Auxiliary heat
aux heat · backup heat · heat strips · supplemental heat
Auxiliary heat is the backup heating a heat pump system uses when the heat pump alone cannot meet the load, most often electric resistance strips in the indoor air handler.
A heat pump system turns on auxiliary heat below its balance point, when the heat pump’s output falls short of the home’s heat loss. Thermostats usually show it as “aux heat”.
Electric resistance strips have a coefficient of performance of 1, so an hour on backup uses more electricity than an hour on the heat pump at the same output. DOE’s Building America guidance stresses keeping resistance backup to a minimum, especially in climate zone 4 and colder.
ENERGY STAR notes that cold-climate heat pumps keep working below 5 °F (−15 °C), but that pairing them with a backup source heats the home most efficiently when temperatures fall even lower. The alternative to strips is a dual-fuel system, in which a furnace takes over.
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