I have an older furnace and am looking to add a c wire for a smart thermostat. Currently behind my thermostat I have red/black wire, however, there are 3 additional wires that are not connected.
At the furnace the red/black are connected (as shown below) and the 3 additional wires are free.
From what I understand the C wire is 24 volt common. I have metered the furnace running/not running and found a post transformer wire that is 24 volt constant. Can I simply connect one of the spare wires to this terminal or is there potential for too much of a current draw blowing the transformer?
Red coil wire is from thermostat.
24v constant is the bottom right black wire (this is where I want to connect the C wire to)
Best Answer
Yes, the terminal at the lower right in the photo is
C.Here's the wiring diagram from the documentation, which I highlighted in different colors.
Here's your wiring, which I highlighted using the same colors.