Auxiliary heat uses electric resistance heating, which is much less efficient than your heat pump. If you have a heat pump, it is best to increase your heat setting by only two degrees at a time to prevent your auxiliary heat from operating. Therefore, a heating system stuck in AUX heat will run up an energy bill. AUX heat turns on to keep you warm. The reason your thermostat has aux heat is that it indicates when gas or electric power has been triggered to help your heat pump reach a warm enough temperature when the weather outside is extremely cold. The reason your thermostat has aux heat is that it indicates when gas or electric power has been triggered to help your heat pump reach a warm enough temperature when the weather outside is extremely cold. The reason your thermostat has aux heat is that it indicates when gas or electric power has been triggered to help your heat pump reach a warm enough temperature when the weather outside is extremely cold. The reason your thermostat has aux heat is that it indicates when gas or electric power has been triggered to help your heat pump reach a warm enough temperature when the weather outside is extremely cold. Heat pump systems have two units - one outside the home and one inside the home. Emergency Heat is typically triggered when it is 35°F and below outside. The reason your thermostat has aux heat is that it indicates when gas or electric power has been triggered to help your heat pump reach a warm enough temperature when the weather outside is extremely cold. The unit outside your home is a heat pump and the unit inside the home is the auxiliary heating system. During freezing weather, the outdoor heat pump will be too cold to quickly heat your home, which is when the auxiliary heating system would kick on. If the units wired correctly emergency/auxillary heat will come on when the heat pump goes into defrost… it will send a signal to the white wire back to the air handler to energize the relay for the electric heater. The reason your thermostat has aux heat is that it indicates when gas or electric power has been triggered to help your heat pump reach a warm enough temperature when the weather outside is extremely cold. The reason your thermostat has aux heat is that it indicates when gas or electric power has been triggered to help your heat pump reach a warm enough temperature when the weather outside is extremely cold. Once the ice on the heat pump has mostly melted, the heat pump goes back to normal and AUX heat turns off. Types of AUX heat (and why it matters) Heat pumps have 2 forms of AUX heat: Electric resistance (default) Gas furnace; The electric resistance option is like the electric coils you see inside your toaster. The reason your thermostat has aux heat is that it indicates when gas or electric power has been triggered to help your heat pump reach a warm enough temperature when the weather outside is extremely cold. The reason your thermostat has aux heat is that it indicates when gas or electric power has been triggered to help your heat pump reach a warm enough temperature when the weather outside is extremely cold. Emergency Heat, also known as “auxiliary heat”, is the second stage of heat that your thermostat runs on when the temperature is too cold for your heat pump to extract heat from the outside.