Wiring guide
Heat Pump Wiring With AUX and E (Emergency Heat)
Separate AUX and E wires are common on older heat pumps. Many smart thermostats have no E terminal, so what happens to that wire decides whether you have a direct swap.
Smart thermostat compatibility
| Thermostat | Verdict | What to know |
|---|---|---|
| Nest Learning (4th gen) | Works: direct swap | Direct swap: every wire has a matching terminal. |
| ecobee Premium | Probably works: verify first | No E terminal. If E and AUX land on the same terminal at the air handler (common), cap E. Emergency heat then uses the aux wire. |
| ecobee Enhanced | Probably works: verify first | No E terminal. If E and AUX land on the same terminal at the air handler (common), cap E. Emergency heat then uses the aux wire. |
| Honeywell T9 | Works: direct swap | Direct swap: every wire has a matching terminal. |
| Amazon Smart Thermostat | Probably works: verify first | No E terminal. If E and AUX land on the same terminal at the air handler (common), cap E. Emergency heat then uses the aux wire. |
Where each wire goes, thermostat by thermostat
| Old wire | Nest Learning (4th gen) | ecobee Premium | ecobee Enhanced | Honeywell T9 | Amazon Smart Thermostat |
|---|---|---|---|---|---|
| R (red) | Rc or Rh | Rc | Rc | R | R |
| C (blue) | C | C | C | C | C |
| Y (yellow) | Y | Y1 | Y1 | Y | Y |
| G (green) | G | G | G | G | G |
| O (orange) | *OB | O/B | O/B | O/B | O/B |
| AUX (white) | AUX | W1 | W1 | W2 / Aux | W |
| E (brown) | E | Verify first | Verify first | E | Verify first |
Colors are typical, not a rule: always go by the letter the wire was connected to. Click a thermostat for its full diagram, parts and app settings.
How to confirm this is your setup
A heat pump setup with separate wires on AUX and E. On many systems the E wire simply connects to the same backup heat as AUX, and it lets the thermostat force backup heat on when the outdoor unit fails. Check the air handler: if E and AUX land on the same terminal, or are jumpered together there, you don’t need a separate E terminal.
Mistakes to avoid
- Capping E without checking whether it drives anything different from AUX. Usually it doesn’t, but some systems use E for a separate relay.
- Assuming a thermostat without an E terminal has no emergency heat. Most offer an emergency heat mode that uses the AUX terminal instead.
Our advice
If E and AUX go to the same place at the air handler, the usual fix is to put AUX on the thermostat’s AUX or W1 terminal and cap E. Nest Learning (4th gen) and Honeywell T9 have a dedicated E terminal if you want to keep it separate.