I recently got a smart thermostat that requires a 'C' wire. My existing thermostat had only W1, W2, R, G. After removing the faceplate, I noticed there was what looked to be an unused yellow wire in what looks to be 18/5 cable connecting the thermostat to the furnace. It was wrapped around the wire bundle at both the thermostat and furnace ends. My thought was to use this wire to connect 'C'.
My furnace is an old Trane XV90 with no 'C' terminal. My understanding is that the 'B/C' terminal is 'common', and I should be able to use that.
Just to test, I used a multimeter on the existing thermostat wires and get 28V on R and any of the other 3. When I connect R and the unused yellow wire, I get 15V and if I connect any of the other wires to the yellow wire, I get 11V.
Knowing very little about electrical systems, I'm not sure what to make of that. You can see the yellow wire in the photo, just wrapped around the cable unconnected to anything.
I've included shots of the furnace electrical diagram and the terminals. Would it be safe to uncoil the yellow wire, connect it to B/C on the furnace, and C on the new thermostat?