Seeing “Aux Heat” on Your Thermostat? Here’s What It Means (and When to Worry)

It’s a typical winter day in Sacramento. You’re just getting ready for work when you notice that your thermostat says “aux heat.”

While it’s normal to see “aux heat” on your thermostat, it may also be a sign your system is working harder than necessary. To figure out if this setting indicates a problem, let’s look at what it means and when you should call for help.

“Aux Heat” in Plain English: Backup Heat for a Heat Pump

A heat pump transfers heat from one location to another. In comparison, furnaces actually create heat.

While modern heat pumps are incredibly efficient and can work in extremely cold temperatures, there may be times when the heat pump can’t keep up. Depending on the design of your heating, ventilation, and air conditioning (HVAC) system, you may have heat strips or a gas furnace installed for backup heat.

“Aux Heat” vs. “Emergency Heat”

Auxiliary heat is functionally different from emergency heat.

• Aux Heat: This heat automatically kicks on to help your heat pump reach and maintain a certain temperature. You’ll normally see it kick on during extremely cold temperatures or during times of heavy demand.
• Emergency Heat: Meanwhile, emergency heat must be manually selected by the user. It’s typically used if you have to shut down your heat pump or if the heat pump isn’t working. Because it costs more to run than the heat pump, its sole purpose is to help you get by until the heat pump is operational again.

Why Your Thermostat Shows “Aux Heat”

There are a few common reasons why your thermostat may show that it is using auxiliary heat.

It’s Cold Outside, and the Heat Pump Needs Help

When it’s extremely cold, auxiliary heat may need to switch on to support your heat pump’s effectiveness and your home’s comfort.

The Thermostat Is Trying To Recover After a Big Temperature Change

If you use big setbacks or need to warm up your home significantly in the morning, your heat pump may require “aux heat” mode to catch up.

Your Heat Pump Is in Defrost Mode

During defrost, the heat pump switches direction in order to warm up the outdoor coils and melt any ice. As a result, your heat pump must use a backup heat source to maintain your home’s temperature.

Your System Is Undersized, or Your Home Loses Heat Fast

If you have an undersized system, significant drafts, or insulation issues, your heating system may rely on auxiliary heat more frequently.

When “Aux Heat” Is Totally Normal (And Nothing To Panic About)

On the coldest mornings of the season, it’s normal to rely on auxiliary heat. Additionally, this mode is used for short periods of time when outdoor temperatures suddenly drop and during the defrost mode.

When “Aux Heat” Is a Red Flag (And Can Spike Your Bills)

You need to schedule professional heat pump repairs if you notice “aux heat” kicks on in the following situations.

• “Aux heat” is on constantly, even in warmer temperatures.
• Your home still won’t warm up.
• “Aux heat” kicks on after minor thermostat changes.
• It seems like your thermostat is stuck in “aux heat.”
• Your vents are producing lukewarm air.
• The electricity bill has spiked without any explainable reason.
• Your HVAC system seems like it is running nonstop.

The Comfort-Cost Connection: Why “Aux Heat” Can Get Expensive

While it’s important to stay warm, backup heat uses more energy than your heat pump’s normal operations. If your backup heat is on more, you’ll end up spending more on your energy costs. It’s essential to perform necessary repairs and schedule routine heat pump maintenance so that your system doesn’t have to rely on backup heat sources.

How To Reduce Unnecessary “Aux Heat” Without Freezing

When outdoor temperatures are freezing, you can’t afford to go without heat. Fortunately, there are a few things you can do to stay warm without relying on auxiliary heat all the time.

Thermostat Habits

Try to make small adjustments and gentle schedules instead of aggressive setbacks and major jumps. Don’t crank the heat all the way up to warm up your home.

Airflow Basics

Ensure your system is operating efficiently by regularly changing the filter. Additionally, clear your vents and returns.

Home Comfort Basics

Avoid heat loss by sealing drafts around your doors and windows. To improve circulation, keep your interior doors open.

Quick Troubleshooting: What To Check Safely at Home

As a homeowner, there are a few questions you can ask to troubleshoot why auxiliary heat is on.

• Did you recently adjust the thermostat’s schedule or add a major setback?
• Is your thermostat turned to “heat” and not “emergency heat”?
• Does the air filter look clogged or dirty?
• Is there uneven heating in specific areas, indicating a duct problem?
• Are there any rugs or furniture blocking your returns?

When To Call a Pro (And What We’ll Typically Look At)

You should call for heat pump service in the following situations.

• The HVAC system is unable to maintain your set temperature.
• You think there is an airflow or duct-related issue.
• Your home’s comfort level has declined, and you’re also experiencing high energy bills.
• You notice that “aux heat” appears frequently on mild days.

Aux Heat Isn’t “Bad”- It’s a Signal

When “aux heat” appears on your thermostat, it’s a sign your system is working the way it should. However, auxiliary heat should only be a backup. If it is running all the time or your home doesn’t stay warm, it’s time to get a professional inspection. Fox Family Heating & Air can check your airflow and HVAC performance to determine the underlying problem.

Book your next HVAC appointment to identify problems and enjoy consistent comfort this winter.

FAQs

Does “aux heat” mean my heat pump is broken?

No, it doesn’t. “Aux heat” comes on when your heat pump is using its backup heat source. It may be due to an especially cold day or because the heat pump is defrosting the outdoor coils. However, you should get help if you notice auxiliary heat coming on all the time or your home struggles to stay warm.

Why does “aux heat” turn on when I raise the temperature?

Sometimes, your heat pump needs an extra boost to get to the right temperature. This is a more common issue when major thermostat changes are involved. By using smaller changes and moderate setbacks, you can avoid relying on auxiliary heat.

Is it normal to see “aux heat” during defrost?

Yes, it is. The heat pump is warming the outdoor coils, so it isn’t heating your home. During this brief interlude, your auxiliary heat keeps your home warm.

How can I keep “aux heat” from running so much?

Moderate setbacks, smaller temperature differences, and consistent HVAC maintenance can help your heat pump perform its best during wintertime conditions.

Will a dirty filter cause “aux heat” to run more?

Yes, it can. If the filter is clogged, your airflow will suffer. This makes your heat pump less efficient, forcing it to rely on auxiliary heat more often.