Heat Pump Thermostat Wiring (5 Wires, No C Wire)

A heat pump with R, Y, G, O/B and AUX has a reversing valve and backup electric heat but no C wire. Heat pumps are exactly where smart thermostats get picky about power.

Detected system: Heat pump, 1-stage compressor, 1-stage aux heat, fan, no C wire · 5 wires

Smart thermostat compatibility

ThermostatVerdictWhat to know
Nest Learning (4th gen)Works with an adapterGoogle says heat-only, cool-only and heat pump systems without a C wire need a C wire or the Nest Power Connector.
ecobee PremiumWorks with an adapterNo C wire: use the Power Extender Kit that comes in the box. It installs at your furnace/air handler.
ecobee EnhancedWorks with an adapterNo C wire: use the Power Extender Kit that comes in the box. It installs at your furnace/air handler.
Honeywell T9Works with an adapterNo C wire: install a Honeywell wire saver at the furnace. It frees up a wire to become C, with the Y wire moving to K.
Amazon Smart ThermostatWorks with an adapterNo C wire: use Amazon’s C-wire power adapter. Per Amazon’s guide, the G wire moves to C and the Y wire moves to K.

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

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

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?

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?

4Your wiring

Your system: Heat pump, 1-stage compressor, 1-stage aux heat, fan, no C wire · 5 wires

Which smart thermostats work with your wires?

Google Nest Learning Thermostat (4th gen)

Works with an adapter
Google says heat-only, cool-only and heat pump systems without a C wire need a C wire or the Nest Power Connector.
Wires from the wallNest Learning (4th gen)Rred · 24 V powerRc or RhYyellow · Cooling, stage 1YGgreen · FanGO/Borange · Reversing valve (O/B)*OBAUXwhite · Auxiliary heatAUX
WireColorJobConnect toNote
Rred24 V powerRc or Rh
YyellowCooling, stage 1Y
GgreenFanG
O/BorangeReversing valve (O/B)*OB
AUXwhiteAuxiliary heatAUX

Install steps

  1. Turn off power to the furnace/air handler (switch or breaker) and the AC condenser. Confirm the old thermostat is dead.
  2. Photograph the old wiring, then label each wire with the letter it came off. Ignore short jumpers.
  3. Install the adapter at your furnace or air handler following its instructions, with power still off.
  4. Connect each wire to the Nest Learning (4th gen) terminal shown above. Cap unused wires one at a time.
  5. Restore power and finish setup in the app using the settings below.
  6. 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 or B (O for most brands; B for Rheem, Ruud and some others).
  • 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
  • Google Nest Power ConnectorSteady power when there’s no C wireCheck 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, no c wire?

Works with an adapter. Google says heat-only, cool-only and heat pump systems without a C wire need a C wire or the Nest Power Connector. See the Nest Learning (4th gen) wiring diagram.

Will the ecobee Premium work with heat pump, no c wire?

Works with an adapter. No C wire: use the Power Extender Kit that comes in the box. It installs at your furnace/air handler. See the ecobee Premium wiring diagram.

Will the ecobee Enhanced work with heat pump, no c wire?

Works with an adapter. No C wire: use the Power Extender Kit that comes in the box. It installs at your furnace/air handler. See the ecobee Enhanced wiring diagram.

Will the Honeywell T9 work with heat pump, no c wire?

Works with an adapter. No C wire: install a Honeywell wire saver at the furnace. It frees up a wire to become C, with the Y wire moving to K. See the Honeywell T9 wiring diagram.

Will the Amazon Smart Thermostat work with heat pump, no c wire?

Works with an adapter. No C wire: use Amazon’s C-wire power adapter. Per Amazon’s guide, the G wire moves to C and the Y wire moves to K. See the Amazon Smart Thermostat wiring diagram.

Other wiring setups