Wyze T stat installation No C terminal on control board

Where does the C wire from the adapter attach on the control board when there is NO C terminal available.

My unit is 20+ years old. It uses a 4 wire cable from the original Tstat to the control board. I have a G, W, Y, and R wires. The adapter has the fifth wire, C.



picture please of the control wiring where the other thermostat wires land.

As you can see, there is no C terminal on the control board.

more pictures. where do the other wires land from the thermostat. also show where the wires go from this board pictured

It’s a 4 wire system (Tstat to control board). G wire to G terminal. R wire to R terminal. Y wire to Y1 terminal. W wire to both W1 and W2. No other pics available at the moment.

It looks to me like the red and white wire in the picture go into a two conductor that goes outside to the condensing unit for the air conditioning compressor. If that’s the case there’s a good possibility that the b is your common but I need a few more pictures

Google research shows the B terminal is normally used for a heat pump valve which I don’t have. Wyze customer support just suggested using the B terminal with out providing an explanation for doing so.

Is the B terminal always going to have 24V available to run the Tstat? I’ve screwed up (and fixed others screw ups) from guessing about the proper connections.

Thanks for your help.

On some older systems b is the common if you’re not sure get a meter and check between r and b you should have 24 vac. if you get me a few more pictures of the back of that board and where they connect to I could probably tell you. Almost always R is 24 vac. When the thermostat decides it needs heat for example the thermostat sends power from R to W and also G. If cooling is needed then it sends R to Y and G. If you have a reversing valve O/B it is energized with the compressor for cooling and not for heating. But depending on the system O/B could be energized for heating and not cooling.
The common wire at the thermostat almost always does nothing except power the actual thermostat, brains and backlight, etc…

Finally found the correct schematic for my unit. Terminals B and R are tied directly to the 24V transformer. I’ve verified with a multimeter that there is 24V available at these terminals when in the heating, cooling, and off cycles. There’s no room to get a picture of the back of the board.

Again, thanks for your help.

I’ve installed the adapter using the B terminal. Using a multimeter, I’ve confirmed that there is always 24V measured between terminals B and R while in the heat, cool, and off cycles.

I progressed to the thermostat install labeling and connecting the wires per the Wye diagram as shown in the pictures. The thermostat does NOT power on when connected. I’ve confirmed that there is 24V between the C and Rh wires.

Hey ther Mike,

Merry Christmas Eve :slight_smile:

If you look at the tutorial closely they it will ask you to label your RH to RC.

Once you do this your unit will turn on.

The Wyze units utilize RC for the power side of the 24v transformer and C to close the loop.


Merry Christmas to you and thanks for trying to help.

I did move the red wire to the Rc terminal and was able to get the thermostat to power up. I was then able to connect to my WiFi network.

The thermostat went through a self diagnosis process. The phone app told me that W/W1 wire was missing. I double checked all the connections, all good. Pulled out the VOM to check the wire’s continuity, good as well.

At this point, I’ve had enough.I’ve ripped it out and reinstalled my original Tstat. This piece of crap is going back to Wyze. I don’t ever recall have such poor technical assistance other than dealing with AT&T which I’ve totally dumped several years ago.

BTW, did you notice that in Wyze’s install video on youtube that the 4 conductor cable shown in the beginning “magically” grew a fifth wire starting at the base plate install…

Hey there Mike,

If you have a multimeter check the below.

The air handler should be utilizing one transformer at 24 volts at the air handler. The 24 volts should be seen when connecting your multimeter leads to Red as the hot side and Y, W and C should close the loop as neutral or common.

I am sorry it is not working :frowning: