AUX Heat vs. Emergency Heat on a Heat Pump Thermostat

Quick answer

AUX heat means your heat pump's backup heat is helping automatically, and EM heat means you've switched to backup heat only. AUX is normal during defrost, cold snaps, and big temperature jumps. If AUX shows most of the time in mild weather, or you're stuck on EM heat, have the system checked.

Key facts

AUX	Backup heat helping, turned on automatically
EM heat	Backup heat only; heat pump off
AUX is normal during	Defrost, cold snaps, big setpoint jumps
When to use EM heat	Short term, when the outdoor unit can't run
Common backup heat	Electric heat strips or a gas furnace

What does AUX heat mean on my thermostat?

AUX heat means your system has turned on its backup heat to help the heat pump. It happens automatically. You didn't do anything wrong, and in most cases nothing is broken.

On most heat pump systems, backup heat is either electric heat strips inside the indoor air handler or a gas furnace in a dual-fuel setup. With electric strips, the heat pump usually keeps running and the strips add extra warmth. With a dual-fuel furnace, the furnace often takes over instead.

Thermostats label it differently — AUX, AUX HEAT, backup heat, or a second stage of heat. Your thermostat's manual shows its exact wording.

What does EM heat (emergency heat) mean?

EM heat, or emergency heat, means you've told the system to stop using the heat pump and heat with backup only. It doesn't turn on by itself. Someone has to select it on the thermostat.

It's meant for times when the outdoor unit can't run — it's damaged, iced solid, or making a bad noise — and you're waiting for a repair. It keeps the house warm in the meantime. Here's how the two compare:

Question	AUX heat	EM heat
How does it turn on?	Automatically	Only when someone selects it
Is the heat pump running?	Usually, with electric strips	No
When is it normal?	Defrost, cold snaps, warming back up	Only while the outdoor unit can't run
What about cost?	Higher while it runs	Higher the whole time it's on

Not sure what's wrong yet?

Start with the safe checks. Many problems are a setting, a breaker, or a filter.

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When is it normal to see AUX heat?

It's normal to see AUX heat for short stretches, and more often in cold weather. These are the common, normal reasons:

Defrost. When the heat pump defrosts its outdoor coil, backup heat often runs briefly so you don't feel cool air.
Cold snaps. As outdoor air gets colder, the heat pump may need help to keep up.
Big temperature jumps. Raising the thermostat several degrees at once can bring on AUX to close the gap faster.
Morning recovery. If the house was set back overnight, AUX may run while it warms back up.

To see AUX less often, raise the temperature a little at a time and skip deep setbacks. ENERGY STAR says heat pumps don't save energy by being turned down when you're away or asleep.

When does AUX heat mean something is wrong?

AUX heat points to a problem when it runs most of the time in mild weather, or the house still won't warm up while it runs. That usually means the heat pump isn't carrying its share of the load. Watch for these signs:

AUX shows almost all the time, even on mild days.
The outdoor unit isn't running, or its fan isn't spinning, while AUX is on.
The outdoor unit stays caked in ice.
Your electric bill jumped with no cold snap to explain it.
The house won't reach the set temperature even with AUX on.

Common causes include a refrigerant leak, a failed outdoor fan motor or capacitor, a defrost control problem, or a thermostat that was set up wrong. These are jobs for a technician.

Why does running on emergency heat for a long time cost more?

Emergency heat usually costs more because electric backup heat makes heat directly from electricity instead of moving it. A heat pump moves heat that's already in the outdoor air. ENERGY STAR says that lets a heat pump deliver up to three times more heat energy than the electrical energy it uses.

So when you run on electric backup alone, you use more electricity for the same warmth. A short stretch while you wait for a repair is fine. Weeks on EM heat can show up on your bill. If your backup is a gas furnace, the cost depends on your gas rates instead.

The fix isn't to put up with a cold house. It's to get the heat pump running again, so EM heat can go back to being a backup.

When should you switch to EM heat on purpose?

Switch to EM heat only when the outdoor unit clearly can't run, and then call for a repair. It's a stopgap, not a setting to leave on. Cold weather alone is not a reason — your system brings on AUX by itself when it needs help. Good reasons to switch:

The outdoor unit is damaged, for example by a falling branch.
It's encased in ice that won't clear after a defrost cycle.
It's making grinding, screeching, or loud banging noises.
The outdoor unit's breaker tripped again after you reset it once.

What can you check before calling about AUX or EM heat?

Try these safe checks first. If they don't solve it, leave the rest for a technician.

1Look at the thermostat mode. If it says EM heat and you didn't mean to set it, switch back to Heat.
2Make sure the thermostat is on Heat, not Auto, and set to a comfortable temperature.
3Check the filter and replace it if it's dirty.
4Go outside and see if the outdoor unit is running and its fan is spinning. Keep hands and tools out of it.
5Clear snow, leaves, and debris away from the outdoor unit. Don't chip at ice.
6Check the breakers. Reset a tripped one once. If it trips again, stop and call.

What does a technician check when AUX heat runs too much?

A technician checks why the heat pump isn't keeping up on its own, starting with the outdoor unit. AUX running too much is a symptom. The goal is to find the cause, so the heat pump can go back to doing most of the work.

Once the cause is found, we explain what's wrong and what it takes to fix it. You get the price before any work starts, and nothing happens until you say yes. A typical check covers:

Whether the outdoor unit, its fan, and its compressor start and run the way they should.
Signs of a refrigerant leak or a low charge.
The defrost controls and sensors, so frost clears on schedule.
The outdoor fan motor and capacitor.
Thermostat wiring and setup, including how and when it calls for backup heat.
The filter and airflow through the indoor unit.
The backup heat itself — electric heat strips or a dual-fuel furnace — and its breaker.

Common questions

Is it bad if my thermostat says AUX heat?

No, not by itself. AUX means backup heat is helping, which is normal during defrost, cold snaps, and when the house is warming back up. It's worth a call if AUX is on most of the time in mild weather.

Should I use emergency heat when it's really cold outside?

Usually not. In cold weather, the system brings on AUX heat automatically, so the heat pump and backup work together. Save EM heat for when the outdoor unit can't run.

How long can I run my heat pump on emergency heat?

You can use emergency heat to stay warm while you wait for a repair, but plan to get the heat pump fixed soon. Electric backup heat usually costs more to run than the heat pump. The longer you stay on EM heat, the more it can add to your bill.

Why does AUX heat come on when I turn up the thermostat?

A big jump in the set temperature tells the system to close the gap fast, so it adds backup heat. Raising the temperature a little at a time can keep AUX off. That's normal, not a sign of a problem.

Does a dual-fuel system have AUX heat?

Yes. In a dual-fuel system, the gas furnace is the backup heat. Your thermostat may show AUX or backup heat when the furnace takes over from the heat pump.

Can AUX heat raise my electric bill?

Yes, when your backup heat is electric. Electric heat strips use more electricity for the same warmth than the heat pump does. If AUX runs a lot without a cold snap to explain it, have the system checked.

Why does my thermostat say AUX but the air feels cold?

That usually means the backup heat isn't working. Electric heat strips often have their own breaker, so check your panel and reset a tripped breaker once. If it trips again or the air stays cold, call a technician.