Wiring guide
Google Nest Learning Thermostat (4th gen) wiring for k wire (Honeywell wire saver)
This setup is possible with the Google Nest Learning Thermostat (4th gen), but a professional install is recommended.
A wire on K means a Honeywell wire saver module is installed at the furnace, combining fan and cooling on one wire to free up a C wire. Only some thermostats understand K. Read the full k wire (Honeywell wire saver) guide for how to confirm your setup and mistakes to avoid.
Wiring diagram
Detected system: 1-stage heat, wire saver on K (fan + cooling), C wire present · 4 wires
Google Nest Learning Thermostat (4th gen)
Pro install recommended| Wire | Color | Job | Connect to | Note |
|---|---|---|---|---|
| R | red | 24 V power | Rc or Rh | |
| C | green | Common (C wire) | C | |
| W | white | Heat, stage 1 | W | |
| K | yellow | Wire saver (fan + cool) | Don’t connect | No K terminal. A pro must remove the wire saver and restore G and Y. |
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
As an Amazon Associate we earn from qualifying purchases, at no cost to you.
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; C → C; W → W; K → not connected.
Do I need a C wire for the Nest Learning (4th gen)?
You already have a C wire, so no adapter is needed.