When you see “AUX HEAT” (or just “AUX”) on your thermostat display, your heat pump’s backup electric heating strips have switched on. This is not a failure — it’s a feature. But there are situations where AUX running is a sign that something is wrong.
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What aux heat actually is
A heat pump heats your home by moving heat from the outdoor air inside — it doesn’t generate heat by burning fuel. This works efficiently down to about 30–40°F outdoor temperature for most standard heat pumps. But as temperatures drop, the heat pump becomes less efficient and eventually can’t produce enough heat to keep up with the house’s heating load.
Every central heat pump system includes a backup heat source for this situation: electric resistance heating strips mounted inside the air handler. These are the aux heat strips — essentially a large electric heater built into the duct system.
When the heat pump needs help, the strips turn on automatically. Your thermostat shows “AUX HEAT” to indicate the strips are running.
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Aux heat vs. emergency heat — the difference
These two modes look similar on a thermostat display but work very differently:
AUX heat (auxiliary):
- Automatic — the thermostat engages it without you doing anything
- Runs alongside the heat pump (both work simultaneously)
- Triggered when outdoor temperatures drop below the balance point, or during a defrost cycle
- Normal and expected in cold weather
Emergency heat (EM HEAT):
- Manual — you switch it on at the thermostat
- Runs instead of the heat pump (heat pump is completely bypassed)
- Use only when the heat pump itself has failed
- Running EM heat while the heat pump is still functional costs 2–4× more per hour than letting the heat pump + AUX run together
The key distinction: if your heat pump is working, leave the thermostat in HEAT mode and let AUX engage automatically. Emergency heat is for when the outdoor unit has failed — not for cold snaps.
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When AUX heat running is normal
Outdoor temperatures below your balance point (typically 25–40°F): the heat pump can’t keep up with demand on its own. AUX supplements it. On a very cold day, AUX may run continuously. This is normal and expected — you’re paying more in electricity, but the system is doing what it’s designed to do.
During a defrost cycle: heat pumps accumulate frost on the outdoor coil at temperatures below ~40°F. Every 30–90 minutes in cold weather, the system runs a defrost cycle — it briefly operates like an air conditioner to melt the frost. During defrost (5–15 minutes), the heat pump isn’t producing heat, so AUX strips run to maintain indoor comfort. Seeing AUX appear briefly and then disappear is normal defrost behavior.
On startup after a large setpoint increase: if you raise the thermostat 5°F or more, the system may engage AUX briefly to catch up to setpoint faster, then disengage once it’s satisfied.
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When AUX heat is a problem
AUX running in mild outdoor temperatures (above 40°F) for more than 15–20 minutes: The heat pump should be handling the load without backup above 40°F in most systems. AUX at these temperatures usually means the heat pump isn’t working properly:
- Heat pump compressor not running (refrigerant issue, capacitor failure, or lockout)
- Reversing valve stuck in cooling mode — the system is trying to cool, not heat → see Heat Pump Reversing Valve: Symptoms and Diagnosis
- Thermostat not calling the heat pump correctly (wiring problem with Y or O/B terminal) → see Heat Pump Thermostat Wiring
AUX on but the house won’t reach setpoint at all: If aux heat is running and the house still won’t warm up, either the strips are insufficient alone (below the system’s design minimum) or they’ve also failed. → See Heat Pump Not Heating.
Outdoor unit is iced over and AUX is running constantly: A heat pump frozen solid can’t defrost itself and heat output drops to zero. AUX then runs indefinitely trying to compensate. → See Heat Pump Frozen Outdoor Unit.
AUX runs on every heating call even in mild weather: The balance-point cutover setting (available on ecobee, some Nest, and Honeywell thermostats) may be set too high — or the heat pump is undersized for the house heating load.
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How to check if the heat pump is running alongside AUX
When your thermostat shows AUX, walk to the outdoor unit:
- Fan running at the top of the unit: you should see and hear the fan spinning.
- Compressor running: you’ll hear a low, steady hum from the compressor. The outdoor unit will also feel warm to the touch on the discharge side.
- Both running: normal. The heat pump is working; AUX is supplementing.
- Outdoor unit completely silent: the heat pump is NOT running. AUX is covering alone.
The heat pump running silently (no outdoor unit) is only normal:
- During a defrost cycle (5–15 min max)
- Below the heat pump’s minimum lockout temperature (often 0°F to -10°F on standard units)
Outside those cases: silent outdoor unit + AUX running in above-freezing weather → call a tech.
