Wiring issue - no control board at Furnace for C-wire adapter

Heat pump set up. See pics for setup info. No control board to wire in c-wire adapter.

Have not labelled anything, but what I believe I should do is to approach as if the wire nuts are the control board. So I would wire in the adapter for the Y, W, R, G, but I am not sure how to wire in the C. I don’t know where to add it. I was originally thinking of adding to the black-to-black wire nut, but am really uncertain.

Thanks for any feedback ahead of time, I’m terrible and electrical/wiring.

At the thermostat

One more, past the wire nuts

Found this wiring diagram that may prove helpful also

Black is common, yes.

Just gonna copy/paste and edit from the other thread with point to point wiring:

I think you should connect your thermostat adapter wire as follows:

You want to make sure all power is off to both the inside and outside units before unscrewing any wire nuts. Have extra wire nuts of the proper size handy, and wire strippers, in case a wire breaks off in the wire nut when you unscrew it. Also, make sure you have a flat screwdriver small enough to fit into the terminal strip on the wyze adapter.

In your image, you have 2 thermostat wires, one runs to your thermostat, and is using all the wires, and one runs to your outdoor unit and has unused wires. in the below instructions, when I tell you to remove a thermostat wire from the wirenut, I am instructing you to only remove the wire that runs to your thermostat, not the wire that runs to the outdoor unit.

Remove the wire nut from the black wires and insert the C wire from the adapter into the connection and reattach the wire nut ( Note: this is the only case where you are only adding a wire, not removing any from the wire nut ).

Remove the green thermostat wire from the green wire wire nut, and attach the G lead from the adapter to the thick green wire from the fan switch with a wire nut. Attach the green thermostat wire to the G terminal on the adapter and tighten the screw.

Remove the red thermostat wire from the thick red and 2 thin red wires wire nut, and insert the R wire from the adapter into that wire nut with the thick red wire and the thin thermostat wire that comes from the outdoor unit and tighten. Insert the red thermostat wire into the R terminal on the adapter and tighten the screw.

I’m not sure about this, your white wire is Aux, not W1, but it might work. Make sure this is where the wyze app wants you to put the W lead for the common wire adapter
Remove the white thermostat wire from the white heating element wire nut, and attach the W lead from the adapter to the white lead from the heating element and the white wire from the outdoor unit using the wire nut. Insert the white thermostat wire into the W terminal on the adapter and tighten the screw.

Remove the yellow thermostat wire from wire nut leading to the yellow wire from the outdoor unit, and insert the Y wire from the adapter into that wire nut with the outdoor unit wire and tighten. Insert the yellow thermostat wire into the Y terminal on the adapter and tighten the screw.

Thank you so much! I’m going to give this a shot Friday or Saturday!

anxiously awaiting your findings, as I’ve received a Wyze today that is convinced that my Electric Furnace is a heat-pump, and my furnace does not have a control board from what I can tell.

your B wire is a C wire.
Tell the wyze app that you have the following wires (from left to right on your current stat):

It works! Wyze turned my Aux into W1 and my G into C. Wired in and everything worked perfectly first time when it powered up and tested. Thanks for your help!


Tried your trick but the thermostat would not power on.

How can I troubleshoot? Below, please find my wires with the suggested labels that you helped with :slight_smile:

On your air handler, is there a blue wire? where does that black wire that was attached to B on your old thermostat go?
You may need to cut power to it and remove the screws that hold the front panel on in order to see the connections.

There should be a control board inside with a layout that looks like this schematic.

Take a picture of it if you can, and let me know where the black thermostat wire goes if it’s not obvious.

Turns out the black wire was actually blue! Opened up my electric furnace to find a hot Blue wire connected into the C pin of the control board, and an uncapped blue wire coming from the braid that runs to my thermostat. Attached the two together. Now the Wyze thermostat works! Thank you so much for your assistance. I would have not been able to do it without your help.


Hey everyone that is struggling with the “basic” non-circuit board units like mine. Hope this picture helps you out. My wiring is as follows:
Rc Red to Red from Transformer
Y1 Yellow to my Reddish wire that goes to the AC compressor
G Green to Green from Relay
W1 White to the Blue/White / Black/White wires that go to the electric heater
C Blue to one of the Brown wires from the Transformer (same as the white wire to the AC compressor)

Since I was running new wire went ahead and installed a terminal block for maximum neatness!

I’m trying to install a Wyze thermostat without a control board and no C wire at the current thermostat. I believe I have located the transformer and there is a post labeled “com” with 2 yellow wires coming off of it but not going to any wire nuts as described in other posts on the subject. Is this the common that needs to be connected to for the thermostat? I also discovered that the wire bundle that goes to the thermostat has an unused blue wire in it (the C wire) could I avoid having to wire in the adaptor if I were to uses this wire and connect it to the “com” post on the transformer?

Not the side of the transformer with the yellow wires, that’s the high voltage side. Show a larger image from farther away (stand back from the furnace a few feet and take an overview image with as much of the internal wiring showing as you can- I can zoom in or ask for a more detailed shot if I need it) and I can guide you in selecting the correct wire. It’s probably the brown wire on the upper right of the transformer, but without seeing more of the wiring, I cant tell for sure.

I’ll do what I can this is reaching in over the top of the air handler.

Hopefully, this is more of what you need.