Auxiliary Heat: The Setting That Quietly Triples Winter Bills
If you have a heat pump and your December bill surprised you, this is almost certainly why.
Auxiliary heat is electric resistance heating that supplements a heat pump when it cannot keep up. It costs roughly three times as much to operate as the heat pump itself. It should engage only below a set outdoor temperature — typically 35 to 40°F in the Valley — but that lockout setting is frequently never configured.
What auxiliary heat is for
A heat pump moves heat from outdoor air into the house. As outdoor temperature falls, there is less heat available to move and capacity declines. At some point the heat pump alone cannot keep up with the house's heat loss.
Auxiliary heat — electric resistance strips in the air handler, essentially large toasters — fills that gap. It is perfectly efficient at converting electricity to heat, one unit in and one unit out, which sounds good until you compare it to a heat pump delivering two to four units of heat per unit of electricity.
That is the entire economics. Auxiliary heat costs roughly three times as much to produce the same warmth.
When it should engage — and when it actually does
In the Phoenix climate, a properly configured heat pump should almost never need auxiliary heat. Our winters produce relatively few hours below 40 degrees, and a heat pump operates comfortably in that range.
What actually happens is that auxiliary heat gets triggered by things other than genuine cold:
- A large setpoint jump. Recovering from a deep setback, the thermostat sees a big gap and calls auxiliary heat to catch up faster.
- No lockout configured. Without an outdoor temperature lockout, nothing prevents strips from running on a 55-degree morning.
- Defrost cycles. Auxiliary heat engages during defrost to offset the cool air. This is correct behavior, but frequent unnecessary defrosts multiply it.
- A failed heat pump. If the compressor, reversing valve, or defrost board has failed, the system falls back to strips entirely and the house stays warm. Nothing feels wrong.
- Emergency heat left engaged. Someone switched to EM HEAT during a cold snap and never switched back.
The failed-heat-pump scenario deserves emphasis
This is the most expensive version and it produces no comfort complaint at all. The heat pump fails, the thermostat calls auxiliary heat, the house reaches setpoint, and everything appears normal.
The homeowner finds out in January when the bill arrives — after two months of heating a house with electric resistance.
The check takes fifteen minutes and should be done every October: set the thermostat to HEAT and confirm the outdoor unit actually runs. If it does not, the heat pump is not working, regardless of how warm the house feels.
Setting the lockout
Auxiliary heat lockout is a thermostat setting that prevents strips from engaging above a specified outdoor temperature. For most Valley homes, somewhere in the 35 to 40 degree range is appropriate.
Setting it requires the thermostat to know the outdoor temperature, which it gets either from an outdoor sensor or, on connected thermostats, from internet weather data.
It is a five-minute configuration change, it is frequently never done, and on a Phoenix heat pump it is worth more than any other single setting.
Emergency heat is not auxiliary heat
They use the same electric strips but they are different modes. Auxiliary heat runs alongside the heat pump when extra capacity is needed. Emergency heat bypasses the heat pump entirely and runs on strips alone.
Emergency heat exists for one situation: the heat pump has failed and you need heat tonight. It is not a "make the house warm faster" setting, though it is used that way constantly.
If you find yourself reaching for emergency heat to stay comfortable, that is a signal the heat pump needs service, not a solution.
What a reasonable Valley winter looks like
On a correctly configured heat pump in Phoenix, auxiliary heat should run rarely — a handful of the coldest mornings, and briefly during defrost cycles. Your heating bill should be modest, because our winters are mild and heat pumps are efficient in exactly these conditions.
If your winter bills are substantially higher than your neighbors' with similar homes, or higher than they were the previous year with no rate change, auxiliary heat is the first place to look. It is usually a setting, sometimes a failure, and almost always fixable.
Frequently asked
What is auxiliary heat on a heat pump?
Electric resistance heat strips that supplement the heat pump when it cannot keep up with demand. They cost roughly three times as much to run as the heat pump itself, so they should engage only below a set outdoor temperature.
Why is my auxiliary heat running when it is not that cold?
Usually the auxiliary heat lockout was never configured, so nothing prevents strips from engaging on a mild morning. Other causes are a large thermostat setpoint jump, a failed heat pump the system is compensating for, or emergency heat left engaged.
How do I stop my heat pump from using auxiliary heat?
Set the auxiliary heat lockout on your thermostat to roughly 35 to 40°F outdoor temperature, keep setbacks to two or three degrees, use a heat-pump-aware thermostat with adaptive recovery, and confirm the heat pump itself is actually working.