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correct wiring for one wire heat + external C wire

IWM3
Community Member

hopefully easy question. Have older hot water boiler with a single thermostat wire (white and black). Previous thermostat required C wire so ran external C wire adapter (single wire, white, red).  Now want to use Nest Learning 3 thermostat. Apparently after trying with just the boiler wire, it does not want to charge the battery sufficiently, so want to hook up previously used external C wire. What is correct placement of boiler wires (white, black) and C wire (white, red). I assume boiler white goes to W1, boiler black to Rh, C white to C, but not sure where C red should go. Or if I have everything incorrect???  thanks for help

 

1 REPLY 1

Markjosephp
Community Specialist
Community Specialist

Hello IWM3,

Thank you for reaching out with your inquiry regarding the wiring of your Nest Learning Thermostat in connection with an older hot water boiler system. We appreciate your detailed description of the existing wiring setup and your attempt to integrate the Nest Thermostat into the system.

It seems you are on the right track with your assumptions. To clarify and guide you through the wiring process, please follow the suggested connections below:

 

  • Boiler White (Thermostat Wire) to Nest W1
  • Boiler Black (Thermostat Wire) to Nest Rh
  • External C Wire White to Nest C
  • External C Wire Red - This wire may not be needed, as the common (C) wire typically only requires one connection. However, to have a better picture, share with us a clear photo of your old thermostat's wiring wiring setup.

 

Keep in mind that the Nest Learning Thermostat is designed to be compatible with a wide range of heating and cooling systems. However, variations in wiring setups may exist, and it's crucial to refer to the Nest Thermostat installation guide.

If you encounter any challenges or have further questions, let me know.

Regards,
Mark