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

A technician wearing a red and blue plaid shirt with blue overalls is working on a heat pump

If you've installed a heat pump in your home, you've made a wise choice. Not only do heat pumps have both heating and cooling capabilities, but they're also designed to improve energy efficiency. And they're fairly easy to use. All you have to do is adjust the device's thermostat, sit back, and enjoy the hot or cool air.

Generally, your thermostat should display the temperature. However, it might read "AUX heat" or "auxiliary heat." Those new to heat pumps might wonder, "What is AUX heat?" or "Is it bad if auxiliary heat comes on?" In this blog, we'll answer these questions by reviewing the following topics:

By the end, you'll know precisely what AUX heat is and what to do about it.

Need help with your heat pump?

The George Brazil team is up for the job. Since 1955, we've provided first-rate heat pump repair services to homeowners across the Phoenix-Metro area. For 5-star service backed by a 100% satisfaction guarantee, call us at (602) 842-0009 or book below.

How Heat Pumps Work

Before we go over auxiliary heat's meaning, it's important to understand how heat pumps work. Put simply, heat pumps are heating and cooling devices that are installed outside the home. There are two main types: air-source pumps (the most common type) and ground-source pumps. As you can probably guess from the name, air-source pumps transfer air in and out of your home. During the summer, the pump removes hot interior air. During the winter, it brings in hot air from outside.

Meanwhile, ground-source pumps absorb natural heat from the ground, which is transferred in and out of the home. Generally, ground temperatures are more consistent than air temperatures. This means ground-source heat pumps are more energy-efficient. However, they're also more expensive to install, so air-source pumps are popular.

Why Your Heat Pump Switches to AUX Heat

In the summer, heat pumps usually have no problem expelling hot air from your home. However, the pump might struggle to bring in hot air from outside when winter hits. For this reason, most homeowners pair their heat pumps with one of the following backup heat sources:

  • Electric resistance coil heaters: These heaters are installed in your home's ductwork.
  • Gas furnaces: These furnaces are heating appliances that run on natural gas. They're more expensive than electric resistance coil heaters but also more energy-efficient.

Now that we've covered how auxiliary heat pumps work, it's time to answer the question: What is auxiliary heat? And what is the difference between AUX heat vs. Heat? If your thermostat says "AUX heat," your home is using your backup heat source (as opposed to the heat pump).

Here are a few reasons why it might do that.

Freezing Outdoor Temperatures

As mentioned above, your heat pump might struggle to pull in hot air when it's too cold outside. Generally, this problem occurs if it's below 32ºF. To help keep your home warm, the heat pump switches to AUX mode.

Thermostat Settings & Indoor Temperatures

Another reason why your heat pump might be in AUX mode is because of your thermostat settings. If it's cold inside your house, you may have set your thermostat to a higher temperature than you normally do. To help fulfill this demand, the heat pump might need the help of a backup source.

The Heat Pump Is in Defrost Mode

If it's extremely cold outside, your heat pump might have iced over. When this happens, the pump goes into defrost mode. This is essentially like cooling mode, which pulls warm air from your home. As the heat pump defrosts, it uses the backup source to keep your home warm.

Cost of Energy Bills in AUX Heat Mode

While AUX heat mode is usually normal, it's important to ensure your device isn't using it unnecessarily. Because your heat pump and backup heat source work together, AUX mode consumes more energy. This leads to increased energy bills — your costs may be up to 50% higher when the device is using AUX heat.

What to Do if AUX Heat Shouldn’t Be Coming On

Are you worried that your heat pump is using AUX heat when it shouldn't be? To determine if AUX heat mode is abnormal, check for the following signs:

  • Warm air: Generally, AUX heat mode is only used when your interior temperatures are too cold. Thus, it should not be on if your home is already warm. Similarly, your heat pump shouldn't need it if the outdoor temperature is over 40ºF.
  • Defrost mode: Your heat pump should only be in defrost mode if the appliance is covered in snow or ice. To check if it's in defrost mode unnecessarily, just look for the defrost light on the device.
  • High energy bills: Pay close attention to your monthly energy bills. Sudden cost increases may indicate a problem with your AUX heat.

If you suspect your AUX heat mode is on unnecessarily, you should contact an HVAC expert. At George Brazil, we have over 60 years of experience executing heat pump repairs. After evaluating your system, we can identify and resolve the problem, resulting in a higher quality, more energy-efficient heat pump.

Need a Heating Repair Expert? Contact George Brazil Today

Our team is available 24/7 and can execute same-day heat pump repairs. No matter the problem, you can trust us to identify it and devise an expert solution. Contact Arizona’s premier HVAC contractor today by calling us at (602) 842-0009 or booking below.