Wiring in thermostat without circuit board, C wire help

Looking for help wiring in my new thermostat to an old hvac unit with no circuit board.

I’ve got 4 wires at my current thermostat:
G, Y, W, R and 2 unused wires wrapped around the bundle Blue and Brown. I found these wires feeding to the unit, as pictured. How do I connect the adapter? If you need more pictures let me know. Thanks.

And heres the rest of the pictures on imgur: Imgur: The magic of the Internet

What do the wires at the thermostat end look like?
Can you get a clearer image of the same wires in the air handler/ furnace?

Here are some more Pictures. Its hard to take pictures because the unit is located in a crammed in spot above my pantry. Hope these help. Let me know. Thanks

I think I see the blue wire in this bundle, but it’s very short… do you have an extra wire nut and any wire that you could use to make a jumper so that you could extend it about 6 inches?

I do have an extra wire nut but I don’t have extra wire. I can get some tomorrow. How do I make a jumper? Thanks so much. You have good eyes!

Your C wire nut is here (technically, this is after a safety switch that will disable it if your drain pan ever gets close to overflowing, but that’s fine, just remember that if you ever find the wyze off for no apparent reason after running the A/C for a long time, you probably have a clogged condensate drain)


You simply need to turn off the power to the furnace, then strip back just enough of the blue wire’s insulation to attach your jumper wire and wirenut. Then attach the other end of the jumper to the black/white wire nut from this image. When you hook up your wyze, tell it that your old thermostat has the following wires:

Rc (red wire)
G (green wire)
C (blue wire)
W1 (white wire)
Y1 (yellow wire)

In the future, it would be a good idea to replace the jumper wire with a 3 amp inline fuse, so that if there ever was a short circuit between hot and common, it would just blow the fuse instead of killing the transformer, You can get an inline blade style fuse holder and 3 amp blade fuse at just about any auto parts store.

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Awesome. Thank you for the detailed write up. I appreciate your help, thanks!

Its working! Thanks!


@speadie help pleaseee. Standard G Y W R wiring at thermostat

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@speadie thanks for quick response. Will test now

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Make sure your turn off the power first, or have a spare fuse handy.

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Power is off

Good. when you’re setting up your thermostat in the app, in order for it to work properly, you need to tell it that your old thermostat has the following wires as well:
G, Rc, W, Y
Remember this if you ever need to reset the thermostat, because if you don’t and tell it you have a C wire and no G wire, the fan will not work. The C adapter uses special signaling on the Y wire from the thermostat to activate the fan, while using the fan’s old wire (G) as the common power return.

Same issues with no C wire or clearly labeled circuit board at unit. Appreciate any help in determining proper wiring to install WYZE stat, either with or without the C wire adapter. There are extra wires (blue, black, dark orange) not in use at the old stat that could possibly be used as the C wire? Just unsure where the C connection is at the furnace to properly provide continuous power to stat? Thanks in advance!

The blue wire should be C.

Tried using the blue as C, but didn’t work? Reinstalled old stat as wired previously and now AC doesn’t blow cold air and hvac unit covered in frost? Turned AC off, left fan ON to thaw, but now I have bigger problems with no AC. Any idea how to fix? Thanks for your help!

Did you change any of the wiring in the furnace?

Did you by any chance try to move wires around at the thermostat or at the furnace with the power on in the furnace? If so, you probably popped this fuse (looks like a 5 amp blade fuse)

You’ve got a Trane TWE048C140C0 air handler, that means the blue wire is C (common). If that’s not working, then your blue wire is probably interrupted somewhere - you could try adding the brown T wire to the blue wirenut in the furnace as it doesn’t appear to be in use for anything.
Once again, do this with the power off, and have a spare fuse handy.

I did blow that fuse while messing with the wiring at the stat, but did replace it. I did attempt what you recommended, I again tried using the blue wire as C but with no luck again. Then placed brown T wire in blue wire nut and tried that, but still no luck. Any other ideas/options? Faulty WYZE stat? However, user error more than likely…

So what wires should I tell the WYZE app install guide that I currently have? Seems like I’ve tried all possible combinations for my setup but maybe not:

  1. G, Y, W, O, R
  2. G, Y, W, O, R, C (tried blue & brown wire)

I’ve yet to try to connect the C-wire adapter, would that possibly work? If so, where would I connect the C wire coming off the adapter at the furnace since I don’t have a labeled circuit board (at least don’t see one that is)? To the blue wire nut? Then again if the blue wire didn’t work as C, I would suspect that won’t work either.

Positive note, I reconnected the old stat again as previously wired after messing with the WYZE stat once more, and the AC is working again. So at least I have cold air.

See various photos of the wiring attempts that I mentioned above and tried, but ultimately failed to power the WYZE stat. Thanks again for all your time and help with this, much appreciated! If you have any other recommendations please send them my way!