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Quick answer: Aux heat (auxiliary heat) is your heat pump’s backup. The thermostat turns it on automatically when the heat pump needs help: in very cold weather, during defrost, or when you raise the temperature a lot. Emergency heat is a setting you choose by hand. It shuts the heat pump off completely and runs only the backup heat. Use it only if the heat pump has failed, because running on backup alone usually costs much more.
If you’ve just moved into a home with a heat pump, the thermostat can be confusing. You’ll see “Aux Heat On” on a cold morning, and there’s a mysterious “Em Heat” option that sounds like something you should press when it’s cold. This guide explains what both labels mean, why they matter for your electric bill, and what to do when you see them.
Aux heat vs. emergency heat at a glance
| Aux heat | Emergency heat | |
|---|---|---|
| Who turns it on | The thermostat, automatically | You, manually |
| Is the heat pump running? | Usually yes; backup adds heat alongside it (in dual-fuel systems the furnace can take over instead) | No, the heat pump is shut off |
| When it’s normal | Cold snaps, defrost cycles, large temperature increases | Only when the heat pump is broken or damaged |
| Effect on your bill | Higher while it runs | Usually much higher if it runs for days |
| What to do | Usually nothing, unless it runs constantly in mild weather | Call for service, then switch back to normal heat |
What "Aux Heat" means
A heat pump heats your home by moving heat from the outdoor air inside. Most heat pump systems also have a second heat source for times when the heat pump alone can’t keep up. Honeywell describes aux heat as the backup heat that turns on to help the compressor maintain your temperature setting (Honeywell Home).
That backup is usually one of two things, according to Carrier:
- Electric resistance heat strips inside the indoor air handler, which work much like the coils in a toaster.
- A gas or oil furnace, in a dual-fuel (hybrid) system.
When aux heat turns on
Carrier lists three common triggers, and notes there’s no single outdoor temperature at which every heat pump calls for aux heat:
- It’s colder than the system’s balance point, the outdoor temperature where the heat pump alone can no longer match your home’s heat loss.
- You raised the thermostat by several degrees, so the system adds backup to get there faster.
- The heat pump is defrosting. During a defrost cycle the outdoor unit briefly reverses, and backup heat keeps cold air from blowing into the house.
Some thermostats also use an outdoor “lockout” temperature. Lennox gives the example of an aux lockout set at 40°F: aux heat is allowed to come on only when the outdoor temperature drops below 40°F (Lennox). Your installer can adjust these settings.
Why aux heat raises your electric bill
Electric heat strips turn electricity directly into heat: one unit of electricity gives you one unit of heat. A heat pump moves heat instead, and Carrier notes that electric auxiliary heat is “100% efficient but much more expensive to operate than a heat pump, which can be 300% efficient.” In plain terms, every hour on electric strips can cost roughly two to three times what the same heat would cost from the heat pump.
Seeing “Aux Heat On” for a while on a frigid morning is normal. Seeing it most of the day when it’s 45°F outside isn’t, and it’s one of the most common reasons heat pump owners get a surprise winter bill. See our heat pump vs. furnace guide for what different heat sources cost per million BTU.
How to use less aux heat
- Avoid deep setbacks. Carrier recommends avoiding large thermostat setbacks at night; a big jump in the morning is a classic aux-heat trigger.
- Use a thermostat that recovers slowly. Programmable and smart thermostats designed for heat pumps can ramp up gradually instead of calling for backup.
- Keep the system maintained. Clean filters and a correct refrigerant charge help the heat pump carry more of the load. Our heat pump maintenance checklist covers what to do.
- Ask your installer to check the settings: aux lockout temperature, the number of degrees that triggers aux heat, and (for dual fuel) the switchover temperature.
- Fix the root cause in cold climates. If the heat pump is undersized for your heating load, it will lean on backup all winter. A cold-climate model sized for heating helps; see do heat pumps work in cold weather?.
What "Emergency Heat" means
Emergency heat is a mode you select yourself. Honeywell explains that the thermostat only runs emergency heat when the system switch is moved to “Em Ht,” and in that mode the compressor shuts down and only the emergency heat operates. Lennox describes it as using your supplemental heat by itself, without the heat pump.
In most homes, emergency heat and aux heat are the same equipment (Honeywell notes this is common): the difference is only who turns it on and whether the heat pump runs too.
When you should (and shouldn’t) use emergency heat
Use it when the heat pump itself can’t run: for example, the outdoor unit is damaged, iced over and not clearing after defrost, or making a grinding noise, and you need heat until a technician arrives.
Don’t use it just because it’s cold outside. A working heat pump with automatic aux heat is almost always cheaper than running on emergency heat alone. Once the repair is done, switch the thermostat back to normal “Heat.”
Frequently asked questions
Is it bad if my thermostat says "Aux Heat On"?
No, not by itself. Aux heat normally comes on in very cold weather, during defrost cycles, and when you raise the temperature by several degrees. If it runs most of the day in mild weather, have the system checked.
Should I turn on emergency heat when it is very cold?
Usually not. A working heat pump brings in aux heat automatically when it needs help. Emergency heat shuts the heat pump off and runs only the backup, which typically costs more. Save it for when the heat pump is not working.
At what temperature does aux heat come on?
It depends on your system and thermostat settings. Carrier says there is no single outdoor temperature that applies to every heat pump. Some thermostats use a lockout temperature; Lennox gives 40°F as an example setting.
Why does aux heat cost so much?
Electric heat strips produce one unit of heat per unit of electricity, while a heat pump can deliver about three. Carrier describes electric aux heat as 100% efficient versus up to 300% for a heat pump.
Are aux heat and emergency heat the same thing?
They usually use the same backup equipment. Aux heat is turned on automatically to help the heat pump; emergency heat is turned on by you and runs the backup alone with the heat pump off.
Sources
- Honeywell Home: "Aux Ht" versus "Em Ht" on your thermostat
- Lennox: Emergency Heat vs. Auxiliary Heat
- Carrier: What Is Auxiliary Heat?
Thermostat menus differ by brand. Check your thermostat’s manual or ask your installer before changing advanced settings. If something here looks out of date, let us know.