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I have a Trane XV80 with a 2 stage motor. I'm replacing a Trane TCONT803AS32DAA thermostat with a Honeywell wifi thermostat thinking it would be an easy switch out. However, the older thermostat had 5 wires; Red = "R" with an "RC" jumper, Yellow = "Y", Green = "G", White = "W1", Blue = "W2".

The Honeywell instructions said to use the Green as "C", then on the furnace control box change Green from "G" to "C" and jumper the "Y" and "G". But the furnace control has a white wire coming from "B/C", make 2 white wires coming from the furnace. All of the other wires seem to correspond with the thermostat wiring.

Any idea on what happened to the "B/C" wire from the furnace? It is not in the wrapped wire bundle hidden in the wall behind the old thermostat? Do I just need to run another wire directly from the furnace "B/C" to "C" on the thermostat?

Old Thermostat

Furnace Wiring

  • The wire connected to the C terminal in the furnace, likely goes to the A/C unit. I'd bet there's a red and yellow wire attached to the Y terminal in the furnace. Follow the red and white wires from the Y and C terminals, and I'd bet they go to a separate cable than the other wires that lead to the thermostat. – Tester101 May 3 '15 at 22:22
  • Tester 101 - I believe you're right and that red and white wires from the Y and C terminals run to a separate cable than those running to the thermostat. My question now is, should I run a wire from the C terminal directly to the thermostat? – jdgrog May 4 '15 at 8:35

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