Wyze thermostat turning on both AC and Heat in Heat mode

My Wyze thermostat is triggering the AC compressor to turn on in Heat mode.

Wires connected on old thermostat:
RC - Jumpered to RH

Wires connected on furnace panel:

Because I have no C-wire at the thermostat, Wyze said I must install the Wyze adapter at the furnace. I did so, however I did not have a Y-wire connected at the furnace panel and also had a C-wire connected. The Wyze setup did not say anything about removing the C-wire on the furnace, so I left that and added the Wyze adapter. Wyze setup said to label the G-wire as C and connect to C on the Wyze Thermostat. When in heating mode, the AC compressor turns on as well as the furnace. Some further testing shows that Fan Mode also triggers the AC compressor.

I removed the C-wire from the furnace panel and moved it to the Y-terminal on the Wyze adapter; That has stopped the AC compressor from turning on in Heat mode and Fan mode, but now it is not turning on in Cool mode.

You have a connection for a C wire on your board already, the left terminal with that dark gray or black wire in it. If you have either a spare wire not used on your old thermostat like that light blue or yellow, you can use that for your C wire and not need the adapter at all. Actually, it looks like that black or dark grey wire connected to C goes into your thermostat cable already, does it come out on the thermostat end? if so, that’s the C wire even if it wasn’t used on your old thermostat.

Only four wires run up to the thermostat,
RC - Jumpered to RH

Since I have no C-wire at the thermostat, Wyze said I need to install the adapter, yet there is a C-terminal on the furnace.

I suspect Y runs out to the AC compressor and returns as C, as there is no Y-wire connected at the furnace terminal and the wires are different gauges and colors.

What about that light blue one wrapped around the cable? I ask because on mine for example, I had a couple extra unused wires that were cut off right where the brown cable insulation was stripped back to. When I pulled a few more inches of cable out of the wall and stripped back some more of the outer insulation, I found the extra wires.

I poked around in the wall a bit and could not see any other wires. The system was obviously upgraded/replaced a few times and maybe spliced along the way as the RC and W1 wires only have what I would describe as tightly-wrapped string insulating them at the thermostat; Y1 and G/C must have been added later as they go through a different hole and have plastic jackets. Tried to tug on them a bit and they are just about at their max reach.

I see. Darn, I’m out of ideas then. When I saw that light blue wire wrapped around the cable I was hoping it went up towards your thermostat figuring if it was in the cable at the furnace end, it had to be at the other end also, even if it was cut off like mine.

That’s what threw me off too; Said I needed the adapter since I didn’t have a C-wire at the thermostat, then opened the furnace panel and saw the C-wire there. Heat is at least functioning now with the C-wire on the furnace connected to the Wyze adapter and it is not triggering the AC compressor in Heat or Fan mode. Won’t need Cooling for several months here in Wisconsin. Worst case I’ll call a pro out, when it isn’t holiday pay.

1 Like

The splice is probably down further the hole.
Can You trace the T wire from furnace. or check the floor below for T wire. Any return air grill. check inside.

I suggest U start follow the wrie rrom furnace panel

I did find the splices above the furnace, but it’s still only two sets of two wires going up to the thermostat. The Wyze thermostat turned off now anyway and won’t turn back on so I’ve re-installed the original one for now and opened a support case.


I have the same wiring and found that I had two yellow wires on the furnace board, thinking both had to be on the C-wire adaptor. This configuration caused my AC compressor to run while in heat mode. I removed the second yellow wire (the yellow wire that was paired with furnace board C-Wire) from the C-wire adaptor and replace it on the furnace board. The system is running correctly.

I believe the yellow wires run the AC and heat separately since I don’t have a heat pump setup.

Hope this insight helps.

I’m pretty sure the adapter only switches common between the G and Y wires, the W wire is just passed through it unswitched, so without a Y wire attached, whenever there is a call for fan (G), it pulls power through the compressor contactor (Y at the thermostat), switching it on, to power the wyze.

I’m kind of having a similar issue. Since I’ve installed the thermostat I’ve noticed that my A/C compressor seems to come on for a split second when the heating is on. House was built in 1986 so original A/C and furnace still in house. Its like a small bolt of electricity is sent to A/C compressor causing it to come on for a second then it shuts off. This seems to happen every 90 seconds as I timed it. Once heating goes off the problem goes away.

Here is photo of my wiring on my control board before I installed the thermostat. Never had this issue with old “dumb” thermostat. I did need to install the Wzye c adapter.
Image show the wiring on my control board before installing thermostat.

Heating and A/C works its just I worry about damaging the A/C unit overtime if it keeps clicking on and off like that.

Any help is much appreciated!

here’s image of my old thermostat wiring

Show how you wired the adapter. most likely you put your AC compressor on the wyze adapter terminals instead of leaving it on the furnace.
It should look like this: In your case the Y wire that goes to the AC compressor is red and the C wire that goes to the AC compressor is white.

correct wiring


Thank you for the diagram. Very helpful! Yes, I did put the AC compressor wire in the wyze adapter terminal instead of leaving it on furnace. Problem solved. Can’t thank you enough!

1 Like

how do you know what yellow wire has to stay on the furnace and what one goes into the adapter. i was having the same problem so i just took it off and put my old on back on.

Share a picture of your wires and I can tell you which is which. I’m looking for the wire bundle that connects to C and comes back as Y. That is your outdoor A/C compressor, and needs to stay on the furnace/ air handler.

So I am guessing that the wire harness For the ac only has two wires the red wire goes with the c and like the whitish yellow one goes on the yellow so I’m guessing that yellow stays on the furnace and then the second harness with all the wires that yellow goes into the adapter right?

if you can strip this back a ways, and splice a wire onto this blue wire, and connect the end to C, then you wouldn’t need to use the C adapter, you would just need to find the blue wire behind your thermostat and connect that to C.

But yes, the 2 wire connection stays on the air handler panel, and the labeled thermostat wires below go into the C adapter, if you use it.