enter image description herethis is a photo of the old board I took out a while back. New one is the same.

I'm trying to add a c wire and don't know where to put it on the board. Can anyone help me?enter image description here


There will be a 24v transformer that supplies power to the board. The thermostat is really just a bunch of switches that controls this 24v supply to toggle on certain relays at certain times.

The transformer has two wires on the low voltage side. One of the wires will become the "R" wire that goes to the thermostat. The other wire from the transformer is the "C" or common wire. The C wire doesn't need to come from the logic board. If your board doesn't have a special terminal to connect a C wire, you just have to splice in to that "other" wire from the thermostat. Also, in many air handlers, the C wire from the thermostat will be bonded to the metal chassis like a ground wire. If that's the case, you can connect your C wire to the screw being used to bond that wire rather than cutting and splicing the wire.

Without much better and more detailed pictures, that is as specific as I can get, but that should hopefully be enough for you to figure it out.

  • Is that second photo what you're referring to? And would I connect the blue to the blue on it? – Renata Galván Nov 8 '19 at 22:02
  • Follow those light blue wires and see what they connect to, but that certainly looks like what you want. If the light blue wire connects to the metal case somewhere, that’s it for sure. – JPhi1618 Nov 8 '19 at 22:05
  • That blue one goes to the case of the transformer. Is that where I can connect it? – Renata Galván Nov 8 '19 at 22:09
  • I got it! Thank you so much! – Renata Galván Nov 8 '19 at 23:07

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