3-Wire Thermostat (Heat + Fan, No C Wire)

Three wires on R, W and G control a furnace’s heat and blower but not AC, and there is no C wire. Some smart thermostats cope; others need extra wire.

Detected system: 1-stage heat, fan, no C wire · 3 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 and too few wires for a furnace adapter. Best fix: run a new 18/5 thermostat cable. Alternative: a plug-in 24 VAC adapter wired to Rc and C, with your existing heat circuit moved to Rh (the ecobee Premium supports split power).
ecobee EnhancedWorks with an adapterNo C wire and too few wires for a furnace adapter. Best fix: run a new 18/5 thermostat cable. Alternative: a plug-in 24 VAC adapter wired to Rc and C, with your existing heat circuit moved to Rh (the ecobee Enhanced supports split power).
Honeywell T9Works with an adapterNo C wire and too few wires for a furnace adapter. Best fix: run a new 18/5 thermostat cable. Alternative: a plug-in 24 VAC adapter wired to Rc and C, with your existing heat circuit moved to Rh (the Honeywell T9 supports split power).
Amazon Smart ThermostatNot compatibleNo C wire and too few wires for an adapter. You’ll need a new 18/5 thermostat cable from the equipment to the wall.

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, fan, no C wire · 3 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 RhWwhite · Heat, stage 1WGgreen · FanG
WireColorJobConnect toNote
Rred24 V powerRc or Rh
WwhiteHeat, stage 1W
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. 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

  • 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 3 wires, heat + fan?

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 3 wires, heat + fan?

Works with an adapter. No C wire and too few wires for a furnace adapter. Best fix: run a new 18/5 thermostat cable. Alternative: a plug-in 24 VAC adapter wired to Rc and C, with your existing heat circuit moved to Rh (the ecobee Premium supports split power). See the ecobee Premium wiring diagram.

Will the ecobee Enhanced work with 3 wires, heat + fan?

Works with an adapter. No C wire and too few wires for a furnace adapter. Best fix: run a new 18/5 thermostat cable. Alternative: a plug-in 24 VAC adapter wired to Rc and C, with your existing heat circuit moved to Rh (the ecobee Enhanced supports split power). See the ecobee Enhanced wiring diagram.

Will the Honeywell T9 work with 3 wires, heat + fan?

Works with an adapter. No C wire and too few wires for a furnace adapter. Best fix: run a new 18/5 thermostat cable. Alternative: a plug-in 24 VAC adapter wired to Rc and C, with your existing heat circuit moved to Rh (the Honeywell T9 supports split power). See the Honeywell T9 wiring diagram.

Will the Amazon Smart Thermostat work with 3 wires, heat + fan?

Not compatible. No C wire and too few wires for an adapter. You’ll need a new 18/5 thermostat cable from the equipment to the wall. See the Amazon Smart Thermostat wiring diagram.

Other wiring setups