A heat pump doesn't make heat the way a furnace does. It moves heat — pulling it out of the outdoor air and carrying it inside. On a mild day that's remarkably efficient, delivering roughly two to three units of heat for every unit of electricity. But the colder it gets outside, the less heat there is to pull, and the pump's output falls just as your house needs more. Aux heat exists to cover that gap.
In most heat pump systems, auxiliary heat is a set of electric resistance coils in the indoor air handler — functionally, a giant built-in space heater. Some systems use a gas or oil furnace as the backup instead (a dual-fuel setup). Either way, the thermostat calls for aux heat automatically when the heat pump alone can't hold your set temperature. Resistance heat is dead simple and totally reliable. It's also the most expensive way to warm a house: one unit of electricity in, one unit of heat out. No multiplier. That furnace-as-backup arrangement is one branch of the wider heat pump vs furnace decision, which mostly comes down to your climate and local fuel prices.
Deep cold. Most heat pumps reach a balance point — often somewhere in the 30s °F, though it varies by system and house — below which the pump can't supply the whole heating load. Under that point, aux heat cycling on to help is the system working as designed.
Defrost cycles. In cold, damp weather, frost builds on the outdoor coil. Every so often the pump briefly reverses to melt it, and aux heat warms the air indoors so you don't feel a blast of cold. A defrost cycle lasts a few minutes. The outdoor unit may steam — that's melted frost, not smoke. One side effect surprises people: when the strips first fire after months off, the dust on them cooks off with a faint scorched smell — normal for a few minutes, but if your furnace smells like burning for longer than that, it deserves a closer look. If that brief cold blast during defrost is what sent you searching, our full diagnosis of a heat pump blowing cold air in winter walks through the thermometer test that separates a normal defrost from a failing valve or low charge.
Big temperature jumps. Raise the setpoint several degrees at once — say, from a 62° night setting to 70° in the morning — and many thermostats fire the aux heat to recover faster. The pump would get there eventually. The thermostat just doesn't want to wait, and it spends your money to save time.
Aux heat becomes a bill problem when it runs long and often in weather the pump should handle alone — roughly, when it's above 40°F outside and the strips are still on, or when aux runs for hours at a stretch. That pattern usually means the heat pump is underperforming (clogged filter, iced or blocked outdoor unit, low refrigerant, a failing part) or the thermostat is calling for backup unnecessarily (wrong mode, aggressive recovery settings). Either way, you're paying space-heater prices to heat the whole house.
| Situation | Aux heat normal? | What to do |
|---|---|---|
| Cold snap, teens or 20s °F | Yes | Nothing — it's designed for this |
| Outdoor unit steaming, aux on briefly | Yes — defrost | Nothing; it should end in minutes |
| Raising the setting 4+ degrees at once | Usually | Raise it 1–2 degrees at a time instead |
| Aux on for hours above 40°F | No | Run the checks below |
| Thermostat set to Em Heat / Emergency | Strips only — most expensive mode | Switch back to Heat unless the pump is broken |
Check and replace the filter. A filter gray with dust chokes airflow, the pump can't move its heat indoors, and the thermostat leans on the strips. If you can't remember when it last changed, that's your answer. Filter sizing and change intervals are covered in our guide to AC filter replacement and MERV ratings.
Look at the outdoor unit. It should be clear on all sides, with the top fan unobstructed. A unit entombed in ice — not a light frost, but solid ice that doesn't clear between defrost cycles — is a service call, not a DIY project.
Confirm the thermostat settings. Mode should read Heat. If someone set it to Emergency Heat, the pump is locked out entirely. On smart thermostats, look for aux heat lockout or balance point settings — set so the strips stay off until it's genuinely cold out. Owners who've recently installed their own thermostat can double-check the configuration against our walkthrough on installing a smart thermostat yourself.
Change how you ask for heat. Raise the temperature in 1- to 2-degree steps, and keep overnight setbacks shallow — 2 or 3 degrees, or none. Watch whether the aux indicator still comes on in mild weather.
Then decide. If aux heat still runs long in mild weather with a clean filter, a clear unit, and sane settings, call an HVAC tech. Low refrigerant, a bad reversing valve, and failing sensors are all on their side of the line.
Deep nighttime setbacks. Programmable-thermostat habits from the furnace era backfire with heat pumps. Dropping to 60° overnight means a long morning recovery, and the thermostat calls the strips to do it. Shallow setbacks or a steady temperature usually cost less — the comparison in smart vs programmable thermostats covers how recovery settings differ.
Cranking the thermostat. A thermostat isn't a gas pedal. Setting 78° to "heat faster" doesn't speed the pump — it just guarantees the aux strips join in and overshoot.
Ignoring a month of warning signs. Lukewarm vent air, longer run times, and a creeping bill are the pump asking for help weeks before the aux heat starts doing the whole job.
Aux heat on your thermostat isn't an error message — it's the backup doing its job in deep cold, during defrost, and on big temperature recoveries. It only deserves your attention when it runs long in mild weather or after a big setpoint jump becomes your daily routine. Filter, outdoor unit, thermostat mode, gentler setpoints: check those four, and you'll know if you're looking at normal operation or a repair bill.
Yes — resistance strips convert electricity to heat one-for-one, while a heat pump in mild weather moves two to three times that much heat per unit of electricity. Short aux runs are a minor cost. Hours of aux heat per day is where bills double.
Common reasons: a defrost cycle, a setpoint raised several degrees at once, a dirty filter strangling airflow, or a thermostat whose aux lockout is set too warm. If it happens above about 40°F with none of those at play, the heat pump itself needs a look.
Aux heat helps the heat pump — both run together, with the strips topping up what the pump can't supply. Emergency heat replaces the pump: the thermostat locks the pump out and heats on strips alone. Aux is automatic and normal; emergency is a manual mode for a broken pump.
Don't disable it outright — in a real cold snap, the pump alone may not hold temperature, and frozen pipes cost more than electricity. Instead, fix the causes of unnecessary runs: clean filter, sane setbacks, and a thermostat lockout set so strips stay off until it's truly cold.
Morning recovery from an overnight setback is the most common trigger. The thermostat sees a house several degrees below the morning setting and fires the strips to close the gap fast. A shallower setback — or holding one temperature — usually ends the pattern.