No C wire at furnace

I’m sure this has been covered but I can’t locate the answer, so
My furnace does not have a C connection. Does the C wire adapter somehow use the existing wiring to send voltage to the thermostat in lieu of a C wire? If I install the adapter per instructions the wire that’s labeled C isn’t hooked up, so in this case is the wire superfluous?

I’ve seen some posts where people had a B instead of a C terminal. If you look at the red and white wire that come from the outside unit, they are connected to the Y (red) and B (white) terminals, so I think you want to connect the C-adapter’s C-wire to the B terminal here.

Also, I see an unused blue wire come out of the bundle that comes from the thermostat. If you have that same blue wire unused coming out of the wall at the thermostat’s side, you can just use that and you won’t have to use the C-adapter at all.

Thank you for the response. I’ll give that a try and report back on this thread.


I agree with Vincent, B should work for the C wire connection. Also, based on your picture, it does look like you have a blue wire you could use for C & eliminate the adapter…as long as that blue wire is also at the thermostat too.

Yes, there is an unused (blue) wire at both ends of the run. I’ll try that this weekend and report back. Thanks you

I finally had time to work on this - no luck. I first connected the C wire adapter and used the “C” wire to connect to the B connection on the furnace control board. That didn’t work. I next connected the free blue wire to the B connection on the furnace and connected the other end to the C connection on the thermostat. Again go luck.
Am i correct that the C wire runs 24v DC to the thermostat to power it in lieu of batteries? If this is the case, I assume I could get any 24v transformer and run my free blue wire from it to the thermostat. Is this correct?
I did test all the connections with a multimeter and didn’t get any indication of voltage on any of the connection points on the furnace board

The C wire is sortof a “ground”, the actual 24v comes from R. Use your multimeter across R & B on the control board to verify the 24v. If no voltage, check for a blown fuse & verify the breaker is not tripped.

Thanks for the help and suggestions. I couldn’t get anything to work so I gave up and sent the thermostat back