I have Weil-McLain gas boiler for hot water heat and going to install WiFi thermostat that requires common 24V 'C' wire. I do have a free wire in the cable but cannot figure out where should I connect it on the boiler, there's no any terminal, marked as 'C', 'common' or similar. The system includes hot water boiler and central AC. Currently I use thermostat with battaries that has 5 wires going in.

  • Just a heads up that there's been a suggestion to make a generic question about how to locate the C wire terminal on the HVAC/Boiler the encompass many similar questions. If you'd like to rephrase this question to be more generic, we can use this rather than considering it a dup once the other question is created.
    – BMitch
    Commented Nov 7, 2013 at 21:11
  • This answer might be helpful.
    – Tester101
    Commented Nov 8, 2013 at 0:42

1 Answer 1


According to the CGa-010 wiring diagram, you'll want to connect the C wire for the thermostat to the terminal pointed to in this image.

CGa-010 Wiring

Notice the transformer is circled in red. The location where the transformer wiring terminates is circled in orange. The "common" side of the transformers secondary is pointed to by a large blue arrow.

  • Thanks a lot, I locate the wire, but didn't connect it yet.
    – Dmitriy
    Commented Nov 12, 2013 at 18:29
  • Is there any difference what transformer use for C in two-transformers system? I have independent boiler and A/C. I'm going to use boiler transformer to power termostat, but doubt if it could influence A/C.
    – Dmitriy
    Commented Nov 12, 2013 at 18:32
  • You'll use the same transformer that supplies the R wire to the thermostat.
    – Tester101
    Commented Nov 12, 2013 at 18:33
  • I have both Rh from boiler and Rc from A/C.
    – Dmitriy
    Commented Nov 12, 2013 at 19:07
  • You'll pair the C from the boiler, with the R from the boiler.
    – Tester101
    Commented Nov 12, 2013 at 20:16

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