What Does AUX HEAT Mean on a Heat-Pump Thermostat?
Last reviewed August 2026
If your thermostat shows AUX HEAT, it usually means your heat-pump system has automatically called for supplemental heat to help meet the home’s heating demand. It does not mean you have an “auxiliary heat pump.” In many residential systems, the supplemental heat is electric resistance heat built into the indoor equipment, but the exact configuration depends on the system and its controls. [1]
AUX HEAT can be normal. The important questions are how often it appears, whether the home stays comfortable, and whether the system is showing any warning signs.
What AUX HEAT Usually Means
Heat pumps move heat rather than creating it in the same way as electric resistance heat. When a system needs additional heating capacity, it may bring on supplemental heat automatically. DOE explains that electric resistance elements are typically added to make up for heating-capacity deficiencies and that their use should be kept to a minimum because they are less efficient. [1]
| Thermostat message or situation | General homeowner meaning | What to do |
|---|---|---|
| AUX HEAT appears briefly during a high heating demand | The system may be using supplemental heat to help maintain comfort. | Observe normal operation; refer to the thermostat/system manual for the exact behavior. |
| AUX HEAT appears after a large temperature change | Some controls may call for additional heat to help the system recover. | Avoid changing system configuration yourself. If it happens frequently, discuss settings and system performance with a qualified HVAC professional. |
| AUX HEAT appears during a system-controlled event, such as defrost, on some equipment | The exact behavior varies by equipment and control strategy. | Consult the exact thermostat and heat-pump manual; do not alter wiring, lockouts, or heating elements. |
| AUX HEAT runs unusually often or for a long time | The system may need professional assessment of setup, capacity, controls, airflow, or equipment condition. | Arrange qualified HVAC service. |
The operating details are system-specific. For example, Trane describes AUX HEAT as supplemental indoor heating stages and says its configuration requires an HVAC technician. That is a useful illustration of why homeowners should use their own system manual rather than copy a setting from a general article. [2]
AUX HEAT Is Not Automatically the Same as EMERGENCY HEAT
The labels AUX HEAT and EMERGENCY HEAT are related but should not be treated as interchangeable. AUX HEAT is generally an automatic supplemental stage. Emergency heat is a system-specific backup mode that may bypass normal compressor heating in certain configurations.
The exact label, icon, trigger temperature, and behavior depend on the thermostat and equipment. A manufacturer’s documentation may use different terminology or configuration rules. Do not change settings, lockouts, wiring, or heat stages based on a generic web explanation.
Use the exact thermostat and heat-pump manuals as the final authority. If the display is unfamiliar or the system does not perform normally, contact a qualified HVAC professional rather than attempting electrical or configuration work yourself.
Why AUX HEAT May Cost More to Run
When supplemental heat is electric resistance heat, it can use more electricity than heat-pump-only operation. DOE advises keeping electric-resistance use to a minimum and notes that heat-pump-specific thermostats can help avoid unnecessary activation through gradual temperature recovery. [1]
That does not mean a homeowner should disable AUX HEAT or make large, sudden thermostat changes. The system may use supplemental heat as part of its intended comfort and protection strategy. Do not rely on a universal cost multiplier, because operating cost depends on the equipment, climate, electricity rate, home, controls, and runtime.
What Homeowners Should—and Should Not—Do
Safe homeowner steps
- Note when AUX HEAT appears, how long it stays on, outdoor conditions, and whether the home reaches the set temperature.
- Check that the thermostat is set to the intended mode and temperature.
- Follow the manual’s instructions for normal, user-accessible maintenance, such as filter replacement where applicable.
- Keep a record of breaker trips, error messages, unusual noises, or unfamiliar odors for the service visit.
- Consult the exact thermostat/system manual before changing ordinary user-facing settings.
Do not do these things
- Do not inspect wiring, fuses, heating elements, relays, panels, fuel systems, or refrigerant components.
- Do not bypass controls or modify thermostat configuration, staging, or lockout settings.
- Do not force emergency heat because AUX HEAT appears, unless the exact manual and a qualified professional instruct you to do so.
- Do not assume another brand’s thermostat instructions apply to your system.
When to Call a Qualified HVAC Professional
Contact a qualified HVAC professional if:
- AUX HEAT runs unusually often or for long periods.
- The home will not warm to the set temperature.
- A breaker trips, an error message appears, or you smell something unusual.
- You hear unusual noises or the outdoor/indoor equipment seems not to operate normally.
- You are unsure whether the display says AUX HEAT, EMERGENCY HEAT, or another alert.
- You want to change lockouts, auxiliary-heat settings, system sizing, or thermostat configuration.
A professional can evaluate the exact equipment, thermostat settings, load, airflow, controls, and installation rather than guessing from a thermostat label.
A Practical Next Step
Before arranging service, write down the thermostat make/model, heat-pump model if available, the settings in use, approximate outdoor conditions, AUX HEAT timing, and any error messages. This information can help the service provider evaluate the system without asking you to access hazardous components.
Sources
[1] DOE FEMP — Purchasing Energy-Efficient Residential Air-Source Heat Pumps