Aux Heat Running
Ecobee Thermostat
Severity: MinorWhat Does This Error Mean?
Ecobee 'Aux Heat Running' shows when the thermostat has activated the auxiliary electric heat strips on a heat pump system.
It is not a fault — it is a status notification.
Aux heat is normal during very cold weather and after long temperature setbacks.
The reason to pay attention is cost: aux heat uses 2-3x the electricity of the heat pump alone.
Affected Models
- Ecobee Smart Thermostat Premium
- Ecobee Smart Thermostat Enhanced
- Ecobee SmartThermostat with voice control
- Ecobee3 lite
- Ecobee3 (heat pump configurations)
Common Causes
- Outdoor temperature below the heat pump's economic balance point
- Setpoint raised more than 1-2°F above current room temperature
- After a long away setback (Smart Recovery or Vacation mode ending)
- Aux Heat Threshold setting too high in installation settings
- Heat pump compressor failing — system relies on aux to keep up
- Defrost cycle running — aux helps temper the cold air
How to Fix It
-
Check outdoor temperature.
If outside is below 30°F, aux heat is expected.
Heat pumps lose efficiency in cold weather and the strips fill the gap.
Once the weather warms, aux usage drops on its own. -
Avoid large setpoint jumps.
Raising the thermostat from 65 to 72 in one step triggers aux heat almost guaranteed.
Raise in 1°F increments over an hour or two and the heat pump alone usually keeps up — at much lower cost. -
Adjust the Aux Heat Threshold.
Settings → Installation Settings → Thresholds → Compressor Heat Min Outdoor Temp.
Setting this lower (e.g. 30°F instead of 35°F) tells Ecobee to keep using the heat pump in colder weather before falling back to aux.
Most modern heat pumps work efficiently down to 30°F. -
Disable Smart Recovery if you do not want it.
Smart Recovery aggressively pre-heats before scheduled times.
It often calls aux to hit the schedule on the dot.
If you would rather wait an extra 15 minutes for heat pump alone, turn Smart Recovery off in Settings → Preferences. -
Check the home report.
On Premium models, the Home IQ report shows aux runtime per day.
Sudden spikes on mild days suggest the heat pump compressor is starting to fail.
Schedule HVAC service if aux usage rises in moderate weather. -
Investigate if aux runs constantly.
Aux running for hours at mild outdoor temperatures usually means the compressor is failing or low on refrigerant.
The heat pump cannot keep up, so aux fills in.
This is expensive and points to system service.
Frequently Asked Questions
How much extra does aux heat cost?
Heat strips run at 100% electrical input.
A heat pump operating efficiently delivers 2-3 units of heat per unit of electricity.
Running aux instead of heat pump alone roughly doubles or triples the heating bill for those hours.
Worth watching.