4-Wire Thermostat Wiring (No C Wire)

R, W, Y and G is the classic furnace + central AC setup found in millions of US homes, one wire short of what most smart thermostats want. The fix is usually an adapter at the furnace, not new wire.

Detected system: 1-stage heat, 1-stage cooling, fan, no C wire · 4 wires

Smart thermostat compatibility

ThermostatVerdictWhat to know
Nest Learning (4th gen)Works: direct swapNo C wire: Nest usually runs on power sharing. If it loses Wi-Fi or its battery drains, add 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: 1-stage heat, 1-stage cooling, fan, no C wire · 4 wires

Which smart thermostats work with your wires?

Google Nest Learning Thermostat (4th gen)

Works: direct swap
No C wire: Nest usually runs on power sharing. If it loses Wi-Fi or its battery drains, add the Nest Power Connector.
Wires from the wallNest Learning (4th gen)Rred · 24 V powerRc or RhWwhite · Heat, stage 1WYyellow · Cooling, stage 1YGgreen · FanG
WireColorJobConnect toNote
Rred24 V powerRc or Rh
WwhiteHeat, stage 1W
YyellowCooling, stage 1Y
GgreenFanG

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. Connect each wire to the Nest Learning (4th gen) terminal shown above. Cap unused wires one at a time.
  4. Restore power and finish setup in the app using the settings below.
  5. Test heat, cool and fan. If a breaker trips or equipment runs nonstop, power off and call a pro.

App settings to choose

  • 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 4 wires, no c wire?

Works: direct swap. No C wire: Nest usually runs on power sharing. If it loses Wi-Fi or its battery drains, add the Nest Power Connector. See the Nest Learning (4th gen) wiring diagram.

Will the ecobee Premium work with 4 wires, 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 4 wires, 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 4 wires, 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 4 wires, 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