Thermostat - Not recognizing GREEN and YELLOW Wires

Well I thought I had a fairly easy setup, but … the latest unit (number 2 of 4) is having issues with the AC. The first unit was a 2 wire heat only setup, and went smoothly.

The unit I am working on now, has the 2 wire (R,W) heat (base board hot water) and 3 wire (Y,G,B) AC. These are separate wire bundles, the AC unit is in the attic, and boiler in the basement.

Installed the Wyze and connected the colors above. Connected the thermostat to the base and the thermostat did not power up and just clicked. The AC unit is 20+ years old, and suspect the C is less than 24V?

Fortunately, I have a C (blue wire ) on the wire bundle from the heat, so I connected the blue wire to the common on the boiler and connected it to the C on the Wyze and the thermostat powered up.

At this point it says the G and the Y wire are “missing”. Going through a test the heat works, but the AC does not.

Obviously the old thermostat works fine, it is not a smart unit, just programmable with manual cool, off,heat switch.

Any suggestions would be appreciated.

Thank you

Pictures of all involved components please. (furnace board with thermostat wires, ac board with thermostat wires, old thermostat hookup, and new wyze hookup)

From your description, it sounds like you are not connecting the Rc wire coming from the AC to anything, is this correct?

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OP might only have one R wire that is common across both. I know on my setup I had the jumpered RC and RH on my old stats and connected the wire to the RC terminal of the wyze. Nothing worked when I connected it to RH on the stats.

Here are the pics, hope it helps. I do not have access to the AC as it is in the attic and currently the hatch to get it is inaccessible.

First is the old thermostat, second is the new and the last is the R,W and B from the boiler.

Guess I can only add one pic, so combined.

Thanks for the response. I definitely have separate wiring, as I had to run the new 5 bundle since there was a short in the old.

Do you have a C wire from the A/C system?
The reason the AC wont work is that it expects the wire that is connected to RC controls the AC system, and you have that RC terminal connected to Rh from the heating system. If you want cooling to work you will need to figure out a way to get Rc to be connected to the AC system- the alternative is to use isolation relays, but those would need to be installed near the A/C system, as they are too bulky to be located near the thermostat.

Wiring with isolation relays would look something like this:

Yes, I have a C wire from the AC. I originally connected that to the C on the Wyze, but it did not power up the Wyze.

Do I connect the R from heat to RH on the Wyze and the C from the AC to the RC on the Wyze.

Wyze always says to connect the R from heat to the RC?

Thank you,

In your case, you would connect R from the AC to Rc, R from the heat to Rh, and C from the AC to C
You need to tell the wyze app that after removing all bridges you have the following wires: Rc, Rh, W, G, Y, C

Well, I will try that … backing up a few steps, I guess I am not sure that my AC has a C wire. It has a BLUE wire (which I thought was a C wire, but when I connected it to the C connector on the Wyze, it did not power up), but it is connected to the RC on the old thermostat.

Why the AC person used that (instead of RED), not sure. I will see if any if there are any other wires in that bundle and what they might be, I am confused as to why I cannot use the C wire from by boiler, I see this snippet in the Wyze Thermostat. I thought a “C” wire is a “C” wire …

Any I will post my results …


Rc is 24vac from the air conditioner transformer. Rh is 24vac from the furnace transformer. C is the other side of the transformer coil, and is unique to each transformer. So the C side of the transformer on your air conditioner won’t necessarily be at the same potential as the C side of the furnace’s transformer, even though they both are 24VAC transformers.

Thank you for all your assistance! It got it working …

This DID NOT work …

In your case, you would connect R from the AC to Rc, R from the heat to Rh, and C from the AC to C
You need to tell the wyze app that after removing all bridges you have the following wires: Rc, Rh, W, G, Y, C

But I connected the R from the AC to Rh and the R from the HEAT to Rc. The app gave me some wiring error, but I skipped and the Test activated heat and AC w/ fan correctly.

Thanks again!

Well, hope it stays working- it sounds like you do not have a C wire on your AC, but maybe both of the transformers use a grounded C, so it is able to find C through the ground path of your house wiring to the other transformer. this might cause more stress on your transformers, so keep an eye on it.

I will monitor … For sure, the 2 transformers are not connected to each other. One is in the basement as part of the control panel (Argo ARM-4P) for the boiler. The other is part of the air handlers (2, one for each side of the house) that is in the attic.

The boiler was in place when we purchased in 1998 and working with programmable 2 wire honeywell mercury thermostats. The AC system was installed in the 2004 in the attic, with vents and returns in the ceilings. At that time, 2 of the 4 thermostats were replaced with programmable thermostats, a unit that could handle both heat and AC, I replaced the other 2 (heat only) mercury units with the same programmable units.

I have 2 more Wyze to install, 1 is a just heat and the other is the same heat/AC setup.

I will be going out of town for 6 weeks and want to set the temp of the house low, be able to monitor and then turn up the heat before we come back. Obviously, hoping to not have a failure of any unit.

Thanks again,