Thermostat guide
Google 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).
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.
What’s different about wiring the Nest Learning (4th gen)
The Nest Learning Thermostat can often run without a C wire by drawing small amounts of power through the heating or cooling wires. It works on many furnace + AC systems, but not on all of them. Heat-only, cool-only and heat pump systems are where it most often causes trouble, and Google recommends a C wire or its Nest Power Connector for those.
It has a single *OB terminal for the reversing valve. You choose O or B in the app. It also has a separate E terminal for emergency heat. Google recommends a professional install if you have separate Rh and Rc wires.
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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 |