What Does Auxiliary Heat Mean on My Thermostat?

December 11, 2017

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First off, this means you have a heat pump (more on that in a moment). Secondly, it means the outdoor temperature is too cold, so your heat pump is using “backup” heat to heat your home.

But don’t worry—it’s usually normal. 

We’ll explain what “backup” heating means, why and when your heat pump switches to it, and how to tell if it’s normal or not...

Why your heat pump switches to aux heat

Your heat pump switches to AUX heat because it can no longer pull in heat from the outdoor air to warm your home.

But to help you understand this switch better, let’s take a quick look at how a heat pump works...

A brief explanation of how your heat pump works

During the winter, your heat pump is basically an air conditioner in reverse. That means it cools air in the summer, but it can also heat air in the winter—to a certain degree.

You see, a heat pump heats your home’s air by extracting heat from outdoor air and bringing that heat inside. Because your heat pump is moving heat (that already exists) instead of creating it, it’s a highly efficient form of heating.

“But how does it heat air if it’s cold outside?”

First off, sometimes the outside air just feels cold, but in reality, there’s still plenty of heat to extract from it. On the other hand, your heat pump does have its limitations. When it’s too cold outside, your heat pump will struggle to heat your home because there’s just not enough heat in the outdoor air.

So how cold is “too cold”? Well, it depends on the region you live in and the unit you are using. Usually once temperatures drop below 35° F, there’s just not enough heat in the outdoor air to heat your home. However, many multi-stage units can run down to 0° F. For reliable heat in cold temperatures, you can pair your multi-stage heat pump with a high efficiency gas-fired furnace.

Related: Do Heat Pumps Work in Cold Climates?

How does AUX heating differ from heat pump heating?

Well, the kind of aux heating you have depends on the kind of heating system you have. For example, most heat pumps have 1 of these 2 types of of backup heating:

  • Electric resistance heating (electric heat strips installed in your ductwork)
  • Gas furnace (called a dual-fuel system)

1 thing to keep an eye out for:

EM heat is NOT the same thing as AUX heat: AUX heat signifies that your heat pump has automatically switched over to backup heating because temperatures are too low. EM heat is an option on your thermostat that you can manually turn on to activate backup heating. You shouldn’t ever use EM heat unless there’s truly an emergency—like your heat pump isn’t working— because backup heating increases energy bills.

When auxiliary heat isn’t normal

Does your aux heat keep coming on, even when it’s above 35° outside? 

That’s a problem.

It could mean one of your thermostats may be stuck or faulty, which is causing your system to be stuck in auxiliary heat mode non-stop.

To avoid a large energy bill (because aux heat is more expensive than heat pump heating), you’ll want an HVAC professional to check your thermostat and fix it ASAP. 

Need help from a Denver heat pump professional?

Give us a call or contact us online. We’ll send over one of our trusted techs to see what’s causing the problem and fix the issue.

Check out our heating repair page to learn what to expect when you schedule a heat pump repair through us.

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