Wire My ThermostatGoogle Nest Learning Thermostat (4th gen) wiring guide
Learns your schedule, supports up to 3 heat and 2 cool stages, heat pumps with 2 stages of aux heat, and dual transformers (pro install recommended).
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What it supports
| Conventional systems | Up to 3 heat / 2 cool stages |
|---|---|
| Heat pumps | 2 compressor stages, 2 aux stages, separate E terminal |
| Dual fuel | Supported |
| Dual transformer (Rh/Rc) | Supported, pro install recommended |
| Power / C wire | Usually runs without a C wire; heat-only, cool-only and heat pump systems need a C wire or the Nest Power Connector. |
| Line voltage / millivolt | Not compatible |
Where your old wires go
| Old terminal | Nest Learning (4th gen) terminal |
|---|---|
| R (single transformer) | Rc or Rh |
| Rh + Rc (dual transformer) | Rh |
| C (common) | C |
| W / W1 (heat) | W |
| W2 (2nd-stage heat) | W2 |
| AUX / W2 on a heat pump | AUX |
| E (emergency heat) | E |
| Y / Y1 (cool) | Y |
| Y2 (2nd-stage cool) | Y2 |
| G (fan) | G |
| O, B or O/B (reversing valve) | *OB |
| K (wire saver) | None |
| L / L/A (service light) | None |
B, X and K depend on your system. Use the tool to check. Confirm with the official guide.
Nest Learning (4th gen) with common wiring setups
| Setup | Verdict |
|---|---|
| 2-Wire Thermostat (Heat Only) | Works with an adapter |
| 3-Wire Thermostat (Heat + Fan, No C Wire) | Works with an adapter |
| 4-Wire Thermostat Wiring (No C Wire) | Works: direct swap |
| 5-Wire Thermostat Wiring (With C Wire) | Works: direct swap |
| Cooling-Only Thermostat Wiring (AC + Fan) | Works: direct swap |
| 2-Stage Heat & 2-Stage Cool Thermostat Wiring | Works: direct swap |
| Heat Pump Thermostat Wiring (5 Wires, No C Wire) | Works with an adapter |
| Heat Pump Thermostat Wiring (6 Wires With C Wire) | Works: direct swap |
| Heat Pump Wiring With AUX and E (Emergency Heat) | Works: direct swap |
| Heat Pump With a B Wire (Rheem / Ruud Style) | Works: direct swap |
| B Wire as Common (Trane / American Standard Style) | Works: direct swap |
| Rh and Rc Wires (Dual Transformer) Thermostat Wiring | Pro install recommended |
| K Terminal Wiring (Honeywell Wire Saver) | Pro install recommended |
Check your own wiring
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
4Your wiring
Tick at least one terminal above and your wiring shows up here instantly.