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What AUX heat costs to run
Electric resistance strips are 100% efficient — every watt of electricity becomes heat. But a heat pump produces 2–4 units of heat per unit of electricity. AUX strips therefore cost 2–4× more per BTU than the heat pump at moderate temperatures.
A 3-ton central heat pump typically has 10–15 kW of strip heat:
- Running 10 kW for 1 hour at $0.15/kWh: $1.50
- Running continuously through a 3-day cold snap (January, zone 5): $30–50 in AUX alone
If AUX is running because it’s 15°F outside, this is expected — it’s the cost of the heating backup working as designed. If AUX is running because the heat pump isn’t working at 45°F, you’re overpaying for heat while also sitting on a failing heat pump.
If you’re replacing a heat pump and want to reduce AUX dependency, cold-climate heat pumps maintain high efficiency down to -13°F or lower, cutting AUX use dramatically. Federal IRA tax credits of up to $2,000 may apply to qualifying cold-climate heat pump installations — see HomeEnergyPlanner for current eligibility details.
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AUX heat on different thermostat brands
Nest (Learning Thermostat, Nest E, Nest 2020+): “AUX” appears beneath the temperature display during auxiliary heat operation. In the Nest app, the current stage is shown in the home screen status. → See Nest thermostat symbols explained.
ecobee: “Aux Heat” appears in the home screen status bar. The Activity Log under System Monitor shows when AUX stages ran and for how long — useful for diagnosing whether AUX frequency is normal.
Honeywell/Resideo T6 Pro, T9, T10 Pro: “Aux Heat On” or “Aux” appears on the display during auxiliary heat operation.
Lennox iComfort, Carrier INFINITY: these communicating thermostats show the current operating stage in the home screen. AUX is displayed as “Aux Heat” or Stage 2 heating.
Basic non-smart heat pump thermostats: many simpler heat pump thermostats (2-stage mechanical) don’t display AUX status. If AUX is running constantly and you have a basic thermostat, you may only notice via the electric bill or by checking the outdoor unit.
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When to call a licensed HVAC technician
- AUX heat running at outdoor temperatures above 40°F for more than 20–30 minutes with the outdoor unit not running
- House won’t reach setpoint even with AUX running in normal cold weather
- Emergency heat indicator appears on the thermostat display without you setting it manually
- Outdoor unit is iced over and not defrosting (see heat pump frozen outdoor unit)
- Electric bill is dramatically higher than prior winters with no change in usage or outdoor temperatures
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FAQ
Is it OK to let AUX run all night during a cold snap? Yes. If outdoor temperatures are below 25–35°F, AUX running continuously overnight is normal. The heat pump alone can’t meet the load at those temperatures. It’s expensive — $1–2/hour or more — but it’s the system working as designed.
My thermostat shows AUX but the house is warm. Is it a problem? Not if it’s cold outside. AUX running in cold weather while the house is at setpoint means the system is doing its job. Check the outdoor unit to confirm the heat pump is also running (it should be). If AUX is on in mild weather (50°F+) and running for a long time, investigate.
Can I turn AUX heat off? In most systems, no — AUX is automatic and can’t be disabled without a wiring change. Some thermostats (ecobee) let you set a balance-point temperature above which AUX is not permitted to engage, which effectively limits its use to very cold days. On a Nest, you can contact Nest support to configure the aux heat lockout threshold.
My thermostat switched to EM HEAT by itself. What happened? Thermostats don’t switch to EM HEAT mode on their own in normal operation — it’s a manual selection. If EM is showing without you setting it, either someone else set it, or there’s a wiring issue with the EM terminal. Check the thermostat settings to confirm, then switch back to HEAT mode. If it keeps reverting to EM: check the O/B terminal wiring at the thermostat base → Heat Pump Thermostat Wiring.
My heat pump is 3 years old but AUX runs a lot in January. Is that wrong? Depends on the heat pump type and your climate. Standard air-source heat pumps lose significant capacity below 30–35°F and need heavy AUX support in cold climates (zone 5–6). Cold-climate heat pumps (rated for -13°F to -22°F) maintain capacity at lower temperatures and need much less AUX. If you’re in a cold climate with a standard heat pump, heavy January AUX use may be normal. If you have a cold-climate heat pump and AUX still runs heavily above 25°F, have the refrigerant charge and reversing valve checked.
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