I have a trane TWE040E13FB2 hooked up to a heat pump. I am wanting to install a smart thermostat. Our current thermostat has 10 wires in it. I do not have a common wire. I also don't see a common wire on terminal block in the furnace. Is there a way to hook this up?enter image description hereenter image description hereenter image description hereenter image description hereenter image description hereenter image description hereenter image description here

  • 2
    Can you upload pictures showing all the wires as they are currently attached to the thermostat and to the furnace? Commented Aug 19, 2021 at 18:43
  • 1
    Yes, can you get us photos of both ends of the thermostat wiring please? Commented Aug 19, 2021 at 23:22
  • Sorry I didn’t add them yesterday. I was just notified of your responses this morning. I have read varying things that T or B could be the common. I have not checked T to have 24v yet. I was told on my unit that B was the common but it is hooked already to valve from heating to cooling I believe.
    – Bbooth
    Commented Aug 20, 2021 at 19:41
  • You probably don't need all 10 wires and the T terminal might be the common so you can probably free up a wire for that. Please post a clear picture of the entire schematic. Also, if you can locate where the two wires from the transformer are connected and trace them both into the handler, one will end up on R, the other will end up somewhere ... and that is C ... you can always splice in. Post pics of that too if you find it.
    – jay613
    Commented Aug 20, 2021 at 20:05
  • You have wires connected to both WH/1 and WH/2 in the unit, but you also have them shorted together. Do you have a secondary or emergency heat module or additional baseboard heat? You have wires connected to S1 and S2 at the thermostat. Those are for outside temperature sensors. Where do the wires go? Do you have those sensors?
    – jay613
    Commented Aug 20, 2021 at 20:08

1 Answer 1


Trane is silly

Trane is sometimes silly and calls their C wires B, which is mildly confusing because other brands sometimes use the B designation for the function that your O wire performs, namely heat-pump reversing control. Anyway, you have a perfectly good C wire in disguise as your B wire, so just use that.

  • Could we find a stronger word than "silly" for wrapping the schematic around both sides of a small metal panel? Anyway this answer is correct, in the photo with your finger in it you can easily trace the 24V common from the transformer to the B terminal where your wiring connects. By the looks of things you have a blue wire connected to it, which at the thermostat is connected to a "B" terminal that may be Trane B or may be everyone else B. Good luck mapping your Trane terminals to your new thermostat ones. Good news, as this answer notes, is that you can.
    – jay613
    Commented Aug 21, 2021 at 13:06
  • @jay613 -- it's a Trane B since we already accounted for the O Commented Aug 21, 2021 at 13:15
  • Thanks for all the help. I did verify the B was the common. I still have a question though. I have the wiring going like this. R/rc > r/rc O > O Y > Y G > G B > C W2>W2 S1 and S2 which I believe are outdoor temp for heat anticipator will not be used and will be wire nutted in wall. Is this ok to do with them? Where will the F (brown) and X2 (Black) wires go?
    – Bbooth
    Commented Aug 24, 2021 at 17:30
  • @Bbooth -- just cap the S1/S2 wires off separately, yes. For the other two wires, we'll need to know what model your old and new thermostats are Commented Aug 25, 2021 at 0:43
  • The current model is a trane tcont802as32daa. manualslib.com/manual/461448/… the new thermostat is a Honeywell RTH9585WF
    – Bbooth
    Commented Aug 25, 2021 at 15:01

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