Auxiliary Heat (AUX Heat)

What is AUX heat?

AUX Heat refers to a heat pump’s back-up heat source, typically electrical resistance. It activates automatically when the demand for heat is highest, providing supplementary heat to keep your home comfortable. Some heat pumps do not use AUX heat at all, and some include it as an optional add-on when installing the system.

AUX heat turns on if the heat pump cannot quickly and efficiently reach or maintain the temperature on your thermostat.

Is it normal for my geothermal heat pump to use AUX heat?

It depends. AUX Heat will likely activate intermittently when the outdoor temperature drops below 15°F, though this depends on how energy-efficient and insulated your home is.

The American Society of Heating, Refrigerating and Air-Conditioning Engineers (ASHRAE) reports that various areas of New York, for example, experience temperatures below 15°F around 1.5–5% of the year — around 130-450 hours total. If AUX Heat turns on during this time, it will do so in short runs and not for the entire duration.

What are some other examples of times when AUX heat will come on?

Significant thermostat adjustments: Your thermostat was set to 60°F while you were on vacation. Now that you’re home, you adjust it back to a more comfortable 70°F. A change in thermostat settings greater than 5°F at a time can trigger AUX Heat.

Sudden weather changes: A cold front arrives, rapidly dropping the temperature outside and pulling more heat from your home. A sudden drop in air temperature returning to your heat pump may trigger AUX Heat.

Human error: Someone left the back door open (again). Now it’s 62°F inside and your thermostat is set to 70°F. Like the previous two, a difference between the air temperature and the thermostat setting greater than 5°F may trigger AUX Heat.

Is auxiliary heat more expensive to use?

AUX Heat is a normal part of heat pump systems and necessary at times, but it uses more electricity than the other heating stages. To maximize savings, it’s important to minimize its usage when possible.

What’s the difference between auxiliary heat and emergency heat?

AUX heat is an additional heat source used to support your heat pump’s operations. Emergency heat doesn’t rely on heat pump functionality at all — it uses a secondary heat source (like electrical resistance) in place of the heat pump.