digital thermostat with finger

As winter sets in and the weather gets colder, you may notice an AUX heat icon on your thermostat screen. What does it mean? Why has it turned on? Will your utility bill skyrocket if you don’t turn it off?

We’ve answered these questions and more in our guide to what auxiliary heat is and what homeowners should do about it.



What Does AUX Heat Mean?

“AUX heat” refers to the backup heating system within your heat pump. This system is also called electric resistance heating. The thermostat inside your house determines when the auxiliary heat turns on.



What Is Auxiliary Heat?

Your heat pump doesn’t create its own heat like a furnace. Instead, it takes heat from the outside air. Auxiliary heat kicks in when the outside temperature drops below freezing (at 32 degrees Fahrenheit). 

In addition to freezing outdoor temperatures, your heat pump goes into emergency heat if ice freezes on its surface. Ice forms on your heat pump if there’s a problem with its heat strips, which keep the outside defrosted. Additionally, your system may sometimes have low refrigerant levels or a problem with its motor fans. All of these issues can cause the auxiliary heat to turn on.



How a Heat Pump Works

One way to understand auxiliary heat is to understand how your heat pump works. Heat pumps use a split system between an outdoor condensing unit and an indoor air handler unit. Below is a breakdown of how most heat pumps operate: 

  1. Refrigerant, a mixture of chemicals in the outdoor condenser, absorbs the heat energy from the outdoor air. 
  2. The refrigerant pressurizes the air using a device called a compressor, heating it and turning it into a vapor.
  3. The heat moves to the inside air handler unit, and the vaporized refrigerant is transported to the indoor unit coil. 
  4. The cold air from your home is blown over the coil and absorbs the heat. As heat leaves the refrigerant, it turns back into a liquid and is circulated back to the outdoor unit.

Inside the air handler is an electric heating element called the electric resistance heating system. This is a supplemental heating source. Depending on your heat pump model, auxiliary heat and emergency heat are both functions of the electric heating element.

This element keeps your heat pump working because if it freezes, you can experience severe problems such as broken fan blades and refrigerant leaks.



Auxiliary Heat vs. Emergency Heat

You may notice your HVAC system using either auxiliary heat or emergency heat. These modes are slightly different, and it’s essential to know which is which.

During freezing weather (anything below 32 degrees Fahrenheit), the outdoor heat pump will be too cold to heat your home efficiently. This is when the auxiliary heating system will kick on. Auxiliary heating turns on automatically to help heat your home more quickly if the temperature drops suddenly.

It’s normal for auxiliary heat to turn on during freezing weather. However, running auxiliary heat for extended periods of time can take a toll on your system and increase your energy bills. If auxiliary heat is turning on despite efforts to make it stop, your heat pump may be malfunctioning.

Heat pumps also require a supplemental heating source. This can be a gas, oil, or hot water backup system. These supplemental heating sources are known as “second-stage” or “backup” heating. The “first stage” heating is the heat pump itself. Emergency heat is when you use your “second-stage” by itself without using your “first stage.”



When Do I Want Auxiliary Heat On?

Auxiliary heat should only be on when the outside temperature is below freezing. It’s not beneficial to run auxiliary heat in other circumstances because it’s much less efficient than your heat pump and can increase your energy bills. 

You should use the emergency heat mode if the heat pump system can’t warm your home and the auxiliary heat won’t reach your desired temperature. If you experience this problem, your heat pump is malfunctioning and you should contact an HVAC professional for repair as soon as possible. Emergency heat can increase your utility bills significantly.



How To Stop Auxiliary Heat From Coming On

Running auxiliary heat for too long may be too much for your unit. You can take the following steps to alleviate your HVAC system and prevent auxiliary heat from turning on. 

Keep Your Thermostat Set to a Low Temperature

Auxiliary heat often turns on because your home’s temperature is set too high. You’re asking your HVAC system to work too hard to maintain a comfortable temperature. Setting the thermostat somewhere between 62 and 68 degrees Fahrenheit is all you should do to get the problem under control.

Shut Off Unused Rooms

If you have a spare area in your home, such as an unused den or bedroom, you can shut the door and close the vents in that room. This should help warm the indoor temperature of the rooms you actually use.

Make Changes to Your Home

The more cozy and warm you can make your home inside, the better. Open your window shades during the day to let the sun warm rooms, layer up with thick clothes, and have plenty of warm blankets on hand. 

Maintain Your Heat Pump

Routine heat pump maintenance can prevent malfunctions that cause auxiliary heat to turn on. During your routine maintenance, an HVAC pro will inspect your heat pump and complete any required cleaning or adjustments. HVAC maintenance is one of the best ways to reduce your supplemental heat.



Contact a Professional

Whether you need to repair your HVAC to get the auxiliary heat to turn off or request maintenance to ensure it doesn’t kick on this winter, contacting an HVAC professional is the best way to deal with auxiliary heat.

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Frequently Asked Questions About Auxiliary Heat



When to Consider a Broader Home Inspection

When purchasing or selling a home, it is critical to complete a thorough inspection to understand the condition of the property. A standard home inspection includes an assessment of a home’s systems and physical structure. After the process, the inspector will provide a report detailing their findings and recommendations.

If you are thinking about buying a home or putting your home on the market, we strongly recommend finding an ASHI home inspector in your area.