What Does “Aux Heat” Mean on Your Thermostat?

Every year, around the end of November, North Florida residents experience the same thing. The first frost date typically brings with it the first morning you turn up the heat, and a new message pops up on your thermostat: “Aux Heat” or “Emergency Heat.” You might ask yourself, what does emergency mean? Is something wrong? Is your system about to fail?

The good news is that most of the time, aux heat is your system doing precisely what it is designed to do. Here’s what it means, when it’s normal, and when it’s a sign that your heat pump needs attention. 

How Your Heat Pump Keeps You Warm

Heat pumps are one of the most popular ways to heat your home in North Florida, and for good reason. Instead of generating heat, a heat pump moves it. In the summer, it pulls heat out of your home. In the winter, that process reverses, pulling heat from the outdoor air and bringing it inside. 

If this sounds counterintuitive, you’re not alone. Let us explain. Even on a chilly day, the outside air still contains heat for your system to capture. As temperatures drop, less heat is available, and your heat pump has to work harder to keep up. That is where auxiliary heat comes in. 

What is Aux Heat?

Auxiliary heat, often shown as “aux heat” on your thermostat, is a backup heat source built into most heat pump systems. In many homes, it’s a set of electric heat strips inside the indoor unit that warm the air directly, much like the coils in a toaster or space heater.

Your system automatically turns on aux heat when the heat pump needs extra help. That usually happens when:

• Outdoor temperatures drop sharply, especially during an overnight cold snap
• You raise the thermostat several degrees at once, asking the system to heat your home quickly
• The heat pump is in defrost mode, temporarily clearing frost from the outdoor unit
• The system is first warming up on a cold morning

In these situations, it’s normal to see aux heat for short periods. Once your home reaches the set temperature or the outdoor unit catches up, the heat pump takes over again.

Aux Heat vs. Emergency Heat

These two settings are easy to mix up, but they’re different.

Aux Heat turns on automatically. Your system decides when it needs extra help. It will switch to the heat pump automatically.

The Emergency Heat setting is something you turn on manually. It shuts off the heat pump completely and runs only the backup heat. It’s not meant for everyday use. It’s meant for true emergencies, like when your outdoor unit is damaged or not working.

If you’ve switched your thermostat to emergency heat, it’s a good idea to call a technician. Running your home on emergency heat for long periods can be costly, and it usually means something needs fixing.

Why Aux Heat Can Raise Your Energy Bill

Heat pumps are efficient because they move existing heat rather than generating it. Electric heat strips, on the other hand, create heat from scratch, which takes much more electricity.

That’s why a few minutes of aux heat on a cold morning won’t make a big difference, but a system that runs on aux heat constantly can lead to a noticeable jump in your energy bill.

Simple Ways to Reduce How Often Aux Heat Runs

• Make small thermostat adjustments. Raising the temperature one or two degrees at a time helps your heat pump keep up without calling for backup.
• Be careful with big setbacks. Dropping the temperature far overnight and cranking it back up in the morning can trigger aux heat. A smaller setback is often more efficient for heat pump homes.
• Use a heat pump–compatible smart thermostat. Many smart thermostats are designed to limit unnecessary aux heat use.
• Change your air filter. A clogged filter makes your system work harder.
• Keep your outdoor unit clear. Leaves, debris, and overgrown plants can block airflow around the unit.

When Aux Heat Is a Sign of a Problem

Aux heat should come and go. If you notice any of these, it’s time to call a professional:

• Aux heat stays on all the time, even on mild days
• Your home struggles to reach the set temperature
• Your outdoor unit is covered in ice that doesn’t clear
• Your energy bill jumps far beyond what’s normal for the season
• The outdoor unit isn’t running when the heat is on

Stay Warm This Winter With Parker Services

North Florida winters may be mild compared to the rest of the country, but when the cold does arrive, you want your heating system ready. At Parker Services, a licensed mechanical engineer leads our team, and we’ve kept Tallahassee homes and businesses comfortable for more than 40 years.

Whether your thermostat shows aux heat more often than it should or you want peace of mind before the next cold snap, our technicians can inspect your heat pump, check your thermostat settings, and make sure your system runs efficiently.

Want to stay ahead of problems all year long? Our Peak Performance Membership includes regular maintenance to help your system run efficiently, last longer, and avoid surprise breakdowns.