No C-wire at thermostat, but control board already has a C-wire connected

I’m trying to install the thermostat and have run into what I believe could be a non-issue, but would like a second opinion. When I pulled my current thermostat I noticed there was no c-wire connected. No problem, I follow the directions and go to install the c-wire adaptor. Here is where I run into my issue.

The control board is behind a secondary panel, and it appears that I’d need to disconnect many wires and physical connections. Luckily, there is a bundle of wires already labeled that come from the control board. So I can use these instead of dismantling my furnace. However, there is already a c-wire connected, and it’s connection does not lead to the thermostat but instead to a third location. I assume this is the AC compressor.

When looking at these wires, I noticed the set of wires leading to my thermostat has a few spares. This leads me to reasoning that this may be a non-issue. Could I just tap into the existing c-wire connection and not need the adaptor at all?

I have tried to label the photographs below and attempted to recreate the diagrams I believe are at play here. I hope it is all clear enough. Much appreciated anyone who can help.


You’ve got it. Connect the blue wire that goes to the thermostat to C on both sides and it should work. If it doesn’t there is likely a splice somewhere where they decided to not hook up the blue wire, but as long as that’s not the case, you shouldn’t need the C adapter.


Wow, that has got to be the best-prepared tech support question I have ever seen in any medium. Very nice, @rm.ayala14 !

And thanks as always to @speadie for his expertise.