Thermostat R and W only but furnace has CYGRW

Can I install the thermostat if the current thermostat only uses R and W but the furnace panel has terminals for CYGRW?

It’s a simple forced hot air furnace not AC.

Thank you

1 Like

As long as you have at least 3 wires for your common, it will work fine.

Thanks Brian

The current thermostat only uses 2 wires R W. Is the 3 wire requirement based on the number of wires available on the furnace terminal or the number of wires used by the existing thermostat?

If I just need to hook up more wires from the furnace which would I use?

Thanks again

I think you would need R,W.C on the furnace connected to Rc,W,C on the thermostat

Hope this helps

Thanks Tomellis, I looked up Rc and that seems to be for a cooling circuit. I did find the following at Ecobee for their smart thermostats. It seems that if there is a C terminal on the furnace control panel that it might be just a matter of hooking that up to the thermostat wire so that I would have R, W, C… I may give this a try tomorrow.

If your thermostat wires connect to terminals marked R and W, installation is as simple as connecting a third wire to the common on the furnace or boiler’s low-voltage Common point. Depending on your furnace, this may be either a terminal marked “C” or “Com 24V”, or you may need to locate where the common can be connected to.

The Wyze gets it’s power from the Rc terminal that is why it has to be hooked up there and not Rh. The thermostat bridges the Rc and Rh internally from what I understand

Good luck!

You are correct! I proceeded with the app installation and it does show moving the R to Rc, the W to W1 and the C remains C.

So I’m hoping that the new wire from the furnace panel from the C terminal should do the trick.

I’ll give it a go tomorrow.

Thanks again,

That is correct, the 3rd wire from the indoor unit to the thermostat is all that’s needed in addition to the two existing wires.

Great! I started the Wyze App installation procedures and when I got to the screen to enter the number and code of the wires in the current thermostat it wouldn’t let me proceed with just R and W. As a result I thought this was going to be a none starter. I’m much more hopeful now.

Thank you

You will need to run a C wire from the thermostat to the furnace, unless you have extra unused wires coming out of the wall near your thermostat. Please show a picture of your thermostat wire.

Fortunately there is an unused wire in the bundle coming from the furnace. Hopefully there are no breaks in the line. If this works :crossed_fingers: the app should be changed to recognize a situation where the thermostat only uses RW and there is an available C terminal on the furnace.

And here’s the original thermostat:

The wyze only says it’s not compatible because it doesn’t think most people are going to want to run a new wire, and it has no way of knowing that you have an unused wire available. If you connect that green wire to common in the thermostat, and tell wyze that your current thermostat uses W, R, and C, it will allow you to set it up.

That makes sense, although "C’ wire wall adapters appear to be working for other smart thermostat installations and I’d assume they would be good for Wyze thermostats too. For a situation that seems to be somewhat common the app responds by saying that “your system is not compatible”. No reason to lose sales when it’s a relatively easy fix.

thanks again

We moved from old to new thermostat (not Wyze) and had the same concerns. We added an external transformer to give us the 12V C (common, I think), and the thermostat works fine.

Our only real issue was the lack of an extra wire from the heater - we had only two. We have mounted the transformer (wall wart) near the transformer and it’s wire runs up the wall. It was cheaper than getting a new cable run between the thermostat and the heater.

It worked! and was extremely easy to install after the minor hiccup of needing to bypass the App and hookup the C wire at the furnace.

thanks for the help.


We did the same moving to a new (not Wyze) thermostat a few months ago. We do not have central cooling and our furnace only had two wires to the thermostat - R and W. It too is a 24v system. So I called customer support, sent them pics of the old thermostat wiring and they replied with “custom” wiring for this thermostat for my situation since there was no cooling wire, I only needed the two as it is internally bridged to C. Problem solved and it has worked flawlessly since.

I think I may have the same issue, but I have a main thermostat that hooked up fine, but our master bedroom has a second thermostat with just 2 wires. I ashoukd also say that I have two furnaces, and I think the contoller in the bedroom runs that secondary furnce. Its kind of a crazy setup. Did you have the third wire or did you somehow trick the thermostat into using just 2?