What Is Auxiliary Heat and Why Did My Thermostat Turn It On?

If you have a heat pump, you may occasionally notice “AUX,” “Aux Heat” or “Auxiliary Heat” displayed on your thermostat. This means your HVAC system has turned on a secondary heat source to help your heat pump warm your home.

What Does “Aux Heat” Mean on a Thermostat?

During normal heating operation, a heat pump uses refrigerant to absorb heat from the outdoor air and transfer it into your home. As the outdoor temperature falls, the system generally becomes less effective at extracting enough heat to keep up with your home's increasing heating needs.

When the heat pump can no longer meet the thermostat setting on its own, auxiliary heat using electric resistance heat strips can turn on to make up the difference.

The thermostat can automatically activate this feature when it determines that normal heat pump operation is not heating the home quickly enough.

Why Did My Thermostat Turn on Auxiliary Heat?

Cold outdoor temperatures are one reason auxiliary heat may turn on, but they are not the only one. You may also see the AUX heat indicator when:

• You raise the thermostat several degrees at once
• The heat pump enters a defrost cycle
• The heat pump is running but cannot bring the home up to the thermostat setting

Is It Normal for Auxiliary Heat to Come on in Arlington and Fort Worth?

During cold snaps or Blue Northers in Arlington and Fort Worth, a heat pump may periodically need auxiliary heat to maintain the temperature inside your home. You may also see the AUX indicator temporarily during a defrost cycle or after making a large increase to the thermostat setting.

Because extended periods of extreme cold are relatively uncommon here, auxiliary heat generally should not need to run continuously throughout the winter. If AUX heat starts coming on frequently during mild weather or remains on for unusually long periods, it may be a good idea to call an HVAC technician to check for problems.

Why Does Auxiliary Heat Use More Electricity?

Electric resistance heat strips use electricity differently than a heat pump. Instead of efficiently transferring existing heat into your home, they use energy-intensive electrical resistance to generate heat directly.

Relying heavily on electric auxiliary heat can increase your home's electricity consumption.

When Can Auxiliary Heat Indicate a Heat Pump Problem?

The AUX indicator becomes more concerning when there is no obvious reason your heat pump should need additional help.

Consider having your system inspected if:

• AUX heat remains on for long periods
• Your home struggles to reach the thermostat setting
• Auxiliary heat is running much more frequently than it did in previous winters
• The outdoor heat pump does not appear to be operating
• Your home still is not getting warm with auxiliary heat running



Restricted airflow, thermostat or control problems, outdoor-unit problems and other heat pump issues can all affect how well a heat pump heats your home.

Is Auxiliary Heat the Same as Emergency Heat?

No. Although both rely on a heat pump system’s backup heating source, auxiliary heat and emergency heat serve different purposes.

Auxiliary heat turns on automatically when the heat pump needs help meeting the home's heating demand. The thermostat decides when to use it and returns to normal heat pump operation when the additional heat is no longer needed.

Emergency heat is a setting available on many heat pump thermostats that allows you to manually rely on the backup heating source instead of the heat pump. It is generally intended for situations when the heat pump cannot operate properly, rather than as a way to make your home heat up faster.

What Should You Do If Your Thermostat Suddenly Says “Aux Heat”?

First, consider what was happening when the indicator appeared. If outdoor temperatures are extremely low, you recently raised the thermostat several degrees or the heat pump is defrosting, temporary auxiliary heat operation may be completely normal.

If AUX heat appears frequently when outdoor temperatures are relatively mild, stays on much longer than usual or accompanies a noticeable decline in heating performance, call Tom's Mechanical at (817) 277-4493 for heat pump service.