Wiring thermostat with separate mercury thermostats for heat and AC

I have two separate mercury thermostats, one for the furnace and one for the central air. I am try to combine into one wyze thermostat. The AC has three wires (Y, G, R). The furnace has two wires (W,R). How do I combine these?

Are the 2 old thermostats in different locations? Post some pictures of your thermostat and control board wiring.

It should work by using R, Y, W, G and you’ll have to use the C wire adapter, unless you have a extra wire to use as the C wire (like if both thermostats are together and use one of the R wires)

Yes, the thermostats were in two different locations. These images will show what I did to get them wired to one battery powered thermostat. Now I need to get this mess into the Wyze thermostat. Thanks!

So I take it you have to flip the switch to go from AC to heat and visa versa?

I’m not a HVAC tech but I’m wondering if you can either connect just one of the R wires & eliminate the manual switch. For instance, using the wiring from the AC, use the C wire adapter, but also connect the W wire for the furnace. Give it a try and see if heat will work like that.

Correct. I flip the switch. I can always run another wire to the Wyze thermostat so that I have 5. I can then run two separate red wires, one to the AC and one to the furnace. If I do this will I have power to the thermostat or do I need yet another wire for power?

The R wire is the power, it’s the C wire that’s needed to complete the circuit for constant power. If you ran a new wire then you wouldn’t need the adapter

Thanks so much. So the C wire needs to a power source? I have been advised to purchase a 24V transformer and run that to the C wire connection. Otherwise, because I don’t have a C wire connection at either the AC or the Furnace I will need a power source on the C wire? Does this sound right?

Thanks again. If I use the adapter instead of running a new wire does the common wire in the adapter need to be connect to a power source (i.e. 24V transformer)?

Compared to AC 110v voltage where you have a hot lead and a ground, the R is 24v and the C is “common” (similar to ground).

I don’t think it’s necessary to use a separate transformer. Usually people with only 2 wires need that.

Do you have a picture of the control board at both AC and furnace?

No, you cannot get common from a stand alone transformer when you want to use the cooling on the wyze.
However, you can run a common wire from the air conditioning unit to the wyze location.
You might be able to tell the wyze you have the following wires:

And it might walk you through setting up the common wire adapter. Do that in the A/C unit and ignore the W terminal for that.

The only downside to this might be it running the fan on your A/C when your furnace runs.
Hopefully wyze will fix the fan logic on furnaces to prevent this behavior.

Good morning. Thanks so much for thinking this through with me. Here’s where I think I’m going. Thoughts?

That should work, but I would think either the AC or furnace could provide the power. The diagram that @speadie provider would be your best solution.

You would need to run a common wire to both the A/C and furnace from the adapter, and the adapter would need to be powerful enough to run the heat and ac subsystems (so at least 40VA)

The current has to flow from the R wire to the C wire, through the loads in your devices… With the adapter providing R, it has to be connected to C inside those devices as well. When the thermostat turns something “on”, it is just sending power from the R wire to that terminal. example: when the thermostat wants to run the fan, it connects R to G and the current to run the fan flows through the fan relay in the air handler, back to common, switching the fan on. Without a common wire going back to the transformer that is feeding the power, it would have to find some other path back to the transformer, or it would not work at all.

Speadie, thank you for your comments. Not sure if you say my original setup but in these configurations there was not a C wire at all. The red wire provide power which was returned to the white, green or yellow and that’s where the circuit ended. I don’t have a C wire on either the Furnace or the AC unit. I am assuming the Wyze C-wire adapter solves the problem you describe. Thoughts?

You do have a C wire on them, you just don’t know which wire it is. I can assist with that. Show pictures of the circuit boards if they have them, or the place where the thermostat wires are attached to the furnace wires if there are not boards. Also show the transformer inside the units. To clarify, you will need to cut the power to the devices and open the access panel on them to expose the internal wiring and take pictures.

Thank you Speadie. What I meant is that I only have 4 wires coming from the existing thermostat (R<W,G,Y), There is currently no Common or Blue wire in the mix. On my furnace connection for this thermostat zone there are only two connections, one for red and one for white. There are no others for this zone. The AC is more confusing but there are three wires running to it, one green, one yellow and one red. Without taking the unit apart more I cannot see a Common wire. I really wish I could.

I can assist you if you share images.

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Speadie, here are the two photos you asked for. I hope this provides the info you need to give me advice. I really appreciate your time.

Pretty sure this is common ( C ) in the Air Conditioner. If you can get a closer shot of that fan control circuit board or the schematic that is hiding under the wiring in the A/C, I can make sure. Or just give me the model number of that air handler, it should be posted on the schematic somewhere.


I hope these help