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.

Detected system: Heat pump, 1-stage compressor, 1-stage aux heat, emergency heat, fan, C wire present · 7 wires

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.

Your wiring is pre-filled below. Change any terminal to match your wall exactly.

1Snap a photo of your old thermostat’s wires (optional)

📷 Upload or take a photo

AI reads the terminal letters and wire colors, then you double-check them. Photos are not saved.

2Tick every terminal that has a wire

Pull the faceplate off your old thermostat. Use the letter printed next to each wire, not its color. Skip short jumper wires between two terminals.

More labels (W3, X, X2, K, PEK, L, S, T, U, HUM, DEHUM)

3A few quick questions

Do you have a heat pump?

YesNoNot sure

A heat pump is an outdoor unit that both heats and cools. Heat pump thermostats usually have an “EM HEAT” or “AUX” setting.

Do any of these apply? Thick wires joined with wire nuts, 120/240 V labels, or electric baseboard heaters (line voltage) Gas wall/floor heater or fireplace that runs without household power (millivolt) Numbered terminals (1, 2, 3, 4 or A, B, C, D), e.g. Carrier Infinity or Lennox iComfort

4Your wiring

Your system: Heat pump, 1-stage compressor, 1-stage aux heat, emergency heat, fan, C wire present · 7 wires

Which smart thermostats work with your wires?

Google Nest Learning Thermostat (4th gen)

Works: direct swap

Wire	Color	Job	Connect to
R	red	24 V power	Rc or Rh
C	blue	Common (C wire)	C
Y	yellow	Cooling, stage 1	Y
G	green	Fan	G
O	orange	Reversing valve (O/B)	*OB
AUX	white	Auxiliary heat	AUX
E	brown	Emergency heat	E

Install steps

Turn off power to the furnace/air handler (switch or breaker) and the AC condenser. Confirm the old thermostat is dead.
Photograph the old wiring, then label each wire with the letter it came off. Ignore short jumpers.
Connect each wire to the Nest Learning (4th gen) terminal shown above. Cap unused wires one at a time.
Restore power and finish setup in the app using the settings below.
Test heat, cool and fan. If a breaker trips or equipment runs nonstop, power off and call a pro.

App settings to choose

Reversing valve (O/B): set to O.
Equipment type: heat pump with auxiliary (electric strip) heat.
Single R wire: the Nest app tells you whether to use Rc or Rh.

What you need

Google Nest Learning Thermostat (4th gen)The thermostat itselfCheck price

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Confirm against the official Google Nest install guide. The app also checks your wiring during setup.

Frequently asked questions

Will the Nest Learning (4th gen) work with heat pump + emergency heat?

Works: direct swap. Every wire has a matching terminal, so it’s a direct swap. See the Nest Learning (4th gen) wiring diagram.

Will the ecobee Premium work with heat pump + emergency heat?

Probably works: verify first. See the ecobee Premium wiring diagram.

Will the ecobee Enhanced work with heat pump + emergency heat?

Probably works: verify first. See the ecobee Enhanced wiring diagram.

Will the Honeywell T9 work with heat pump + emergency heat?

Works: direct swap. Every wire has a matching terminal, so it’s a direct swap. See the Honeywell T9 wiring diagram.

Will the Amazon Smart Thermostat work with heat pump + emergency heat?

Probably works: verify first. See the Amazon Smart Thermostat wiring diagram.