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Auxiliary Heat vs Emergency Heat: What Your Heat Pump Thermostat Is Telling You
-
By
Michael Haines
- Sep 12, 2026
What AUX and EM HEAT mean, when each one runs, why backup heat costs more, and how an inverter heat pump calls for help less often.
Last updated September 12, 2026 | By Michael Haines, Founder, AC Direct
Auxiliary heat is backup heat, usually electric resistance strips, that a heat pump system adds automatically when the heat pump alone cannot keep up or during defrost. Emergency heat is a manual thermostat setting that turns the heat pump off and runs only the backup heat, typically at 1 unit of heat per unit of electricity.
- AUX is automatic and temporary. EM HEAT is manual and stays on until you switch it off.
- Seeing AUX on a cold morning, during defrost or after raising the setpoint is normal.
- Electric resistance heat has a COP of 1; a heat pump typically delivers more than 3 in mild weather.
- Use emergency heat only when the heat pump has failed or a technician tells you to.
- AUX that runs constantly points to sizing, settings, setbacks or a fault.
What is the difference between auxiliary heat and emergency heat?
Auxiliary heat runs alongside or in place of the heat pump automatically, whenever the thermostat or controls decide the heat pump needs help. Emergency heat is selected by a person, shuts the heat pump's compressor off, and leaves the backup heat to warm the house by itself until the setting is changed.
Both settings usually use the same hardware. The ACEEE Summer Study paper on heat pump thermostats describes supplemental or auxiliary backup heat as usually provided by electric resistance heating elements, which can defrost the coils, boost heat delivery and make up for insufficient capacity on cold days.
Think of a football team. AUX is the coach sending in a backup for a few plays while the starter catches his breath. EM HEAT benches the starter for the rest of the game. Only one should happen every Sunday.
For where backup heat fits with capacity, defrost and dual fuel, see our hub on inverter heat pumps in cold climates.
| Factor | Auxiliary Heat (AUX) | Emergency Heat (EM HEAT) |
|---|---|---|
| How it turns on | Automatically, by the thermostat or system controls | Manually, by the homeowner or technician |
| What runs | Backup heat, often together with the heat pump | Backup heat only; the heat pump compressor is off |
| When to use it | No action needed; it covers cold snaps, defrost and recovery | Only when the heat pump has failed or is shut down for service |
| Operating cost implication | Higher for the minutes or hours it runs | Highest, because every unit of heat comes from resistance |
When does auxiliary heat turn on?
Auxiliary heat turns on when the heat pump cannot hold the setpoint by itself, typically on very cold days, during defrost cycles, and when the thermostat is raised several degrees at once, such as recovery from a night setback. Many systems also block it above a set outdoor temperature.
The ACEEE paper notes that auxiliary heat can be triggered by the difference among the target, outside and inside temperatures, that a well-insulated and sealed home reduces the need for it, and that the extent of nighttime setback affects how much auxiliary heat runs. Defrost is the other common trigger. The Consortium for Energy Efficiency explains that in ducted systems supplemental heat may come on during a defrost cycle to prevent a gap in heating.
The thermostat is not panicking when AUX lights up. It is doing arithmetic: the house wants more heat than the heat pump is delivering right now, and the gap has to come from somewhere.
Why does auxiliary heat cost more to run?
Auxiliary heat costs more because electric resistance strips convert electricity directly into heat at a COP of 1, while a heat pump moves heat from outdoor air and typically delivers several units of heat per unit of electricity. Every hour of heat supplied by strips instead of the compressor uses more electricity.
The CEE guide puts the numbers side by side: an air source heat pump typically has a COP greater than 3 and can maintain a COP greater than 1.75 even at 5°F, while electric resistance has a COP of 1. The same guide calls auxiliary heating costly and inefficient and advises consulting an HVAC expert if you see frequent use.
Field data shows how much it can matter. In NREL's cold climate study of variable-capacity heat pumps, auxiliary heat lowered the overall system COP by more than 30% at two sites, while six sites with little auxiliary heater use saw heating COP lowered by less than 7%. Same technology, very different winters on the electric bill.
We will not print a dollar figure for AUX. It depends on your rate, your strips and your house. The ratio does not change.
When should you use emergency heat?
Use emergency heat only when the heat pump itself cannot run: a failed compressor or outdoor fan, an outdoor unit damaged or buried in ice, or a technician's instruction while waiting for a repair. Emergency heat is not a cold-weather setting, and switching to it on a cold night usually raises the electric bill without improving comfort.
Here's why people reach for it anyway. EM HEAT sounds like the setting for an emergency, and a zero-degree night feels like one. But a working heat pump on a cold night is still the cheaper source of heat, and ENERGY STAR notes that a cold climate air-source heat pump will continue working at temperatures below 5°F. Turning the heat pump off because it is cold outside is like parking the car because the road goes uphill.
If you do switch to emergency heat, call a qualified technician, and switch back to normal heat once the repair is done. The setting does not turn itself off.
How does an inverter heat pump reduce backup heat use?
An inverter heat pump reduces backup heat use by raising compressor speed as the outdoor temperature drops, so it delivers more heat on cold days before the thermostat needs strips. A heat pump with more capacity retained at low temperatures covers more of the home's heating load with the compressor alone.
