Wiring guide
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.
Smart thermostat compatibility
| Thermostat | Verdict | What to know |
|---|---|---|
| Nest Learning (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. |
| ecobee Premium | 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). |
| ecobee Enhanced | 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). |
| Honeywell T9 | 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). |
| Amazon Smart Thermostat | 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. |
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 | Rh | Rh | Rh | R |
| W (white) | W | W1 | W1 | W | W |
| G (green) | G | G | G | G | G |
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
Three wires on R, W and G, with no Y. Your furnace has a blower you can switch on by itself, but the thermostat was never wired for air conditioning. If you do have central AC, the cooling wire may be connected somewhere else, so check the furnace control board before you assume.
Mistakes to avoid
- Moving G to C to power the new thermostat and losing the ability to run the fan by itself. That trade-off is fine for some people, but it should be a choice, not an accident.
- Guessing which terminal a wire goes to at the furnace board from its color. Read the letter on the board instead.
- Leaving a bare copper end touching the wall plate after capping an unused conductor.
Our advice
Look behind the wall plate and at the furnace for extra conductors that are folded back and unused. A spare wire in the same cable can become your C wire with no new cable. Connect it to C at both ends.