How-to guide

Heat Pump Using Aux Heat Too Much? Causes and Fixes

Updated 2026-09-30

Auxiliary heat is the backup heat in a heat pump system: electric resistance strips in the air handler, or a gas furnace on a dual-fuel system. It is much more expensive to run than the heat pump itself when it is electric, so a thermostat that shows “Aux heat” for hours is worth looking into. Often it is doing exactly what it should. Sometimes a setting or a wire is wrong.

When aux heat is normal

• Defrost. In cold, damp weather the outdoor coil ices up. The heat pump reverses for a few minutes to melt it, and the strips come on to stop cold air blowing into the house. Short bursts every hour or so are normal.
• Very cold weather. A heat pump’s output falls as the outdoor temperature drops. Below its balance point it can’t keep up alone, and aux heat makes up the difference. Cold-climate heat pumps have a much lower balance point than older models.
• A big jump in the set temperature. Raising the setpoint by several degrees, or coming out of a large overnight setback, can make the thermostat call for aux heat to catch up quickly.

Settings to check

• Equipment type. The thermostat must be set up as a heat pump. Set up as a furnace with AC, it heats with the W wire, which on a heat pump is the strip heat. See settings to check after installing.
• Aux heat lockout (also called “aux heat max outdoor temperature” or “aux heat upper temperature”). This stops aux heat above an outdoor temperature you choose. If it is off or set high, aux heat can run on mild days. Ask your installer for your heat pump’s balance point, or start somewhere in the 35 to 45 °F range and adjust.
• Staging. Some thermostats bring on aux heat after the heat pump has run for a set time or when the room is a couple of degrees below the setpoint. Allowing a longer time or a larger gap before aux heat keeps the heat pump doing more of the work, at the cost of slower recovery.
• Setbacks. Large overnight setbacks save less with a heat pump than with a furnace, because recovery can use aux heat. Smaller setbacks, or the thermostat’s smart recovery feature, usually work better.
• Emergency heat. Make sure the thermostat is in Heat mode, not Emergency heat. Emergency heat uses only the backup heat. See E and emergency heat.

Wiring mistakes that cause it

• A W2/Aux wire on the wrong terminal, or the Y wire missing so the thermostat never calls the compressor. Compare against our heat pump wiring guide.
• A reversed O/B setting. The heat pump cools when asked to heat, the room keeps getting colder, and the thermostat calls for aux heat to make up for it. Warm air in cooling mode, or cold air in heat mode, points to this. See O/B explained.
• E and W2 jumpered at the thermostat when the new thermostat has a single Aux/E terminal. That is usually correct, but check the thermostat’s instructions for your model.

Heat pump problems (call a technician)

If the settings and wiring are right, go outside while the thermostat is calling for heat:

• The outdoor unit isn’t running: check its breaker and the disconnect box next to it. If those are on, the fault is in the outdoor unit (contactor, capacitor, control board).
• It runs but the coil is a solid block of ice that doesn’t clear after a defrost cycle: a defrost control, sensor or refrigerant problem.
• It runs but the air from the vents is only lukewarm even on mild days: often low refrigerant or a failing compressor or reversing valve.

Those need a licensed HVAC technician. A thermostat can’t fix them, and aux heat covering for a failing heat pump is expensive.