Auxiliary heat may be normal in cold weather, defrost or large temperature recovery. It is suspicious when it remains active in mild weather after setpoint is reached. Record the outdoor temperature and whether the outdoor unit is running. Raise the setpoint in small steps and see whether the auxiliary heat drops out.
What this covers
- Residential heat pumps with thermostat-displayed AUX or EMERGENCY HEAT
What it does not cover
Differential
Likely causes of “Heat pump auxiliary or emergency heat stays on”, and how to tell them apart
- 1
It is cold outside and the heat pump needs help
What separates it Auxiliary heat is on at low outdoor temperatures, the outdoor unit runs and the house is slowly heating.
How to confirm Record the outdoor temperature and whether the outdoor unit is running.
- 2
A large setpoint jump makes the system call for extra heat
What separates it Auxiliary heat runs after you raise the temperature by several degrees at once, and stops when the house recovers.
How to confirm Raise the setpoint in small steps and see whether the auxiliary heat drops out.
- 3
The thermostat is set to Emergency Heat
What separates it The display shows Emergency or Aux Heat, and the outdoor unit is not running by design.
How to confirm Return to the Heat mode and see whether the outdoor unit starts.
- 4
The outdoor unit is not working or is iced up
What separates it The outdoor unit is silent or covered in ice, and auxiliary heat runs continuously in mild weather.
How to confirm Look at the outdoor unit from a distance; a silent or iced-over unit needs service.
Before you touch a setting
Record this first — changing settings destroys the evidence
- The outdoor temperature, the thermostat mode and the recent setpoint changes.
- Whether the outdoor unit is running and its condition.
- How long the auxiliary heat stays on after the setpoint is reached.
Try in this order
0 of 4 completedCapture the exact state
ReversibleRecord outdoor temperature, thermostat mode, setpoint change and outdoor-unit operation.
Separate the likely fault domains
ReversibleReturn to Heat rather than Emergency Heat, reduce large setpoint jumps and inspect exterior airflow.
Correct only the proven, accessible condition
ReversibleCorrect mode or schedule; arrange service for a non-running outdoor unit or persistent auxiliary heat.
Verify once before going farther
ReversibleHold a steady setpoint and confirm AUX drops out after recovery.
Stop boundary
When this stops being a self-service repair
- !Arrange service for a non-running outdoor unit or auxiliary heat that stays on in mild weather, because electric resistance heat is expensive to run.
Evidence ledger
Sources behind this answer
Exact questions
Common questions
Is auxiliary heat the same as emergency heat?+
Auxiliary heat runs alongside the heat pump when needed. Emergency heat runs the backup heat on its own and is used when the outdoor unit is not working.
Why is my electric bill high with auxiliary heat?+
Electric resistance heat costs several times as much as heat from the heat pump. Small setpoint steps and a working outdoor unit keep it running less.
What is auxiliary heat?+
Backup heat, often electric, that supports the heat pump when it is very cold or defrosting.
Is it normal for it to be on?+
In very cold weather, yes. Constant use in mild weather points to a fault or setting.