E Terminal on a Thermostat: Emergency Heat Wiring

E turns on backup heat without the heat pump, for when the outdoor unit fails. On many systems, E and AUX run the same heat strips and are tied together at the air handler.

ecobee and the Amazon Smart Thermostat have no E terminal: emergency heat mode uses the aux heat wire instead. If your E and AUX wires land on the same terminal at the air handler, the E wire can be capped. If they go to different places, get a pro to confirm before capping it.

Where it connects on each smart thermostat

Heat pump with AUX + E

Wires: R, C, Y, G, O, AUX, E · open in the tool

ThermostatE goes toWhole setup
Nest Learning (4th gen)EWorks: direct swap
ecobee PremiumVerifyProbably works: verify first
ecobee EnhancedVerifyProbably works: verify first
Honeywell T9EWorks: direct swap
Amazon Smart ThermostatVerifyProbably works: verify first

Other wire letters