Thermostat reads Auxiliary Heat

Those first few cold days of the year always take our heat pumps by surprise. What does it mean when your thermostat reads auxiliary heat? That “AUX Heat” mode is usually an indicator that it’s freezing outside. Your heat pump is using a secondary back-up heat source to keep your home at your set temperature. If, however, your thermostat reads auxiliary heat when it is not cold outside, be sure to call one of our professional HVAC technicians here at LHT Cooling, Heating & Refrigeration, Inc.

How your heat pump works: 

Heat pumps are not furnaces. They are air conditioners that can work in reverse to heat your home during the winter by moving heat from outside air into your home. Yes, even when it’s freezing outside, there is heat outdoors that can be “pumped” into your home.

When is AUX Heat needed?

While heat pumps work just fine in most temperatures, they do struggle when outside temperatures drop below 40 degrees. The following 2 reasons can coincide together.

1. When the heat pump can’t draw enough heat, it switches to AUX Heat because there is not enough heat outside to keep your home at the desired temperature.
2. When the temperature gets to freezing, ice can build up on your outdoor unit. The heat pump defrosts by going from “heat” mode to “air conditioning” mode. In other words, it uses the warm air in your home to melt the ice. Unfortunately, cool air is now being pumped into your home through the vents. And nobody wants cool air blowing during freezing temps. AUX Heat turns on to keep you warm. Once the ice on the heat pump is mostly melted, it will go back to normal and turn off AUX Heat.

Types of AUX heat (and why it matters)

Heat pumps have 2 forms of AUX heat:

1. Electric resistance (default)
2. Gas furnace

The electric resistance option is like the electric coils you see inside your toaster. Running this form of AUX heat for too long can get pricy, so many homeowners opt for the second form of AUX heating: a gas furnace.

Note: A heat pump combined with a gas furnace is also called a “duel fuel” or “hybrid heat” system.