Why is my auxiliary heat staying on?

A heat pump moves heat into your home. Its auxiliary heat is a second heat source that the controls can bring on when needed. Many systems use electric heating elements inside the indoor unit. A dual-fuel system uses a furnace as its backup heat source.

The AUX message shows that backup heat has been requested. It does not, by itself, mean the heat pump has failed or confirm that every heating component is working correctly.

• Colder weather: the home may need more heat than the heat pump can supply on its own.
• Defrost: backup heat may help maintain indoor warmth while the outdoor coil clears frost.
• Warming the house back up: a large thermostat increase or recovery from a lower scheduled setting can bring on backup heat, depending on the controls.

There is no single outdoor temperature at which every system needs AUX. Equipment capacity, the home’s heat loss and thermostat settings all affect when it operates.

Look at the pattern rather than a fixed number of minutes. AUX during a cold morning warm-up is different from repeated use on mild days with an unchanged thermostat setting.

Arrange a check if the house keeps falling below the set temperature, AUX use has increased noticeably or the outdoor unit stays heavily iced. A new thermostat or a recent settings change is also useful information. Restricted airflow, a heat-pump fault or control settings could be involved; the display alone cannot identify the cause.

1. Check the mode and schedule. Confirm the thermostat is set to Heat. Look for a recent temperature increase or a schedule bringing the house back up from a lower setting.
2. Keep a comfortable, steady setting. Avoid repeatedly raising it to try to speed things up. Don’t disable backup heat or change installer settings to make the AUX message disappear.
3. Check airflow. Replace a dirty accessible filter with the correct size and type. Keep supply vents and return grilles clear.
4. Observe the outdoor unit from a safe distance. Note lasting ice or unusual sounds. A stopped fan can be part of defrost, so one glance is not a diagnosis. Don’t open panels or chip ice off the coil.

If the rooms also feel cold, our guide to a heating system blowing cold air explains what to check next.

Electric backup heat generally uses more electricity to deliver the same amount of heat than a working heat pump. The difference comes from moving heat rather than generating it with resistance elements. Extended electric AUX use can therefore add to heating costs.

The effect depends on how long it runs, the equipment and your electricity rate. A dual-fuel system has a different cost comparison because its backup uses fuel. The goal is to avoid unnecessary backup operation while keeping the home comfortable.

AUX and Emergency Heat can use the same backup equipment, but they are different operating modes. AUX is called automatically. Emergency Heat generally stops heat-pump heating and relies on the backup source.

Don’t switch to Emergency Heat simply because it is cold outside or AUX appears. Follow your system’s instructions or a technician’s advice if the heat pump has stopped working.

If AUX keeps running in mild weather, your home is losing temperature or the pattern has changed without an obvious reason, request a heat-pump check. Share the outdoor conditions, room temperature, set temperature and when you notice the AUX message.

If you also notice a burning odor, follow our burning-smell safety guidance before running another cycle.

Haven serves Fort Mill and surrounding areas. We can check the heating system and its controls to understand why backup heat is being called on.