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I'm currently trying to add a Amazon smart thermostat to my Trane UH2B060A9V3VA furnace. When I had my AC service earlier in the year I mentioned the project to my heating and cooling guy and though I don't have a C wire he said I could add the thermostat to my system as long as I got a c wire adapter. He told me if I brought the thermostat and adapter he'd set it up when he serviced the furnace. He was to come today, but there was a mix up and a new guy who was comfortable setting it up came out. I figured I'd set it up myself, but as I was following the instructions I realized I have a W2 and a w wire and the adapter only has a spot for the W wire. I did some research and apparently I have a 2 stage furnace. My main question is, is there anyway I can still set up the new thermostat or is it simply incompatible with my system? I'm including some photos of the wiring at the furnace and thermostat. And suggestions would be much appreciated:)enter image description here

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  • I'm still trying to figure it out, but I had the understanding the furnace was just gas. Here's a wiring diagram if that helps. manualslib.com/manual/825715/…
    – Kevin P.
    Dec 28, 2022 at 6:19
  • Also I had read this "A two stage furnace works like this: your thermostat calls for heat and the furnace kicks on low. After a predetermined amount of time, if it has not satisfied the temperature, it kicks up to high until the temperature is satisfied. For this extra stage, you need to connect one additional wire to your furnace and your thermostat." That lead me to believe it was normal for a two stage two have 2 w wires. Is that incorrect?
    – Kevin P.
    Dec 28, 2022 at 6:27
  • Thank you, that is what it says, it needs both W1 for stage 1 and W2 for stage 2. That leaves us with G-wire that controls the Fan only. I was looking how to repurpose the G wire to become the C wire. By doing so you will no longer have the Fan only function, but the fan will run with heating or cooling. However I can not find the C terminal on the control board, can you ?
    – Traveler
    Dec 28, 2022 at 7:12
  • I have the Honeywell C wire adapter google.com/url?sa=t&source=web&rct=j&url=https://… this turns G into C and y I to k. My problem is the adapter doesn't have a W2 spot meaning the there wouldn't be able to control stage 2. Now I could put a wire nut W2 and just have w1 going to the thermostat and switch the controller board to single stage thermostat but not sure I'd want to do that.
    – Kevin P.
    Dec 28, 2022 at 16:53
  • Also I'm wondering if I could leave W2 going directly two and from the control board and just put w1, g, r, g, and y on the adapter. But not sure if that would be safe.
    – Kevin P.
    Dec 28, 2022 at 16:55

1 Answer 1

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So many signals, so few wires

The issue here is you have the terminal, but you don't have enough wire to support what you want to do. Multi-stage heating isn't very common in residential and the standard 4 or 5 conductor thermostat cable that builders used to install just doesn't have enough conductors for each signal.

Look at the 2-conductor cable (red/white) that goes off to your air conditioner compressor. That is taking the cooling (Y) signal from your control board/thermostat and closing it with the B/C to complete a circuit. That strongly suggests that B/C is the "common" terminal on this board.

To be sure - test for voltage with a multimeter between the B/C terminal and the furnace frame. You should get 0 VAC. Then test between the B/C terminal and the R terminal. You should get around 24-30 VAC.

However....

You only appear to have a 5-conductor cable going to your thermostat. Your problem here is not the lack of a C terminal, but the lack of available conductors to connect it to the thermostat. The handy-dandy HVAC-installer's helpful friend in size AA steps in to make the "dumb" thermostat work, but what you really needed was a 7-conductor cable.

Bottom line: you can't power that thermostat without replacing the cable.

Except...

Except... in the case of the Honeywell C-wire adapter. Per the documentation, it converts a 4-wire (R,W,Y,G) thermostat cable to carry 5 signals (R,W,Y,G,C) over 4 wires (R,W,K,C). This supposes that you have a thermostat on the other end that is capable of supporting the K wire in place of G and Y.

While I have never hooked one up that way so I don't have hands-on experience with that adapter, both the documentation on the Honeywell adapter and the Amazon smart thermostat support this configuration.

But... what about W2?

In the case that you are using the C-wire adapter to convert the "big 4" wires to 5 signals, you would simply leave W2 connected directly to the furnace board W2 terminal, bypassing the adapter. That's the way it's done for folks who have other "extra" wires, such as O/B for heat pump.

The way it would be connected would look like this:

Furnace:                                            Thermostat:

W2  --------------------------------------------------------- W2
W1  ----- W |---------| W ----------------------------------- W
R   ----- R | Adapter | R ----------------------------------- R
G   ----- G |         | C ----------------------------------- C
Y   ----- Y |         | K ----------------------------------- K
B/C ----- C |_________|
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  • Thank you so much for your answer. I did some research as confirmed that B/C is common (it's how trane often labels it as the common wire is normally blue). The thermostat I want to use is an Amazon smart thermostat. The Honeywell adapter adds a C wire (pics above). Would it be possible to add all the wires R, W, Y, and G to the adapter but keep the W2 wire connected directly bypassing the adapter. I found a Manuel online that said I can connect the two stage to a one stage thermostat if I use a jumper between W & w2 so I also considered just putting a wire nut on the W2 and using a jumper.
    – Kevin P.
    Dec 28, 2022 at 21:44
  • Here's the Manuel concerning thermostat hookup manualslib.com/manual/1018294/…
    – Kevin P.
    Dec 28, 2022 at 21:50
  • The diagram you reference (bottom of page 27: Hooking up a 2-stage heat furnace to a 1-stage thermostat) would allow you to use the thermostat, but defeats the purpose of the 2-stage heat. By jumping W to W2, you force both stages to come on any time there is a call for heat. In my opinion, keeping the regular "dumb" thermostat and leaving the 2 heat stages in place would be the better option. How far away is your furnace from the thermostat? If there is any way you could get the wire there, you could return the C-wire adapter, buy a spool of 18/7 and have money left over.
    – Chris O
    Dec 28, 2022 at 22:06
  • I agree, and don't really like the jumper idea. The furnace and thermostat are pretty far apart and I'd probably have to go up above the ceiling to get the wire there and not something I'm comfortable doing myself. No way I could leave the W2 wire connected as is and use the adapter for the other wires? Thanks for your advice it is much appreciated.
    – Kevin P.
    Dec 28, 2022 at 22:15
  • I'm leaning towards trying it, but haven't been able to find any information on someone trying it and don't want any electrical problems.
    – Kevin P.
    Dec 28, 2022 at 23:19

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