What Is Emergency Heat on a Heat Pump, and When Should You Use It?

What Is Emergency Heat?

Emergency heat (EM HEAT on most thermostats) is a manual setting that turns off your heat pump's outdoor unit and heats the house with backup heat only, usually electric heat strips in the indoor air handler. It's meant for times when the heat pump itself can't run, such as when it's damaged, iced solid or waiting on a repair. It works, but it costs a lot more to run than normal heat pump operation.

Most heat pump thermostats have this setting, and plenty of homeowners have never touched it. That's fine. In a mild climate like ours, you may go years without needing it.

Emergency Heat vs. Auxiliary Heat: What's the Difference?

These two get mixed up because they usually use the same equipment: the backup heat strips. The difference is how they turn on and whether the heat pump keeps running.

• Auxiliary heat (AUX) is automatic. The thermostat turns on the strips alongside the heat pump when it needs help: on a very cold morning, during a defrost cycle, or when you raise the temperature several degrees at once. The heat pump keeps running, and the strips shut off when the boost isn't needed.
• Emergency heat (EM) is manual. You choose it. The heat pump's compressor is locked out, and the backup heat does all the work until you switch back.
• Seeing AUX on the thermostat now and then is normal. Seeing EM means someone turned it on.

Why Is Emergency Heat So Expensive to Run?

A heat pump doesn't create heat. It moves heat that's already in the outdoor air into your home, so in mild weather it delivers more heat than the electricity it uses. Electric heat strips work like a big toaster: each unit of electricity becomes one unit of heat, and no more.

The U.S. Department of Energy notes that a heat pump can cut the electricity used for heating by roughly half compared with electric resistance heat like baseboard heaters and electric furnaces. Running on emergency heat throws that advantage away. A day or two on EM heat while you wait for a repair is fine. Weeks of it will show up on your power bill.

Some homes have a dual-fuel system, with a gas furnace as the backup instead of electric strips. In that case EM heat runs the furnace. The same rule applies: it's a backup, not the main mode. If you're weighing that kind of setup, see our comparison of a heat pump vs. gas furnace in Alabama.

When Should You Use Emergency Heat?

Emergency heat is for a heat pump that can't or shouldn't run. It isn't for a cold night. Your heat pump and its auxiliary heat already handle cold snaps together, automatically, so switching to EM heat just because it's freezing outside only raises the bill. Good reasons to use it:

• The outdoor unit is iced solid and the ice isn't clearing. Our guide to a frozen heat pump in winter covers what to do.
• The outdoor unit is damaged, for example by a fallen limb or a storm.
• The outdoor unit is making grinding, screeching or banging sounds.
• The compressor runs but the outdoor fan isn't spinning.
• A technician has diagnosed a problem and told you to run on EM heat until the repair is done.

Why You Shouldn't Leave Emergency Heat On

Emergency heat won't harm your system, but leaving it on has real downsides.

• It costs more: Every hour on EM heat is an hour of heating at the least efficient rate your system has.
• It hides problems: If the heat pump has failed, EM heat keeps you warm and you might not notice for weeks. Meanwhile a small repair can turn into a bigger one, and a refrigerant leak keeps leaking.
• It's easy to forget: Some thermostats show only a small EM light or word. If your winter bill suddenly jumps, check the setting.
• Switch back when the reason is gone: Once the ice has cleared or the repair is done, go back to normal HEAT.

What If Your System Only Heats on Emergency Heat?

If the house only warms up on EM heat and normal HEAT blows cool air, the heat pump side isn't working. The backup strips are doing all the work. That's a repair call, not a setting to live with.

Common causes include a failed outdoor fan motor or capacitor, a bad contactor, a stuck reversing valve, low refrigerant from a leak, a defrost control problem, a compressor failure, or a thermostat that's wired or configured wrong. Our guide on a heat pump blowing cold air walks through the safe checks first.

Keep running on EM heat so the house stays warm, and schedule heat pump repair. Tell the technician how long it's been heating only on EM. It helps with the diagnosis.

Thermostat Setup: Getting AUX and EM Heat Right

High winter bills on heat pump systems often trace back to the thermostat, not the equipment. A thermostat that's wired or programmed wrong can bring on the strips too often, or never let the heat pump do its job. If you're not sure yours is set up right, we can check it, or install a heat pump thermostat and configure it properly. Things to look at:

• Heat pump mode: The thermostat has to know it's controlling a heat pump with auxiliary heat, not a furnace and AC. After a thermostat swap, check this first.
• Reversing valve setting (O or B): Set wrong, the system heats when it should cool and cools when it should heat.
• AUX lockout or balance point: Many programmable and smart thermostats let you set an outdoor temperature above which the strips won't run. Set right, it keeps the heat pump doing most of the work.
• Small setpoint changes: Raising the temperature several degrees at once can call for aux heat. Go up a degree or two at a time, or use a thermostat with a smart recovery feature that ramps up gradually.
• Emergency heat wiring: Some systems need a dedicated terminal or setting for EM heat to work. If choosing EM does nothing, have it checked.

Keep Emergency Heat for Emergencies

Think of EM heat like a spare tire. Good to have, fine for a short trip, not how you want to drive every day. A fall heat pump maintenance visit tests the heat strips and confirms AUX and EM heat both work before you need them.

If your heat pump is stuck on emergency heat, or you're not sure whether it should be, call My Affordable Air at +1 (327) 210-5999. We'll find out why, explain it plainly, and give you an upfront price before any work starts.