What Does Aux Heat Mean?

By Michael Anderson, 28 years of experience · November 10, 2026 · 6 min read

What does aux heat mean? It means your heat pump is using supplemental heat because the pump alone is not meeting the thermostat’s demand. Call us for heat pump service if aux stays on and your home is not warming.

Need help now? (469) 369-0748

What Does Aux Heat Mean on a Heat Pump?

“AUX” stands for auxiliary heat: backup heat that helps a heat pump warm your home when the pump cannot keep up with the thermostat’s request by itself. It is usually supplemental electric heat, though some heat pumps use another backup setup.

Seeing the indicator does not automatically mean your system is broken. The thermostat turns auxiliary heat on when it needs extra heating capacity, so the key clues are how long it stays on, whether your rooms warm up, and what the outdoor conditions are like.

Why Aux Heat Turns On During a Heat Pump Cycle

A heat pump moves heat rather than making all of its heat inside the house. When the system needs more output than the pump can provide, auxiliary heat can assist. These common situations help you tell normal backup heat from a possible problem.

When Aux Heat Is Normal—and When It Needs Attention

Use the pattern, not the indicator alone, to decide what to do. A brief period of auxiliary heat with steady indoor warming is different from a system that runs for a long time without bringing the temperature up.

• Likely normal: brief support — AUX appears during a cold stretch, after a substantial thermostat adjustment, or while the outdoor unit defrosts, then turns off as the home warms.
• Check the system: weak airflow — Air from the vents feels weak or several rooms are not receiving much airflow. Inspect the filter and make sure supply vents and return grilles are not blocked.
• Arrange service: heat does not recover — The AUX indicator stays on while the indoor temperature stalls or falls, or the system repeatedly stops and starts. Make a note of the thermostat setting and what the display shows.
• Stop troubleshooting: unusual equipment behavior — The outdoor unit is unusually noisy, visibly damaged, or repeatedly trips a breaker. Do not open equipment panels or keep resetting a breaker; contact an HVAC technician.

Need help now? (469) 369-0748

Seven Safe Checks When Your Thermostat Shows AUX

These owner-level checks can identify a simple airflow or setting issue without taking apart the heat pump. Stop at any step that requires opening the equipment or handling wiring.

1. Read the thermostat display — Check that the system is set to HEAT, not COOL or a special mode. Note the room temperature, set temperature, and whether AUX or EM HEAT appears.
2. Check the temperature difference — Compare the set temperature with the room reading. If you raised the setting sharply, return to a reasonable target and see whether the system begins warming the home without an extended demand.
3. Look at the air filter — Find the filter slot and check whether the filter is visibly loaded with dust. Replace it with the correct size and airflow direction if it is dirty; then check whether airflow improves.
4. Clear supply vents and return grilles — Move furniture, curtains, or other items away from registers and returns. Check for stronger, more even airflow after clearing them.
5. Look at the outdoor unit from outside — Check for leaves or other loose debris around the unit, but do not reach inside or chip ice from the coil. A light frost can occur during operation; heavy ice that remains in place needs professional attention.
6. Notice whether the system is cycling — Listen for whether the heat pump runs steadily or repeatedly starts and stops. Write down when AUX appears and whether the indoor temperature changes; that pattern helps a technician diagnose the issue.
7. Recheck the thermostat after the system runs — Confirm whether the room temperature is moving toward the setting and whether AUX has switched off. If the house stays cold or the display continues to show AUX, schedule heat pump service instead of opening the unit.

What a North Texas Cold Snap Can Mean for Aux Heat

Auxiliary heat may run more during a sharp cold snap because the heat pump has less heat available outdoors. The National Weather Service reports that DFW Airport reached -2°F on February 16, 2021, during the winter storm; that extreme event shows why backup heat can matter when temperatures plunge. See the National Weather Service’s DFW winter storm record for the event details.

That does not mean every AUX display signals a failure, or that the same setting is right for every home. Watch whether the system is warming the rooms and whether the indicator goes away as demand eases. For advice on a different thermostat setting, read what emergency heat means on a thermostat.

Aux Heat and Emergency Heat Are Different Settings

Auxiliary heat usually comes on automatically to assist the heat pump. EM HEAT, or emergency heat, is a separate thermostat mode on many systems; selecting it typically tells the system to rely on backup heat instead of using the outdoor heat pump. Exact operation depends on the equipment and thermostat.

Do not switch to emergency heat just because AUX appears. If the outdoor unit is not operating, the system is not heating, or you are unsure what your thermostat mode controls, get the equipment checked. Our guide to using emergency heat during a Texas ice storm covers that specific situation.

Need help now? (469) 369-0748

When Persistent Aux Heat Calls for Heat Pump Service

Call for service when AUX stays on for an extended period and the home will not warm, the heat pump repeatedly shuts down, or you see persistent ice on the outdoor coil. Share the thermostat readings, how long the indicator has been on, and any changes you noticed in airflow or cycling.

We inspect heating or cooling problems for $29. A heat pump service visit can help identify whether the issue is airflow, controls, the outdoor unit, or another part of the system. Contact us to arrange an inspection.

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