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Why Does My Heat Pump’s AUX Heat Keep Coming On?

If you’ve been running your heat pump in heat mode and you’ve noticed that your thermostat is reading “AUX heat,” you’re probably wondering:

  • What does that mean?
  • Why is the AUX heat turning on?

AUX heat refers to your heat pump’s “backup” electric resistance heating. This heat setting should only kick on when the heat pump can’t produce enough heat to warm your home or if your heat pump goes into defrost mode.

We know you have questions, so below we’ll go into more detail about:

First, let’s take a closer look at how your heat pump works.

Need a professional’s help to diagnose your heat pump? Just contact us and we’ll send one of our Phoenix techs right over.

1. How does a heat pump work?

Generally, heat pumps work by transferring outside heat into your home via a process called heat transfer.

Your heat pump absorbs heat from the outdoor air via refrigerant lines and carries that heat inside where it’s used to heat your home’s air.

Process of heat traveling through a heat pump

Heat pump process

Heat transfer is the most efficient (and cheapest) type of heating, and it’s your heat pump’s main solution for heating your home.

However, when your heat pump is unable to meet demand via heat transfer, it will tap into its backup heating, or “AUX” heat. For some homeowners, the “AUX” heating source will be electrical resistance heating, which consists of large wire heating elements that are heated by electricity.

Note: In some cases, your heat pump’s AUX heat could be a gas furnace, but since Phoenix typically has mild winters, some AZ homeowners will have electric resistance as their AUX heat.

The important thing to remember is that “AUX” heat is fast and dependable but it’s also more expensive than heat transfer heating. In fact, electric resistance heating is about 50% more expensive than heat transfer.

Below, we’ll look at the situations in which your heat pump is unable to heat your home via heat transfer (and may need to use AUX heat).

2. It’s normal for your heat pump to switch to AUX heat when…

...The temperature outside is below 35°

Usually, once outdoor temperatures fall below 35°, a heat pump will struggle to keep your home warm and will automatically switch to AUX heat.

As we mentioned above, heat pumps absorb heat from the outdoor air and then move that heat indoors. And when outdoor temperatures drop too low, there’s simply not enough heat in the outdoor air to keep your home warm.

So before you call us at (602) 842-0009, check the outdoor temperatures. If it’s an unusually cold Arizona day, it’s normal to see AUX heat on your thermostat.

...The thermostat is calling for a 3° or higher temperature rise

Most heat pumps are designed to automatically switch to AUX heat when the indoor temperature is 3 degrees colder than the thermostat setting.

For example, let’s say that you wake up to a cold home (62° F) and immediately raise the thermostat to 68°. Because the temperature rise is more than 3 degrees (it’s 6 degrees in this scenario), the system will automatically turn on the AUX heat to help your home meet the desired temperature faster.

So first, check to see if your thermostat only says AUX when you raise the thermostat 3 degrees or higher than the current temperature of your home. If so, that’s normal.

Can I change the AUX trigger point?

As we mentioned above, most heat pump systems are designed to switch to AUX heat when the indoor temperature is 3 degrees colder than the thermostat setting.

This 3-degree “trigger point” is programmed into the thermostat by the manufacturer. If for some reason you wanted your heat pump to switch to AUX heat when there was a 1-degree difference or a 5-degree difference, you’d need to hire a professional to change the temperature differential on the heat pump board of the condenser.

HOWEVER, as we also mentioned above, electrical resistance heating (AUX heat) is more expensive than heat transfer. That said, even though electrical resistance heating warms your home quickly, we would caution you against triggering your system’s AUX heat unless absolutely necessary.

...The heat pump is in “defrost mode”

If ice starts to build on your heat pump’s outdoor unit, your heat pump will undergo a “defrost cycle” to melt it. During this time, the system will rely on AUX heat to keep your home warm.

Although this situation is rare in Arizona, it’s common in colder parts of the country. When outdoor temperatures reach freezing temperatures, the outdoor coils can begin to freeze over with ice. When that happens, the heat pump will reverse the flow of refrigerant so that hot refrigerant is sent to the outdoor unit (to melt the ice).

And, because a heat pump stops heating your home during this process, some models will automatically switch to AUX mode until the defrost cycle is over.

Signs that your heat pump is in defrost mode include:

  • Steam and/or water coming from the outdoor unit
  • The fan in the outdoor unit has stopped running
  • A blinking light on the unit to indicate it’s in defrost mode (only on some heat pump models)

3. What to do if your heat pump in AUX mode when it shouldn’t be?

If you are not experiencing one of the 3 situations listed above but your heat pump is running on AUX heat, your heat pump likely needs a professional repair.

As we mentioned above, AUX heat usually costs up to 50% more than heat transfer, which means if your system is running on AUX heat when it shouldn’t be, your heating bill is likely going to be a lot higher than you’d like it to be.

Our suggestion would be to reach out to a professional as soon as possible. A skilled tech will be able to determine what the issue is and fix it before you continue to spend money on high energy bills.

Pssttt...if you live in Phoenix or surrounding areas, we can help you determine what’s wrong with your heat pump and fix it.