Wire letter guide
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.
When to use emergency heat
Emergency heat is for when the outdoor unit can’t run: it is damaged, iced solid, or waiting for a repair. Because it heats with electric strips (or the furnace on dual fuel) and never uses the heat pump, it is usually far more expensive to run. Use it until the heat pump is fixed, then switch back to normal heating.
Seeing the aux heat indicator come on during cold snaps or defrost cycles is normal. Emergency heat is something you switch on yourself.
Where the E wire goes on each thermostat
- Honeywell Home T9: has its own E terminal, so an E wire can be connected directly.
- Google Nest Learning Thermostat (4th gen): supports a separate emergency heat wire; the table below shows where it connects.
- ecobee and Amazon: no E terminal. Emergency heat mode runs the aux heat output instead, which is why the E wire can often be capped (see below).
Checking whether E and AUX are joined
With the power off, open the air handler and find where the E and AUX (or W2) wires from the thermostat cable land. If both go to the same terminal, or are joined together with a jumper, they run the same heat, and the E wire can be capped when your new thermostat has no E terminal. If they go to different terminals or relays, the system has separate emergency heat wiring, and it is worth having a technician confirm how it should be connected.