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Aux heat (auxiliary heat)

Aux heat is a backup heat source that a heat pump thermostat turns on automatically when the heat pump needs help.

Editorial staffEdited by heatpumpwiki

History 1

Aux heat is backup heat that the thermostat brings on automatically alongside or in place of the heat pump. It is usually electric resistance elements in the ductwork or baseboards, or a furnace or boiler in a hybrid system.Source 1

It runs when the heat pump cannot meet the load on its own (below the thermal Thermal and economic balance points), and it may run during Defrost. Lennox notes "the thermostat may engage auxiliary heat during the defrost cycle."Source 3

DOE warns that "many heat pump controls are incorrectly wired to automatically turn on the auxiliary strip heat if the indoor thermostat is changed by more than 3 degrees F."Source 2

Aux heat is different from Emergency heat, which you select by hand. See Heat pump backup heat and lockout temperatures.

Sources

  1. 1.Heating and cooling with a heat pump. Natural Resources Canada. Accessed Oct 25, 2026
  2. 2.Air-Source Heat Pumps (archived copy of energy.gov/energysaver/air-source-heat-pumps, captured June 20, 2026). U.S. Department of Energy. Accessed Oct 25, 2026
  3. 3.Heat Pump System Information, User Guide (507657-01). Lennox. Accessed Oct 25, 2026

More in Glossary

  • AHRI certificateAn AHRI certificate shows the certified efficiency ratings for one exact combination of outdoor and indoor heat pump equipment.
  • Btu (British thermal unit)A Btu is the heat needed to raise one pound of water by one degree Fahrenheit, and heat pump capacity is given in Btu per hour.
  • COP (coefficient of performance)COP is the heat a heat pump delivers divided by the electricity it uses, at one set of conditions.
  • DefrostDefrost is the cycle in which a heat pump briefly reverses to melt frost off its outdoor coil.
  • EER2EER2 is a cooling efficiency rating at a single hot test condition, in Btu per hour of cooling per watt.
  • Emergency heatEmergency heat is a thermostat mode that turns the heat pump off and heats with the backup source only.