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I am trying to install a new wifi stat. The wiring seems to all be there but apparently not connected correctly. So far I have tried using the green wire from the furnace, the blue and the white from the air handler, each as the C wire, all without luck (doesn't turn on when power is restored at breaker).

Behind the old stat, the upper cable goes to the air handler while the lower cable goes to the furnace. The white, brown, and blue wires in the upper cable were unused in the old stat while the lower cable's green wire was also unused.

In the air handler (a trane twe048c140c0), the brown and blue wires are unused. Do I need to connect either to one of the existing wire nuts?

I'm not sure why the furnace isn't providing 24V either, but possibly because the relay switch is bad? The furnace works fine otherwise.

Here is an image album displaying all wiring that leads to the stat from the air handler and furnace: https://imgur.com/a/uecMa

Thanks.

EDIT: Solved. There were two white wires hidden in the loops. None of the three spare wires running to the air handler were connected. Connected blue and stat powers on.

  • Make that solution an answer and you'll get a +1 from me :) – ThreePhaseEel Sep 26 '17 at 0:41
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I had the same issue. All the instructions i found said connect one of the unused wires to the C terminal on the furnace and to the C terminal on the stat. Problem for me was there was no C terminal. I finally found a thread that was specific to my brand and it said connect the furnace side to the chassis. I had already come to the same conclusion after studying my wiring diagram but had continued looking just to be sure. I Did that and it works fine.

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To follow up:

The photos don't show it too well, but I noticed a single white wire would have been too long to make sense in that wrap of loops. There ended up being two white wires hidden in the loops. The one already tied to the blue common wire nut runs to the compressor. The other that runs to the stat was stripped bare and a little rusty, which made it look brown and fooled me while working in a 100°F tight space. None of the three spare wires running to the air handler were connected. Connected blue to blue and stat powers on.

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