Exactly how does C wire adapter work?

Hey, I’m decently handy and just bought 3 thermostats to replace the 3 in my house. 2 of the 3 had a C wire so install was easy. The 3rd only had 4 wires (R, W, G, and Y.) We’re talking single-stage gas heat and straight cooling systems here. Pretty simple.

No problem, I thought. I’ll just use the C wire adapter for that 3rd system. Hooked up the C wire adapter inside the air handler as described. Then I hooked up the thermostat on the wall, connecting the 4 wires to their appropriate terminals. Powered the system on, and the stat just clicks every few seconds.

Well, I thought, I have another C wire adapter…maybe something is wrong with this one. So I hooked it up, same way. Same thing. The stat clicks every few seconds but never powers on.

In order to get up and running, I removed the C wire adapter and hooked everything back up the way it was before I started, except I connected the G wire to the C terminal at the furnace control board, and then at the stat I connected the G wire to the C terminal. Thermostat powers right up and works great, I just don’t have independent fan control. Not really a problem as I never run the fan by itself.

But my question is, how’s the C wire adapter supposed to work, and why did 2 of them not work right? I’m guessing that when the stat isn’t calling for heat, cool, or fan, the adapter is supposed to change one of the wires to carry continuous current from the C terminal, and then when the stat calls for something, it quickly flips back to carrying the normal signal.

I guess the question is, if I wanted to hook the C wire adapter back up and test it, if I get my voltmeter out, which terminals should I check to confirm I’m getting 24 VAC when the system is at rest, vs. when the stat is calling for heat, AC, or fan?

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C adapter works by adding logic to control both the fan and compressor on the Y wire, allowing the G wire to be used as C.
When you hook up the C adapter, you need to tell the wyze that you are doing that, and make sure that the wire that used to be connected to G on your thermostat is now connected to C on your thermostat.

Here’s a schematic:Thermostat wiring diagram, voltages? - #28 by demiller9


Ah, that was the missing piece. I had the G wire in the G terminal when I was testing with the adapter. Makes total sense that we’d need the G wire on the C terminal of the stat in order to provide power to the stat. Thank you.