Looking to install a C-wire for my power stealing nest. I have 5 wires up at the thermostat and same at the boiler. I am using red and white at the thermostat however it looks like blue and white it’s being used at the boiler. enter image description here

See below.

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thermostat wire enter image description here

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  • Can you get us a clear shot of the wiring diagram label please? The photo of it you posted is rather blurry... – ThreePhaseEel Feb 7 at 15:59
  • upload a new photo – Astoria718 Feb 7 at 18:14

The soldered C terminal is labeled on the transformer board. To confirm you should be able to follow that wire to a ground terminal. You might have to cut and splice that wire.

That being I'm not convinced from the blurry diagrams that the standard furnace wiring setup will work with your boiler, after connecting the C wire you may need to connect the R from the boiler transformer board to the Rc terminal on the nest and use the Rh/W1 on the nest as dry contacts for the two wires currently connected to the old stat.

  • Thanks - I added a better shot. – Astoria718 Feb 7 at 16:48
  • The black wire goes into the relay receptacle which I do not think is being used. There are two bare wires black and yellow in the box behind the transformer. (i.stack.imgur.com/xXodp.jpg) (i.stack.imgur.com/SWVZa.jpg) – Astoria718 Feb 9 at 0:00
  • Any final thoughts with the update picture and details? – Astoria718 Feb 12 at 0:11
  • Clearer diagram confirms that R is 24vac and C is Common similar to labelling on a standard forced air furnace, but still leery that Y,G, and W on the circuit board don't trace out to similar functions, and the function of "cable to damper" leaves much to imagination. Hopefully Nest has your boiler in their configuration database. – NoSparksPlease Feb 12 at 1:01

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