able to heat the supply air temperature above 85°F.  See Damper Failure section for troubleshooting damper failure.

Airflow Issues

Other airflow issues can produce a Heating Failure Alarm.

Airflow blockage:

Airflow blockages can generate a Heating Failure Alarm by reducing the airflow across the coil.  The reduction in airflow can cause the high limit safety to trip on the unit.  Check ductwork downstream of unit for closed fire or smoke dampers.  Verify balancing dampers are set properly.

Air being short circuited back to unit:

Broken ductwork can cause supply air to be dumped directly into the ceiling return plenum.  This will short circuit warm air back to the unit causing the high limit safety to trip.  Check ductwork downstream of unit for breaks or leaks.  If unit has a bypass damper, check damper for proper operation.

Airflow not reaching the intended space:

Occasionally ductwork can be crossed up with another zone.  This might cause the unit to receive return air from a different zone than the supply air is being sent to.  This could generate a heating failure alarm if the return air is too cold and the unit is not able to deliver 85°F supply air.