Trying to add a 'C' wire to my thermostat

Question

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?

Answer 1

When working in the blower compartment note that furnace power is shut off automatically by a safety switch. You can override the switch by pressing it in, and must do so in order to take meaningful voltage measurements with the compartment door open. When bypassing the safety switch take care so that if the blower motor were to turn on any long hair, clothing, or other items would not be sucked in and cause injury.

There's a legend in the schematic that says B/C is common (a box at the right-hand edge, middle section). The schematic also shows the 24 volt transformer TNS connects to the red and blue wires in that white 12-position connector (positions 7 and 11 according to the pinout diagram at lower-left).

The easy test is to measure voltage from R to B/C -- if it's around 28 then yes, B/C is probably your C terminal and you could use that spare yellow conductor to carry it up to the thermostat.

If you want to be a little more cautious in confirming the idea, measure AC volts between the R terminal and first the red, then the blue wire. You should find 28 volts from R to one of those and 0 volts from R to the other. The finding should be opposite when measuring B/C to the wires: 0 volts to one and 28 volts to the other. This confirms that B/C is common or "C wire."