I had an old thermostat with wiring as below image: enter image description here enter image description here

I wired Air Handler Trane (Model# TWE037E13FB2) in the attic just like it was before: Air Handler Wiring 1 Air Handler Wiring 2

Here is a wiring diagram of Air Handler: enter image description here

However, my Nest has few wires that I am not sure where to connect: enter image description here

Does anyone know:

  1. Whether I connected White wire properly, since it was a jumper wire before
  2. What should I do with Blue, Brown and Orange wires?
  • Can you post the wiring diagram for your air handler please? Commented Sep 2, 2019 at 3:27
  • Is this a plain heater and A/C, or is it a heat pump or something exotic? Commented Sep 2, 2019 at 4:02
  • @Harper it is a heat pump
    – Angelina
    Commented Sep 2, 2019 at 4:21
  • @ThreePhaseEel I updated my question to include a Wiring Diagram of Air Handler.
    – Angelina
    Commented Sep 2, 2019 at 18:23
  • Can you get us a wiring diagram for your outdoor unit? That brown wire has me puzzled... Commented Sep 2, 2019 at 20:54

1 Answer 1


The blue wire goes to C, not B

Some Trane systems, yours included, use a non-standard labeling where the 24V common wire is labeled B, not C. We can tell this from your wiring diagram as well as the wiring on your old thermostat. As a result, since the blue wire went to your old thermostat's B terminal, it needs to go to C instead on the Nest. This frees up the O/B terminal on the Nest to get connected to the orange wire from the thermostat, which connected to the O terminal on your old control.

As to the brown wire? While it might be possible to figure that out with more information, your system should work fine without it connected, since it was not hooked up to your old control. The white wire being unhooked, though, is a sign that the emergency/auxiliary electric heat was not used on the old system. Perhaps it was disabled intentionally due to an issue with it, so I would pay attention for signs that the emergency heat is running when it shouldn't.

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