What does auxiliary heat mean on my thermostat?

​Have you ever noticed the phrase “aux heat” on your thermostat? This heat pump thermostat setting means auxiliary heat is being used.

What does auxiliary heat mean? Your heat pump will defer to “aux heat” on its own when the temperature is too cold outside for the unit to warm your home to the desired temperature. While you don’t want this setting to be on all the time, it doesn’t necessarily mean you need to call an HVAC expert and shell out a bunch of money.

Before we jump into what auxiliary heat means, let’s talk heat pump basics. Understanding your unit will help you recognize when aux heat and emergency heat are necessary and when they may indicate a system malfunction.

What is a heat pump?

A heat pump warms your home by pulling in heat from the outdoors, but it can also cool your home by expelling heat from your living space to the outside. Heat pumps don’t create heat or cold air by themselves. They transfer the warmth or coolness from one place to another.

This type of system uses electricity and refrigerant to create a comfortable temperature within your home. To transfer heat from one place to another, the refrigerant flows back and forth from the indoor unit (your air handler) to the heat pump condenser (your outdoor unit).

In mild climates, heat pumps are an energy-efficient way to heat and cool your home. A backup heating source is needed, however, for moments when the temperature suddenly drops. When those freezing temperatures occur, auxiliary heat makes its seasonal debut.

What is auxiliary heat?

Auxiliary heat is a secondary heating function your thermostat automatically switches to when it’s too cold outside for your heat pump to extract warmth from the air. This type of heating is often triggered when the outdoor temperature drops to near or below-freezing temperatures.

Auxiliary heat will also turn on when your heat pump goes into defrost mode. It does this to prevent blowing cold air into your home while the outdoor condenser is thawing.

How does auxiliary heat work?

Your auxiliary heat turns on when your heat pump needs assistance reaching your set indoor temperature. The heat pump will automatically energize its auxiliary heat strip to help warm your home.

Your thermostat plays a big part in determining when auxiliary heat is activated. When the temperature inside your home drops three or more degrees below your set limit, your thermostat will kickstart the auxiliary heat.

This type of heating is also activated if your heat pump becomes covered in ice or snow. Ice can form when there is a problem with your heat strips, which work to keep your equipment defrosted. Another trigger could be low refrigerant levels or a malfunction in your outdoor motor fans.

An indicator on your thermostat should alert you when your heating has switched to auxiliary heat. This can be in the form of a small aux light or a digital reading that says “aux heat” on your display.

What to do if auxiliary heat turns on?

Here are a few tips and tricks for dealing with auxiliary heat. If you can avoid using this type of heating for your home long-term, your energy bill and wallet will thank you.

• Auxiliary heat should be a temporary setting and automatically turn off once your home warms up. Check back in 30 minutes or so to see if it has turned off on its own.
• Only turn your heat up two degrees at a time. Auxiliary heat can be triggered if your home is three degrees lower than your set temperature.
• Do not turn on emergency heat unless it’s actually an emergency. Give auxiliary heat time to work its magic. Emergency heat requires a lot more energy and is more expensive.
• Make sure your aux heat indicator turns off when the outside temperature reaches 40 degrees or higher. If it remains on, you may have a malfunction in your heat pump and need to call an HVAC service provider.

Auxiliary heat vs. emergency heat

Auxiliary heat will kick on automatically when the outside temperature takes a sudden dip and your heat pump can’t keep up with warming your space. Emergency heat must be manually turned on.

The heat pump is your home’s primary heating source, but when the weather gets too cold, a secondary heating source is needed. Emergency heat is used to give the heat pump a break and avoid system damage (plus, it will warm your home in sub-freezing temperatures).

The emergency heat setting on your heat pump should be used sparingly, typically when the thermostat temperature continues to drop after the heat pump has run for a long time. Weather this cold means your heat pump is rendered useless. There simply isn’t enough heat in the outdoor air for your unit to absorb and carry into your home.

When the emergency heat is turned on, the radiant heat strips in your system are ignited. You’ll stay warm, but emergency heat mode will cause a noticeable spike in your electricity bill. Run this function only when the temperature inside your home takes an extreme drop and turn it off when the temperature is within 3 degrees of your desired setting.

If the temperature outside is 40 degrees or above, but your home won’t stay warm if not in emergency heat mode, call your local HVAC expert. Your system may be frozen or need repair. It’s best to have it checked before temperatures take another dive.

Auxiliary heat will automatically turn off when your home or environment has warmed up enough for your heat pump to function normally. Remember, your emergency heat is not automatic and will require you to physically turn it on and off.

Both auxiliary and emergency heat should be temporary settings. They use a lot of energy and will increase your electricity bill.

Signs your heat pump may need repairs

It’s important to perform regular maintenance checks to ensure your heat pump is working properly, especially as the colder months roll in. There are a few DIY fixes that can save you some money, but most repairs should be handled by a professional heat pump repair technician. 

You can find more information about common heat pump malfunctions and steps to solve these issues here.

Heat pump will not turn on

Make sure your heat pump is receiving power and check that your thermostat is set to heat. If everything is set correctly, your heat pump start capacitor may be broken or there could be an issue with your reversing valve.

Heat pump not heating home

See if your unit is blocked by snow, ice, or fallen shrubbery. Your air filter may also need to be cleaned or changed if it is clogged and unable to efficiently let air through.

If your unit and filter are free of debris, the problem could be low refrigerant levels. For your safety, refilling and recharging the refrigerant should be done by an EPA-certified HVAC professional.

Heat pump is constantly running

A continuously running unit will hike up your electricity bill and shorten the lifespan of your heat pump. Check your thermostat settings and clean your air filter. If that doesn’t solve the problem, your compressor contactor may be broken.

Best thermostat setting in winter

The best temperature to set your thermostat to in the winter is 68 degrees while you’re at home. When you’re away, we recommend lowering your thermostat by seven to 10 degrees.

This fluctuating temperature provides both comfort and cost savings for your electricity bill. According to Energy Star, you can save up to 10% a year in heating costs by lowering your temperature to around 60 degrees during the night or when you’re away.

To maintain your cost savings and keep your aux heat from turning on as you warm up your living space, we recommend increasing your thermostat by two-degree intervals. This will help avoid tripping your thermostat’s failsafe feature and starting your auxiliary heat cycle.

How to better insulate your home

Keep your home at a steady temperature and your heat pump from putting in extra work by properly sealing your living space from the chilly air. Energy.gov has some great recommendations for sealing air leaks to keep you comfortable all year round.

Here are a few of our favorites:

• Caulk and weatherstrip doors and windows that may leak air.
• Ensure where your plumbing, ducting, and electrical wiring comes through the walls, floors, and ceilings is properly sealed.
• Inspect dirty spots in your insulation for air leaks and mold. You can use a low-expansion spray foam or install house flashing where you find issues.
• Replace single-pane windows with more efficient double-pane windows.
• If you have a fireplace, make sure the flue damper is tightly closed when it is not in use.
• Seal air leaks around fireplaces, chimneys, furnaces, and water heaters with fire-resistant materials like sheet metal, sheetrock, or furnace cement caulk.

Stress less about auxiliary heat

It’s likely at some point throughout the winter you’ll see “aux heat” on your thermostat. Now you know there’s no need for immediate panic. In all likelihood, your heating system is working just as it should and needs a little extra power to get your home back to where you’re most comfortable.

Keep an eye on your thermostat year-round for any warning signs of larger issues with your HVAC system. Remember to perform regular checks and maintenance, and everything should continue to run smoothly.

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