Aux Heat vs. Emergency Heat: What Your Thermostat Is Telling You

Aux heat and emergency heat both use your heat pump's backup heat, but they aren't the same thing. Aux heat turns on automatically to help your heat pump when it needs a boost. Emergency heat is a setting you choose by hand, and it's meant for when the heat pump itself can't run.

Knowing the difference can save you a surprise on your power bill.

What is aux heat?

Aux, short for auxiliary, is backup heat that works alongside your heat pump. In most homes in our area, it's a set of electric heat strips inside the indoor unit. Some homes have a gas furnace as the backup instead.

Your thermostat turns on aux heat by itself when the heat pump needs help, such as:

• On very cold mornings, when the heat pump can't pull enough heat from the outdoor air
• When you raise the thermostat several degrees at once
• During a defrost cycle, to keep cool air from blowing through the house

Seeing "Aux" on your thermostat now and then is normal. You don't need to do anything.

What is emergency heat?

Emergency heat, often shown as "Em Heat," shuts the heat pump off completely and runs only the backup heat. You turn it on yourself. The system won't switch to it on its own.

Use it when the heat pump has stopped working or is damaged, such as when the outdoor unit is frozen solid, a tree limb has hit it, or it's making grinding noises. It keeps the house warm until a technician can get there.

Why shouldn't I leave emergency heat on?

Electric heat strips use much more electricity than a working heat pump to make the same amount of heat. Leaving emergency heat on for days can raise your power bill noticeably, and it can hide a heat pump problem that keeps getting worse.

If you've switched to emergency heat, plan on calling for service soon. Once the heat pump is repaired, switch the thermostat back to Heat.

Why does my aux heat come on so often?

Occasional aux heat is expected. If it runs most of the day on mild days, something may be off:

1. Big thermostat jumps. Raising the temperature several degrees at once can call for aux heat. Small changes keep the heat pump doing the work.
2. Thermostat setup. Some thermostats are set to bring on backup heat too early. A technician can adjust the settings.
3. A dirty filter or blocked outdoor unit. Both make the heat pump work harder.
4. Low refrigerant or a failing part. If the heat pump can't keep up on an average day, it needs a diagnosis.

When should I call a pro?

Call us if you've had to switch to emergency heat, if aux heat runs constantly on mild days, or if your power bill jumps without explanation. Call Aero Air Services at (888) 409-2376 or request service online. See our heat pump repair page for more. We show up. We clean up. We back our work.

Frequently asked questions

Is it bad if my thermostat says aux heat?

No. It means the backup heat is helping the heat pump, which is normal on cold days and during defrost.

What temperature should I switch to emergency heat?

Emergency heat isn't based on temperature. Use it only when the heat pump isn't working. On a cold day, a healthy system brings on aux heat by itself.

Does emergency heat mean my system is broken?

Not on its own. It's just a setting. But if you needed to use it, the heat pump likely needs service.