Why Does My Heat Pump Use Auxiliary Heat?
If your thermostat says AUX HEAT, the heat pump is calling in a backup or supplemental heat source to help the primary system keep up. That doesn't automatically mean the heat pump has failed.
A brief display during cold weather, a big temperature recovery, or a defrost cycle can all be normal. AUX that stays on in mild weather, arrives with weak heat, or shows up while the outdoor unit isn't working is worth a closer look from us.
Cold weather or defrost
The backup stage can help once outdoor conditions cut into capacity, or while the outdoor coil clears frost.
A big temperature change
A large setback, or a return after time away, can ask the system to raise the home several degrees fast.
Long or unusual runtime
Persistent AUX, weak airflow, poor comfort, or an idle outdoor unit is a reason to call us for a diagnosis.
Three normal reasons AUX can turn on
The backup stage should be temporary when the heat pump is running well. Duration, frequency, and comfort tell you more than the label alone.
Very cold outdoor conditions
A heat pump moves available heat from outdoor air into the home. As it gets colder outside, below freezing especially, the equipment and the home may need supplemental heat to close the gap.
A large thermostat recovery
If the house sits several degrees below the new setting after a deep setback, a power interruption, or time away, the controls may bring on AUX to recover faster.
The outdoor coil is defrosting
In cold, damp weather, frost can build up on the outdoor coil. The heat pump briefly switches modes to clear it, and backup heat keeps the indoor vents from blowing cold air during that window.
What the backup stage is doing
What does AUX heat mean on a thermostat?
AUX is short for auxiliary heat. It's backup heat - a second stage that supplements the heat pump when the primary system can't close the gap between the current indoor temperature and the thermostat setting fast enough.
Many air-source systems use electric resistance heat strips inside the air handler. A dual-fuel system may use a gas or oil furnace as the backup source instead. The exact control logic depends on your equipment, thermostat, settings, and home, so we can't give you one outdoor-temperature number that applies to every system.
- The heat pump remains the primary heating source in normal operation.
- AUX may start automatically based on outdoor temperature, indoor temperature difference, or recovery time.
- The thermostat message alone can't tell you whether a component has failed.
Does auxiliary heat use more electricity?
Usually, yes, when the backup source is electric resistance heat. The heat pump moves available heat, while resistance strips turn electricity directly into heat. A short assist during a 5 to 15 minute defrost is a different animal than resistance heat carrying the home for hours.
The thermostat message can't predict a dollar amount. The effect depends on the backup source, equipment capacity, outdoor weather, thermostat settings, your utility rate, and runtime.
- Keep the filter in good condition and keep supply vents and returns open.
- Avoid large temperature jumps unless the system is configured for rapid recovery.
- Keep leaves and snow out of the outdoor unit airflow path, but never cover the unit.
- Don't change advanced lockout or balance-point settings without equipment-specific guidance.
Is AUX heat the same as Emergency Heat?
No. AUX heat is automatic supplemental operation. Emergency Heat is a manual thermostat setting that tells the system to rely on the backup source because the heat pump needs service or isn't working correctly.
In Emergency Heat, the outdoor compressor is normally taken out of the heating job entirely, and the backup source becomes the main source. That can keep the house warm for a while, but it's not the setting to reach for just because AUX appeared for a few minutes.
- Use the equipment instructions if the outdoor unit has stopped or is heavily iced.
- Call us for service when the indoor temperature is falling or the heat pump can't maintain comfort.
- Don't keep switching modes to see whether the problem disappears; that can hide symptoms we'd need to see.
What you can check without opening the system
Stop at ice, water, unusual noise, a safety warning, or anything that needs a panel removed. Watching and noting what you see is useful. Guessing inside the cabinet can create a second problem.
- 01
Read the thermostat
Note the indoor temperature, setpoint, whether AUX is steady or intermittent, and which mode the thermostat is using.
- 02
Check the filter
Replace or clean a visibly dirty filter according to the filter and equipment instructions.
- 03
Open the air path
Make sure supply vents and return grilles aren't blocked by furniture, rugs, boxes, or storage.
- 04
Observe the outdoor unit
From a safe distance, note whether the fan runs, whether the unit is iced, and whether leaves are blocking airflow.
- 05
Record the pattern
Write down the weather, rooms affected, filter condition, and whether the home is gaining temperature.
Let the pattern decide the next step.
One brief AUX event during a cold snap or defrost cycle may be normal. Persistent AUX paired with poor comfort is a different story. We test the system instead of guessing at a failed part.
Explore heat-pump repair- AUX stays on through long stretches or shows up in mild weather.
- The outdoor unit doesn't start, is heavily iced, or keeps entering defrost.
- The home cannot gain temperature or the vents put out weak or cool air.
- The message returns after a filter change or a thermostat adjustment.
Note when the message started, outdoor conditions, rooms affected, filter condition, thermostat setting, and anything you see, hear, or smell. A photo of an error code or visible ice helps us too.
Questions homeowners ask next
Should AUX turn on every time it is cold?
It can, when the heat pump needs a hand, but the expected pattern depends on the equipment and controls. If it runs constantly or looks unusual, we should check it.
Can a dirty filter cause AUX to run?
Restricted airflow can cut delivered heat and hurt performance. Check the filter first, then call us to diagnose the full pattern if AUX keeps going.
When should I call for heat-pump service?
Call us when AUX stays on for long stretches, shows up in mild weather, the outdoor unit never starts, the home cannot gain temperature, or the same message returns after a filter change.
Can I leave Emergency Heat on?
Emergency Heat is a temporary backup mode, not a substitute for finding out why the heat pump isn't doing its job. Follow the equipment instructions and call us for service.