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C Wire Gas Boiler Wiring

bbnyc Member Posts: 1
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.
Thanks !


  • STEVEusaPA
    STEVEusaPA Member Posts: 6,132
    edited February 2019
    Follow a green wire and send the common from the thermostat to the ground screw.
  • 991blk
    991blk Member Posts: 1
    I have the same need for C wire. Can you post where to connect for C wire?
  • EdTheHeaterMan
    EdTheHeaterMan Member Posts: 5,652
    @bbnyc, your boiler has two black wires that go to the R and W terminals on the thermostat. If Weil McLain were user friendly, one of those black wires would be Red and the other would be White. But that would make it too easy. One of the black wires should be red and the other should be the white. You may need a multi-meter to determine which is which. It is not important if you don’t use a thermostat that requires a C (common) wire.

    By using a thermostat that requires a common , it now is important to know which is which.
    Edward F Young. Retired HVAC ContractorSpecialized in Residential Oil Burner and Hydronics
  • EdTheHeaterMan
    EdTheHeaterMan Member Posts: 5,652
    edited January 2021
    It appears that the hand drawn diagram you made will operate properly. If the L4006 or the LWCO ever trips, the Nest will show an error message “ no power to thermostat “. I think it is E72 but I’m not 100% sure of the error number.
    Edward F Young. Retired HVAC ContractorSpecialized in Residential Oil Burner and Hydronics