What does "aux heat" mean on a heat pump thermostat?
Aux heat is the backup that switches on when the heat pump alone cannot keep up. Why it appears, what it costs, and when it points to a problem.
Published 3 min read
In short
"Aux heat" on a heat pump thermostat means auxiliary heat is running: a backup source, most often electric resistance strips in the indoor air handler, adding heat because the heat pump alone is not meeting the demand.
Some aux heat on the coldest days is normal for a system sized below the home's full heating load. Because resistance strips have a COP of 1, long hours of it cost far more than the heat pump, so frequent aux heat in mild weather is worth investigating.
A heat pump system usually has two ways to heat: the heat pump itself, and a backup. When the thermostat shows “aux” or “auxiliary heat”, the backup is running, alone or alongside the heat pump. Often the backup is a bank of electric resistance strips in the indoor air handler; in a dual-fuel system it is a furnace.
Why does aux heat come on?
The basic reason is capacity. An air-source heat pump delivers less heat as the outdoor air gets colder, while the house loses more. Below the balance point, where the two meet, the heat pump cannot hold the set temperature on its own and auxiliary heat makes up the difference.
How often that happens depends on how the system was sized. DOE’s Building America guidance describes a range: a heat pump sized to the cooling load may need backup for much of a cold-climate winter, with a balance point around 40 °F (4 °C), while one sized to the full heating load at the local design temperature may not reach its balance point until −15 °F (−26 °C) or 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.
How much does it cost?
Electric resistance strips turn each unit of electricity into one unit of heat, a coefficient of performance of 1. A heat pump at the federal minimum seasonal rating, HSPF2 7.5, averages a COP of about 2.2 over a standard heating season. For the same heat, an hour on strips uses roughly twice the electricity of an hour on the heat pump, or more in mild weather when the heat pump’s COP is higher.
That is why DOE’s Building America guidance stresses keeping resistance backup to a minimum, especially in climate zone 4 and colder.
When is aux heat a sign of a problem?
Aux heat on a very cold night is expected for many systems. It is worth a closer look when:
- it runs often in mild weather, well above what you would expect the balance point to be;
- electricity use jumps without a change in weather or habits.
Possible causes range from settings to equipment faults, and a technician can check which. Useful questions for the visit: what is this system’s balance point, at what outdoor temperature is auxiliary heat allowed to run, and is the heat pump delivering its rated capacity?
Can I turn aux heat off?
Not without consequences: if the heat pump cannot meet the load and the backup is prevented from running, the house cools below the set temperature. Whether your thermostat and equipment allow any limit on auxiliary heat, and at what outdoor temperature it would make sense, is a question for the installer, who knows the system’s capacity and the home’s heat loss.