How-to guide
Aux Heat vs Emergency Heat: What’s the Difference?
By Ismail Farooq · Updated 2026-10-01
If you have a heat pump, your thermostat probably shows two kinds of backup heat: auxiliary (aux) heat and emergency (EM) heat. In most homes they use the same heater, usually electric resistance strips in the indoor air handler. The difference is when they run and whether the heat pump runs too.
| Aux heat | Emergency heat | |
|---|---|---|
| How it turns on | Automatically, when the thermostat decides it’s needed | Manually: you choose it as a mode |
| Heat pump | Usually keeps running alongside it | Turned off |
| Typical display | “Aux Heat On” or “Aux” | “Em Heat” or “Emergency” |
| When it’s normal | Defrost, very cold weather, recovering from a big setback | The heat pump is broken or can’t run |
| Running cost | Higher while it’s on, but only part of the time | Highest: all heat comes from the backup |
Why the cost difference matters
A heat pump moves heat from outdoors instead of making it. In mild weather it typically delivers two to four units of heat for every unit of electricity it uses. Electric strips turn one unit of electricity into one unit of heat. So heating the house on electric emergency heat can cost two to four times as much as running the heat pump, for the same warmth.
When aux heat is normal
- Defrost. In cold, damp weather the outdoor coil ices up. The heat pump runs in reverse for a few minutes to melt it, and aux heat comes on so the air from the vents isn’t cold.
- Very cold weather. Below its balance point, the heat pump can’t keep up on its own, and aux heat makes up the difference.
- A big jump in the setpoint. Raising the temperature by several degrees, or recovering from an overnight setback, can bring aux heat on to catch up.
If aux heat runs often in mild weather, check the settings and wiring in heat pump using aux heat too much.
When to use emergency heat
- The outdoor unit has stopped working, is damaged, or its breaker keeps tripping.
- The outdoor coil is a solid block of ice that doesn’t clear after defrost cycles.
- A technician tells you to use it until a repair is done.
Don’t switch to emergency heat just because it’s cold outside. The thermostat already brings on aux heat when the heat pump needs help, and it keeps using the cheaper heat pump for as much of the work as it can. Emergency heat doesn’t harm anything, but it costs more, and in some homes the backup strips are sized only to help the heat pump, so on their own they may not keep the house as warm in very cold weather.
Remember to switch back to Heat once the heat pump is repaired. An emergency heat mode left on by accident is a common reason for a surprising electric bill.
Dual fuel: when the backup is a furnace
If your heat pump’s backup is a gas, oil or propane furnace, aux heat and emergency heat both mean the furnace runs. On a dual-fuel system the heat pump and furnace normally don’t run at the same time: the thermostat switches to the furnace below a set outdoor temperature. Whether the furnace is cheaper than the heat pump depends on your fuel and electricity prices. See our heat pump and dual-fuel guide.
How aux and emergency heat are wired
On the old thermostat, aux heat is usually on a terminal labeled AUX, W2 or sometimes W1, and emergency heat on E. At the air handler, both wires often go to the same strip heat terminal, which is why some thermostats join them with a jumper.
Many smart thermostats have no separate E terminal. They offer emergency heat as a mode that uses the aux heat terminal. If your old thermostat had separate wires on AUX and E, see heat pump wiring with AUX and E and the E terminal page before connecting them.
Settings that control aux heat
- Aux heat lockout (or “aux heat max outdoor temperature”): keeps aux heat off above an outdoor temperature you choose.
- Staging: how long the heat pump runs, or how far the room drops below the setpoint, before aux heat joins in.
- Compressor lockout (dual fuel): the outdoor temperature below which the furnace takes over.
These are covered in settings to check after installing. To see how your own wires map to a new thermostat, use the wiring tool.