If your Ecobee’s auxiliary heat runs too often, the usual causes are thermostat settings that let aux heat start too early or at too high an outdoor temperature, large temperature setbacks that the heat pump can’t recover from quickly, or a heat pump that’s undersized or not working properly. Adjusting a few settings in the Ecobee’s installation menu usually fixes it, and can cut your winter electric bill noticeably because backup strip heat costs 2 to 3 times more to run than the heat pump.
Why aux heat matters
Aux heat (usually electric resistance strips in the air handler) helps the heat pump on very cold days and during defrost. Seeing it occasionally is normal. Seeing it on mild days above roughly 35 to 40°F usually means the settings are too aggressive or something is wrong. See heat pump emergency heat for the difference between aux and emergency heat.
Settings to check
Menu names vary slightly by model and software version; on most Ecobee thermostats, they’re under Main Menu > Settings > Installation Settings > Thresholds (some require you to acknowledge a warning).
| Setting | What it does | Suggested starting point* |
|---|---|---|
| Aux heat max outdoor temperature | Aux heat won’t run above this outdoor temperature | 35 to 40°F (lower in mild climates) |
| Compressor min outdoor temperature | Heat pump won’t run below this | Set low (for example 0 to 10°F) on cold-climate heat pumps; follow your contractor’s advice |
| Heat differential / temperature difference to engage aux | How far below setpoint the room must fall before aux runs | 2 to 3°F rather than 1°F |
| Compressor to aux delta / time | How long or how far behind the compressor must be before aux helps | Increase the delay so the heat pump has time |
| Smart Recovery | Starts heating early to reach scheduled temps; can bring on aux | Turn off, or use smaller setbacks |
| Aux heat simultaneous operation | Whether aux can run alongside the heat pump | Enabled is usually more comfortable; follow your installer |
*Starting points only. The right values depend on your heat pump’s capacity, your climate and your comfort. If your contractor set specific thresholds, ask before changing them.
Other causes
- Large setbacks: dropping the temperature 6 to 8 degrees overnight forces aux heat in the morning. Try 2 to 3 degrees.
- Dirty filter or iced outdoor unit: see heat pump covered in ice.
- Low refrigerant or a failing heat pump: if the heat pump isn’t producing warm air, the thermostat calls for aux.
- Undersized heat pump: see our heat pump size calculator.
- Wiring: aux heat (W2/AUX) wired to the wrong terminal. See thermostat wiring colors.
How to check how much aux heat you’re using
The Ecobee app and web portal show equipment run time, including aux heat, under Home IQ or system monitoring reports. Compare aux hours with outdoor temperatures to see whether it’s running when it shouldn’t.
Have a Nest instead? See Nest thermostat using aux heat too much.
If your ecobee isn’t heating at all, or shows “Calibrating”, see ecobee not heating.
Frequently asked questions
Is it bad if aux heat comes on?
No. It’s normal during defrost and on very cold days. It becomes a problem when it runs often in mild weather.
What temperature should aux heat turn on?
Many homes set the aux heat max outdoor temperature around 35 to 40°F, but cold-climate heat pumps can often go lower.
Can I turn aux heat off completely?
You can limit it with thresholds, but on the coldest days you may need it to stay comfortable.
Disclaimer: this guide is for general information only. Turn off power at the breaker before touching thermostat wiring. Contact a licensed HVAC professional for electrical diagnosis.