Auxiliary Heat on a Heat Pump
Auxiliary heat is supplemental heat that the HVAC system can turn on automatically when the heat pump alone is not meeting the current heating requirement or during certain operating events such as defrost.
In many all-electric systems, auxiliary heat is electric resistance heat.
In dual-fuel systems, the second heat source may be a furnace depending on the control design.
Source: Carrier - What Is Auxiliary Heat?
Safety: Use only homeowner controls described in the equipment manual; leave installer settings, wiring, and backup-heat components to a qualified technician. Do not bypass safeties. For smoke or suspected fire, leave the building, stay outside, and call 911 from a safe location. For burning smells or repeated breaker trips without signs of fire, stop troubleshooting; switch the thermostat Off only if safe and arrange service.
When Does AUX Heat Turn On?
Common reasons include:
- very cold outdoor temperature
- house load exceeds heat-pump output
- defrost cycle
- large thermostat recovery
- control strategy
- heat-pump fault
The thermostat/control decides when auxiliary heat is needed.
Is Auxiliary Heat Normal?
Yes, sometimes.
Brief or strategic use is not proof of a problem.
Investigate when auxiliary heat:
- runs nearly constantly in moderate weather
- appears far more often than before
- accompanies poor comfort
- causes a sharp kWh increase
- runs because the heat pump itself is not operating
Why Is Electric AUX Heat Expensive?
Electric resistance heat has roughly one unit of heat delivered per unit of electrical energy at the point of use.
A heat pump operating at COP 2 or 3 can deliver two or three units of heat per unit of electricity.
Therefore unnecessary resistance-heat runtime can materially increase electricity use.
AUX Heat During Defrost
During defrost, the heat pump temporarily changes operation to clear frost from the outdoor coil.
Some systems use auxiliary heat to prevent uncomfortable indoor air during the event.
That can be normal.
AUX Heat vs. Emergency Heat
They are not the same user behavior.
Auxiliary heat is normally controlled automatically.
Emergency heat is usually a manually selected thermostat mode that tells the system to rely on the backup heat source rather than normal heat-pump operation.
Why Is AUX Heat Always On?
Possible causes:
- extreme weather
- large thermostat setback/recovery
- heat pump undersized
- heat pump lost capacity
- outdoor unit not running
- control/sensor problem
- bad staging settings
- excessive building heat loss
Leave installer/service control parameters to a qualified technician; use only homeowner controls described in the equipment manual.
How Do You Reduce Unnecessary AUX Heat?
Safe homeowner steps:
- use steady thermostat settings
- avoid large rapid setpoint increases
- keep filter clean
- maintain clear airflow
- fix known building-envelope problems
If AUX heat remains excessive, the control/system design needs professional review.
Does a Cold-Climate Heat Pump Need AUX Heat?
It may still have backup heat, but a strong cold-climate system can cover a larger share of the heating load at low outdoor temperatures.
The right design uses actual low-temperature capacity and Manual J load.
Get Excessive AUX Heat Checked
If auxiliary heat is running constantly without an obvious extreme-weather explanation, have the system and controls diagnosed.
Understand repair costs and next steps
Review diagnosis and repair cost factors, then prepare useful observations for a qualified technician.