I would like to buy a smart thermostat and I am confused about the C wire. I went to check my furnace in the attic and I saw a white wire connected to the C, I was excited as I thought that that was the C wire, but then behind the thermostat, I have only 4 wires connected, a white one connected to W, a yellow connected to Y and green connected to G. There is another cut blue wire that is not connected. Is that the C wire? If the blue wire behind the thermostat is the C wire, why I don't see any blue wire connected to the furnace? I am attaching a picture in the hope that someone can help me. Thank you in advance
Somebody was thoughtful
The good news is that somebody was thoughtful and ran a thermostat cable with extra wires in it, so you have the spare wire you now need. So, take that blue wire, land it on the C terminal on your furnace board alongside the existing C connection from your air conditioner (if you need to extend it, just get some blue 18AWG TFFN and nut it to the existing blue wire, then land this pigtail on the C terminal screw, being careful not to undo the existing connection of course), hook the blue wire up to the C terminal on your new thermostat, turn the furnace back on, and enjoy!