Looking to add C wire into my furnace. Heat is currently running on two wires with older thermostats. Luckily the in wall wiring has three wires.

Any help to point out the C wire for my setup? Or tips to find it?

see image

enter image description here


  • 2
    Possible duplicate of How do I identify the C terminal on my HVAC? – Daniel Griscom Jan 31 '19 at 17:08
  • The thing in the top right looks like a transformer. The primaries go to the white and black wires. I think one of the secondaries goes to the red wire and the other goes directly to the relay below the transformer. That's where (I believe) the C wire (if one existed) would attach. Since there's no terminal there, you'd have to figure out how to attach it. If I'm understanding how things work, you can test by measuring the voltage from the red wire to the suspected terminal, and if it says 24VAC under all conditions, you've found it. But I could be wrong. Be safe. – Duston Jan 31 '19 at 20:02
  • Can you provide a make and model number for the thing in your photo? – ThreePhaseEel Feb 1 '19 at 0:31

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