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Why Does My Thermostat Say AUX Heat?

When winter hits, most of us rush to the thermostat to activate our heating systems. Normally, your thermostat should simply display the desired temperature. If you have a heat pump, however, it might show the words "AUX Heat." While this is usually nothing to worry about, in some cases, it can indicate a problem with your thermostat. In this article, we'll cover what "AUX heat" means by going over the following topics:

Read on to learn more about heat pumps and discover the answer to the question, "why does my thermostat say "AUX Heat"?

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Make sure your heating system is in top-notch condition before winter hits by scheduling an appointment with MSP Plumbing Heating Air. Whether you need to install a new heat pump or get a same-day repair on your thermostat, you can trust us to get the job done right the first time. Call us today at (651) 228-9200 to set up a diagnostic visit.

How Does a Heat Pump Work?

Before we discuss AUX heat, let's go over how heat pumps work. Heat pumps are heating appliances that are installed outside your home. They may be divided into two types:

  • Air-source heat pumps
  • Ground-source heat pumps

The most commonly used types are air-source heat pumps, which transfer air between the outdoors and indoors. During warm months, the heat pump removes hot air inside your home and expels it outside, consequently cooling the interior. During cold months, the heat pump takes the hot air from outside and transfers it inside your home, making it warmer.

Ground-source heat pumps work similarly to air-source pumps — however, instead of using the outdoor air, they rely on the ground temperature. Because the ground temperature usually remains consistent, ground-source heat pumps are more energy-efficient. However, they come with more expensive installations, which is why most homeowners opt for air-source pumps.

Why Does a Heat Pump Use AUX Mode?

White programmable Emerson brand thermostat showing that it's set to 70 degrees Fahrenheit and AUX on the display screen.

Is “AUX” showing on your thermostat’s display screen?

Heat pumps are designed to heat and cool your home all year round. However, when it's especially cold out (below 32ºF), they may struggle to pull hot air from outside to heat your home. That's why pairing your heat pump with a backup heat source is usually recommended. Most homeowners choose between two sources: electric resistance coil heaters and gas furnaces.

Electric resistance coil heaters are cost-effective heating sources installed directly into your home's ductwork. Gas furnaces, meanwhile, are heating appliances that run on natural gas. They require more expensive installations but help reduce energy costs over time.

If your thermostat is in AUX mode (or "auxiliary mode"), your heat is using one of your backup heating sources. This is usually due to one of the following reasons:

  • It's too cold outside
  • The heat pump has iced over
  • You set your thermostat to a high temperature

In most cases, heat pumps switch to AUX mode because it's too cold for the pump to extract hot air from outside. Sometimes, your heat pump may switch to AUX mode because it has iced over. When ice builds upon the outdoor unit, it switches to cooling mode, which pulls warm air from your home to defrost it.

Another reason your heat pump may be using AUX heat is that you've set the thermostat higher than normal. The heat pump may rely on the help of auxiliary heat to reach the desired temperature. No matter the reason for the AUX mode, it will usually turn off automatically when it's no longer needed.

When AUX Mode Isn’t Normal

A woman with short brunette hair, wearing a dark blue dress with a light blue cardigan, and a blue banded Apple watch is paying her bills with her silver laptop and credit card.

Aux mode could indicate a problem if higher energy bills accompany it.

AUX mode is typically just a normal part of your heat pump's operating system. However, if your heat pump is using AUX heat when it's warm outside or switching to AUX mode frequently, there may be something wrong with your heating system. To diagnose a heating problem, look for the following signs:

  • AUX heat is on when it's warm out
  • Your energy bills are too high
  • Your home is too warm
  • The defrost light is on

The biggest indication that something is wrong is when your heat pump is in AUX mode while it's warm out. Generally, it should not be using AUX heat if it's higher than 40ºF outside. Another sign to look out for is higher energy bills than normal, which means AUX heat is running too often. Also, keep an eye on the temperature inside your home. If it feels too warm and your thermostat is not responding to temperature changes, your AUX heat is probably running.

Finally, check your outdoor unit to see if the defrost light is on. Your heat pump should not be in defrost mode unless it's covered in ice or snow. Otherwise, it's using AUX heat unnecessarily.

If you suspect an issue with your heat pump's AUX heat, don't ignore it. Not only can AUX mode make your home too warm, but it may also increase your energy costs. At MSP, our experienced technicians have worked with a wide range of heat pump makes and models, as well as electric resistance coil heaters and gas furnaces. After an initial evaluation, we can let you know the problem and perform an expert heat pump repair. We can also install a new thermostat if needed.

Schedule a Visit With a Heating Expert Today!

MSP technician wearing a blue company shirt, blue company baseball cap, and khakis, rolling out a blue carpet in a Minnesota home

Are you experiencing problems with your heating system? At MSP Plumbing Heating Air, we have over a century's worth of experience in repairing thermostats and heat pumps. Our commitment to quality, convenient scheduling options, and fast response times has earned us a 5-star reputation in the Twin Cities and surrounding areas. Schedule a visit today by calling us at (651) 228-9200!