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.
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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 Nov 07 '13 at 21:11
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[This answer](http://diy.stackexchange.com/a/10527/33) might be helpful. – Tester101 Nov 08 '13 at 00:42
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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.
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.
Tester101
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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 Nov 12 '13 at 18:32
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You'll use the same transformer that supplies the `R` wire to the thermostat. – Tester101 Nov 12 '13 at 18:33
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Got it. I just realized that termostat doesn't have Rh, it has R and Rc. Sorry for bothering and thank you again. – Dmitriy Nov 13 '13 at 15:49