If your thermostat shows AUX HEAT or you've spotted an EM HEAT setting, you're not alone — these confuse a lot of North Carolina heat-pump owners. Here's what they actually do and when to worry.
A heat pump doesn't burn fuel — it moves heat from the outside air into your home. It's very efficient in our mild Piedmont winters, which is why heat pumps are so common here. But on the coldest days, it sometimes needs a hand. That's where auxiliary and emergency heat come in.
Auxiliary heat is backup heat (typically electric heat strips, or a furnace in a dual-fuel system) that kicks in automatically when the heat pump can't keep up — on very cold days, or when you bump the thermostat up several degrees at once. Seeing AUX during a cold snap or a big temperature swing is normal. It costs more to run than the heat pump alone, so it shouldn't run constantly in mild weather.
Emergency heat is a manual mode you turn on. It shuts the heat pump off entirely and runs only on the backup heat. It's meant for one situation: the heat pump itself has failed (for example, the outdoor unit is iced over or broken) and you need heat until it's repaired. Because it runs on the expensive backup the whole time, you shouldn't use EM heat as a normal setting.
If your heat pump runs on aux/emergency heat far more than the weather warrants, ices over, or stops heating, have it checked. For a system that's aging or struggling every winter, it may also be worth weighing heat pump vs. furnace or repair vs. replacement.
Auxiliary (AUX) heat turns on automatically to help the heat pump on very cold days or big temperature swings. Emergency (EM) heat is a manual mode you switch on that shuts the heat pump off and runs only on backup heat - meant for when the heat pump has failed.
Only when the heat pump itself isn't working - for example, the outdoor unit is iced over or broken - and you need heat until it's repaired. It's not meant to be a normal setting because it runs on expensive backup heat.
Yes, during cold weather or when you raise the thermostat several degrees at once. What's not normal is aux heat running constantly in mild weather, which can signal a problem.
Common causes include low refrigerant, an iced-over outdoor unit, a failing defrost cycle, reversing valve, or sensor, or simply a very cold stretch. If it persists in mild weather, have it checked.
Yes. Emergency and auxiliary heat usually run on electric heat strips, which cost more to run than the heat pump itself, so relying on them raises your power bill.
Related:
Heat Pump vs. Furnace →
Heat Pump Service →
Repair or Replace? →
Ductless Mini-Splits →
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