I would like to install a Sensi thermostat which requires a c-wire. I am planning to run a new thermostat wire, since the old one was 18/2. I am trying to figure out the correct way to wire the c-wire for the new thermostat wire, if possible. The wiring diagram for the boiler can be found here:


The set up for my boiler is on page 30:

Wiring Diagram

Also, see the similar question How to connect c-wire to Burnham Series 2 boiler?. It seems in this case that 24v power was able taken from an extension plug for unused accessories, which is not applicable to this situation as far as I can tell.

Thanks for any help that you can provide!

Edit: Here are some additional pictures:

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  • Can you post photos of the wiring compartment on your boiler please? – ThreePhaseEel May 5 at 0:28
  • Thanks for the reply, @ThreePhaseEel. I have posted some additional pictures- let me know if there is anything that I've missed that would help! – user1001938475 May 5 at 2:52

Connect C to the wire-nutted bundle of yellow wires

According to the manual for your unit, the C-wire coming off your transformer is a yellow wire (we know this because the manual has it connected to ground, among other things), that is joined with that green grounding wire and a bunch of other yellow wires, including one to the vent damper plug. Given that the vent damper connector is likely the plug we see at the bottom of your photos, based on the wire colors going to it, we can then deduce that the junction of yellow wires in your photos is where your C wire should connect to.

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  • It is working properly. Thank you! – user1001938475 May 9 at 14:27

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