I have an old thermostat with only 2 wires connected to my Weil-McLain CGA Series 2 boiler. I would like to replace it with a Nest thermostat. I need to add a C-Wire in order for it to work. Luckily, the cable coming from my thermostat already has a 3rd wire that I can use. I just need to find where to connect the C wire on the boiler side. In attached picture, blue pen drawing shows existing wirings. The Low Water Cut Off has 4 wires, and one is already connected to the C Common terminal. Can I just connect my thermostat’s C wire to the same terminal? (Green pen in drawing) Thanks!

Aquastat is I believe Honeywell L4006E1067. Low Water Cut Off is GuardDog LB-24E. Orange and blue nuts connectors in picture connect all the above limits + thermostat in series to the thermostat connector on boiler. All other limits and external devices are mounted per manual diagram.

Thermostat wiring diagram

Boiler wiring from Manual

Boiler wiring picture

Zoom on upper wiring

Zoom on lower wiring

Additional Wiring picture

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  • Do you have any field-fitted limits wired in series with the spill switch and thermal fuse on your boiler? – ThreePhaseEel Feb 1 '19 at 4:35
  • In fact, can you post a photo of the wiring on your boiler? – ThreePhaseEel Feb 1 '19 at 4:37
  • Also, what model is your Aquastat? – ThreePhaseEel Feb 1 '19 at 4:39
  • Hi ThreePhaseEel, I added the missing info. Thanks a lot – Brice Feb 1 '19 at 5:15
  • 1
    Does the wire to the thermostat happen to have 4 wires? If so you could use a separate transformer to power the Nest. It looks like the way everything is wired now, even if you had a C-wire the nest could loose power if the aquastat or LWCO is tripped. (not an expert on your type of system tho) – JPhi1618 Feb 1 '19 at 5:35

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