Aux heat on a thermostat refers to the backup heating system within a heat pump, also known as electric resistance heating. It is a secondary heating function that automatically switches on when the temperature outside is too cold for the heat pump to extract warmth from the air. This type of heating is often triggered when the outdoor temperature drops to near or below-freezing temperatures. Auxiliary heat will also turn on when the heat pump goes into defrost mode to prevent blowing cold air into the home while the outdoor condenser is thawing. The term "aux" is short for auxiliary, and it indicates that a secondary heat source, either gas or electric, has been deployed to help the heat pump reach the desired temperature when the weather outside is extremely cold. Aux heat is designed to support the heat pump to become more efficient as opposed to taking over completely, so it doesnt cost as much to run as emergency heat. If the aux heat indicator remains on when the outside temperature reaches 40 degrees or higher, there may be a malfunction in the heat pump, and an HVAC service provider should be called.