Fixed-speed equipment runs out of room early. For a typical single-speed heat pump, Oak Ridge researchers report that heating capacity at minus 13°F typically falls to between one-fifth and two-fifths of its rated capacity at 47°F, forcing supplemental resistance heat. Oak Ridge research on variable-capacity heat pumps found that intentionally sizing them for heating loads could drastically reduce the need for supplemental electric resistance heat, and an Oak Ridge cold climate heat pump field test in Ohio operated down to minus 13°F and eliminated resistance heat use.
The catch is that the inverter is only part of it. In the NREL study, where every home had a variable-capacity heat pump, auxiliary heater energy exceeded 40% of compressor-based heating energy at 5 of 11 all-electric sites, caused by an undersized system, auxiliary heat after defrost and during setback recovery, and a control or sensor fault. For how cold-rated equipment keeps heating in deep cold, read how heat pumps work in very cold weather, and for households keeping a gas furnace, our guide to the dual fuel heat pump.
If your current system leans on its strips every winter, the equipment conversation starts with capacity at low temperatures. AC Direct ships inverter heat pump units nationwide to homeowners and contractors, and your installer's load calculation should decide the size.
How should a thermostat be set up for a heat pump?
A heat pump thermostat should be configured for heat pump operation with auxiliary heat identified, a steady setpoint instead of deep setbacks, and outdoor temperature limits that keep backup heat locked out in mild weather. Heat or cool mode is better than auto, and setpoint changes should be small and gradual.
Configuration comes first. The ACEEE paper notes that for homes with heat pumps, the thermostat must be able to identify the auxiliary heat function. Then set it and leave it: ENERGY STAR advises that heat pumps do not save energy by turning the temperature down when you are away or asleep, and recommends avoiding Auto mode. The CEE guide recommends avoiding abrupt temperature changes of 3°F or more during unoccupied hours, and NEEP's consumer guide warns that deep setbacks of cold-climate heat pumps may cost you energy and money.
Lockouts are the installer's job. The NEEP cold climate specification asks manufacturers of some heat pump types whether the compressor locks out at a low ambient temperature, what the sequence of operation for electric heat is, and whether electric heat can be disabled above a certain outdoor temperature. Ask your installer the same three questions about your system.
Picture two neighbors with the same heat pump. One drops the thermostat eight degrees every night and raises it at 6 a.m. The other leaves it alone. The first wakes up to a lit AUX indicator and a warm hallway on strip heat. The second wakes up to a warm hallway. If you are replacing controls, compare heat pump thermostats that support auxiliary heat staging, and read our guide to thermostats and controls for inverter units.
What does it mean if auxiliary heat runs constantly?
Auxiliary heat that runs constantly means the heat pump is not carrying the load it should. Common causes are an undersized system, a thermostat or lockout setting that calls strips too early, deep setbacks, a refrigerant or airflow problem, a sensor or control fault, or a heat pump that has stopped running entirely.
The CEE guide lists auxiliary heating mode constantly displayed on the thermostat among the signs a heat pump is not working properly, along with a setpoint that is never reached and frozen coils, and it recommends having a qualified HVAC professional check the unit.
In practice, check that the thermostat is not on EM HEAT, that the outdoor unit is running and clear of ice, and that filters are clean. Beyond that, call a technician.
Auxiliary heat is a backup the system calls in automatically, and seeing it occasionally is normal. Emergency heat is a manual override for a broken heat pump. The best way to see less of both is a heat pump with enough low-temperature capacity, a steady setpoint and backup heat settings your installer has configured on purpose.
Backup heat is a good insurance policy. It is a terrible heating plan.
Frequently Asked Questions
Is it bad if my heat pump is running on auxiliary heat?
Auxiliary heat running occasionally is not bad. It normally appears during defrost, on very cold days and when the setpoint is raised several degrees. Auxiliary heat that runs for most of the day, or appears in mild weather, costs more to operate and should be checked by a qualified HVAC technician.
Should I switch my heat pump to emergency heat when it is very cold?
No, a working heat pump should stay in normal heat mode during very cold weather. The system adds auxiliary heat automatically when needed. Emergency heat turns the heat pump off and runs only resistance heat, which costs more to operate. Reserve emergency heat for a heat pump that has failed or is awaiting repair.
What temperature does auxiliary heat come on?
Auxiliary heat has no single activation temperature. It comes on when the heat pump cannot hold the setpoint, during defrost, or when the setpoint jumps, and installers can set an outdoor temperature above which backup heat is locked out. The right lockout depends on the heat pump's low-temperature capacity and the home's heat loss.
How much more does auxiliary heat cost than a heat pump?
Auxiliary heat usually costs several times more per unit of heat than a heat pump in mild weather. Electric resistance strips have a COP of 1, while an air source heat pump typically exceeds a COP of 3 in mild conditions. The exact cost depends on your electric rate, strip size and run time.
Why does my thermostat show AUX in the morning?
A thermostat often shows AUX in the morning because the setpoint rose after a night setback, or because the coldest outdoor temperatures and a defrost cycle arrive around dawn. Keeping a steady setpoint, or using small gradual changes, usually reduces morning auxiliary heat use in homes with heat pumps.
Does a cold-climate heat pump still need auxiliary heat?
Most cold-climate heat pump systems still include auxiliary heat or a furnace for defrost, extreme cold and equipment faults. A cold-climate heat pump sized to the home's heating load may run backup heat rarely. ENERGY STAR Cold Climate models must keep at least 70% of their 47°F capacity at 5°F.
