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Why Does My Thermostat Say “Aux Heat”?

Turning your thermostat to “heat” came with an unexpected “AUX” function appearing. What does it mean?

The “AUX Heat” mode is usually an indicator that it’s freezing outside, and your is heat pump is using a secondary backup heat source – your auxiliary heat—to keep your home at your set temperature.

If the “AUX” signal shows all the time when it’s not freezing outside, you need to contact a professional HVAC technician to fix your heat pump.

We’ll explain how a heat pump works and why it needs that backup heat source.

How your heat pump works

Heat pumps are not furnaces. They are air conditioners that can work in reverse to heat your home during winter. How? By moving heat from outside air into your home. (Yes, even in cold Minnesota winters there’s heat outdoors that can be “pumped” into your home.)

Here’s an entertaining instructional video that breaks down how a heat pump works.

When is AUX heat needed?

While, heat pumps work just fine in most temperatures, it struggles when outside temperatures drop below 40° F—which is most of the time during Minnesota winters

When temperatures go below 40° F, heat pumps need to use AUX heat. Why? There are 2 reasons that can coincide together – and often do—in Minnesota:

1) When the heat pump can’t draw enough heat. On super chilly nights in Minnesota, the heat pump switches to AUX heat because there’s not enough heat outside to keep your home at the desired temperature.

2) When the heat pump ices over. When it gets to freezing temperatures, 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. But, cool air is now being pumped into your home through your vents. Nobody wants air conditioning during freezing temps!

AUX heat turns on to keep you warm. 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:

  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 Minnesota 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.

Contact a Minnesota tech when AUX heat malfunctions

If your thermostat stay AUX heat mode even when outside temperatures are not freezing, there’s a problem. Call a local heat pump repair company ASAP.