Wiring guide
Google Nest Learning Thermostat (4th gen) wiring for b wire used as common
Good news: the Google Nest Learning Thermostat (4th gen) is a direct swap for this setup. Every wire has a matching terminal.
On many Trane, American Standard and Carrier furnaces, the B terminal on the old thermostat is actually the common wire. Put it on O/B and you can blow a fuse or damage the thermostat. Read the full b wire used as common guide for how to confirm your setup and mistakes to avoid.
Wiring diagram
Detected system: 1-stage heat, 1-stage cooling, fan, C wire present · 5 wires
Your B wire is treated as the common (C) wire. That is normal on many Trane, American Standard and Carrier systems. If it lands on a B or O terminal at the equipment, stop and recheck.
Google Nest Learning Thermostat (4th gen)
Works: direct swap| Wire | Color | Job | Connect to | Note |
|---|---|---|---|---|
| R | red | 24 V power | Rc or Rh | |
| B | blue | Common (C wire) | C | |
| W | white | Heat, stage 1 | W | |
| Y | yellow | Cooling, stage 1 | Y | |
| G | green | Fan | G |
Install steps
- Turn off power to the furnace/air handler (switch or breaker) and the AC condenser. Confirm the old thermostat is dead.
- Photograph the old wiring, then label each wire with the letter it came off. Ignore short jumpers.
- Connect each wire to the Nest Learning (4th gen) terminal shown above. Cap unused wires one at a time.
- Restore power and finish setup in the app using the settings below.
- 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
Where does each wire go on the Nest Learning (4th gen)?
R → Rc or Rh; B → C; W → W; Y → Y; G → G.
Do I need a C wire for the Nest Learning (4th gen)?
You already have a C wire, so no adapter is needed.