I'm looking to add a C-wire for my thermostat. My current thermostat has two wires, W and R. But there is another tucked in blue wire. That blue wire isn't connected to the boiler. I'm looking for help on where on the terminal can I connect this wire to make it into a C-wire.

The manual for the boiler can be found here

Thanks for reading

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EDIT 2: Added the labels for the Terminal and Relay

enter image description here Transformer label

enter image description here Relay Terminals part 1

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EDIT 3: Closer view on Relay

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  • Can you post any wiring diagram that may be present on the boiler itself? – ThreePhaseEel Nov 17 '20 at 4:55
  • @ThreePhaseEel Thanks for the reply! Unfortunately I can't find any other diagrams on the boiler itself. – Jeremy Bertin Nov 17 '20 at 18:09
  • Can you get us a clear shot of the labeling on the control transformer then? – ThreePhaseEel Nov 18 '20 at 1:06
  • @ThreePhaseEel Yeah sorry, I added a few more pictures. Hopefully it helps. Thanks – Jeremy Bertin Nov 18 '20 at 2:55
  • Sadly, none of the additional photos help me figure out what's going on with the transformer's wiring -- I need to see the labeling on the transformer for that – ThreePhaseEel Nov 18 '20 at 3:02

The transformer label says "PRIMARY: BLK & WHT". So its black and white wires are the 120 V ("primary") connections. Its other wires are the "secondary", ie the 24 V connections. The theory is simple enough: one of the transformer secondary wires should lead directly to the R terminal; the other one is C. I'm not certain but it looks like the secondary wires might both be blue.

Figuring out which is which, and sorting out whether your thermostat wires are reversed, is a little more complicated. Start by disconnecting the thermostat from its wiring. We'll re-connect it later.

Next let's identify which of the thermostat terminals is the R terminal. Start at either of the terminals and trace the wiring, working toward the transformer. It might help to also start at the transformer and trace one of the wires, working toward the thermostat terminals. Eventually you'll find a continuous wiring path between one of the secondary wires and one of the thermostat terminals. That terminal is the R terminal. If your W wire is connected to it then swap the R and W wire connections.

Finally, track the C connection from the transformer secondary out to some place that's convenient to tap. Just follow it along until you get to a terminal where the C wire could be reasonably attached. Ideally it should be on the right-hand side of that barrier which divides the low-voltage thermostat stuff from the mains-voltage stuff. When you do find a good location, connect your spare (blue?) thermostat conductor there.

Now it's time to test. With the thermostat still disconnected turn the system power on. Measure AC voltage between the thermostat R and W wires and between the R and C wires. You should find something like 24-28 volts between each of those pairs, and about 0 volts between W and C. Next, short R to W to call for heat. The system should begin heating in its usual way and continue as long as R and W are shorted.

With those checks passed, go ahead and connect the thermostat.

  • According to the documentation for the transformer in the OP's boiler, the secondary wires are blue and yellow – ThreePhaseEel Nov 19 '20 at 0:01
  • Yeah the transformer's secondary yellow wire goes directly to the thermostat's W wire ( I will swap the W and R wires like you suggest). So that would mean that the transformers C would be the blue wire. There is a white wire on the right-hand side of the barrier separation that can be followed back to the blue wire secondary on the transformer. So based on what you said I believe I can tap into that wire for the thermostat's C. – Jeremy Bertin Nov 19 '20 at 0:24
  • Thank you for the very detailed answer and explaining it in a way that is eazy to follow. I believe that with this information I should have what I need to figure it out. I will try to work it out this weekend. Thank you so much, I really appreciate it. Thanks to all who read my post and thanks @ThreePhaseEel for all the help as well. – Jeremy Bertin Nov 19 '20 at 0:26
  • Worked like a charm, thanks again! – Jeremy Bertin Nov 22 '20 at 6:43

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