Why Does My Thermostat Say Aux Heat When I Turn On My Heat Pump

Sometimes when you turn your thermostat to “heat” the “AUX” function switches on. Why?

Answer: Normally this is fine. When it’s unusually cold outside, your heat pump needs to use a backup heat source—your auxiliary heat—to keep your home at your thermostat’s set temperature. But if AUX heat stays on all the time, you have a problem.

We’ll explain this as well as show you why your heat pump needs auxiliary heat, but first you need to understand how a heat pump works…

A heat pump doesn’t create heat like a furnace. Instead, it moves heat from the outside air into your home. Basically, it is an air conditioner that works in reverse.

This somewhat cheesy video does a good job of explaining the process in more detail.

99% of the time during our Phoenix winters, this process works just fine. But then, there are always exceptions…

At very low temperatures (usually below 32° F) heat pumps may need to rely on auxiliary heat. This happens for 1 or 2 reasons (or often both):

The heat pump can’t get enough heat
In the Phoenix area, this is very uncommon. But we do have the occasional winter night with a low in the 20s. On these nights, the heat pump switches to auxiliary heat because there just isn’t enough heat to make running the heat pump worth it.

There’s ice on the heat pump
While also rare, ice is the more common reason for a Phoenix heat pump to use AUX heat. When outside temperatures are around or below freezing, moisture from the air collects and freezes on your outdoor unit.

For more on why ice builds up, check out our article, Why is my heat pump covered in ice?

To melt the ice, the heat pump switches to defrost, which temporarily puts the heat pump into air conditioning mode. Defrost pulls heat from your home to melt the ice. But since your heat pump is now in cooling mode, cool air would be coming out of your vents.

No one wants that!

To prevent this, the auxiliary heat turns on and heats the air coming out of the vents. Once the ice has been sufficiently melted, your heat pump will resume normal operation and your auxiliary heat will turn off.

The end of the video we mentioned above also does a great job detailing how this works.

If your thermostat stays in auxiliary heat even when temperatures outside rise, you should call a heating repair company. Auxiliary heat uses electric resistance heating, which is much less efficient than your heat pump. Therefore, a heating system stuck in AUX heat will run up a monstrous energy bill.

George Brazil has been keeping Phoenix-area homes safe, comfortable and energy efficient since 1955. Contact us online for more information about any of our services.