3 wire thermostat

I have a 3 wire thermostat and wondering if this will work with the Wyze Thermostat. On my existing thermostat the wires are W, Y, R/RC

For both heating and cooling, you will need 4 wires.

You are missing the C wire. Needed to have full-time power going to your Smart Thermostat. I think Wyze sells a device to use when you don’t have a C wire.

I see a wire not being used. It looks Pink. Any others? Red is 24V power, Green is Indoor Fan. But I see it’s going into Y which is Cooling. The Black Wire going into W is Heating. So It looks like you don’t have a separate wire to control the Indoor fan. It just auto comes on with heating and Cooling.

So any Thermostat is basically just a switch. Old ones had Mercury and a spring that moved to temp. When it got too cold, the mercury would move and make contact with 2 electrical points and turn on the heat.

These cheap Temp controllers and the Smart ones have Relays. These cheap Thermostats run on batteries. The don’t require much power. They have some relays. A relay is just a switch. When it’s powered up, it allows power to flow through it. If you take that R wire and touch it to the Y wire, your cooling would turn on. The Red wire will send 24V down to Y and turn on Cooling. The Relay does the same thing. It doesn’t take much power to do that.

A Smart Thermostat requires more power and it needs non-stop power. So it would have 24V coming in, but there is no Common, or Ground and so it won’t power up. Just like your power outlet. You need 120V power and Common at least to power whatever you plug into the wall. The same goes with a Smart Thermostat. So I see an extra wire not being used, pushed behind. If you go back to your heater and find that multi-wire cable, and find that color wire, and it’s not being used for anything. You can connect that to the C terminal, there may be another wire there, but you can add a wire to it. That new Pink wire would then go to C for Common/ground for your Smart Thermostat mounting plate.

I did something similar at my own house. I didn’t have a C wire. I found an extra wire behind to use and connected it up in my Heater electrical panel to the C terminal. make sure to unplug/Kill the power going to the heater first!!! I also did the same for a friend’s house. Their heater was up in the attic. Pulled the panel off and connected the extra wire there. Worked out great!!!

Of course, when going into settings, You’ll be setting it up that the fan comes on automatically. It doesn’t have its own control wire. Having this extra fan control helps with things like running the fan longer when the heat is turned off to not waste that heat. Or running just the fan to help circulate the air around your house.

Thanks, that helps and I will have a look to see if I can run the extra wire