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I am adding a Nest (not Nest Thermostat E or Nest Learning Thermostat) to a 2-wire boiler system. Fortunately, the existing thermostat wire has free conductors. So I connected a 3rd wire from the C terminal of the thermostat to the 24V transformer Common.

So far, so good. It seems to be working well. But when I go into the Nest settings, I see the Nest Power Connector has not been configured. I am not using the NPC. I am providing 24V from my boiler.

My question is this:

Should I disable the NPC in the thermostat settings? This would imply this setting is enabled only when explicitly using the NPC. Or should it be enabled if the C wire is connected by any means?

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  • The Nest is looking for 24 Volt on R and Common wires.
    – Traveler
    Jan 20, 2023 at 20:23
  • @Ruskes So I should have 3 wires connected to Nest - C, W & R, correct? C is wired to 24V common, W = 24V Common & R = Zone valve (yellow).
    – Bassman
    Jan 20, 2023 at 21:17
  • Nope, the W is for heating signal the R and C go to transformer
    – Traveler
    Jan 20, 2023 at 22:52
  • @Ruskes I think I get it: W is unchanged. R & C now get fed from the external 24V transformer. But what do I do with the existing R wire? Originally the thermostat had R & W. If I simply disconnect the original R, it doesn't work. Do I connect it to R terminal along with xformer's R? I'm afraid this will fry something.
    – Bassman
    Jan 25, 2023 at 14:50
  • Your original thermostat was battery operated, so did not need the C. However it used the R (24 Volt) to send signal to the heating on W. The new thermostat needs R+C for it self to work. Yes you can conect the old R to the new R
    – Traveler
    Jan 25, 2023 at 18:45

2 Answers 2

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Your original thermostat was battery operated, so did not need the C since it got the operating power from batteries.

However it uses the R (24 Volt) to send signal to the heating on W.

The new thermostat needs R+C for it self to work.

Yes you can conect the old R to the new R

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If it's working and not giving you power error codes, so far as I can tell from the documentation you can completely ignore that setting.

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