Dual thermostat swap wiring questions

Hello there. In the process of swapping out 2 thermostats with Wyze stats.

Here’s my situation:

  • 2 thermostats, one upstairs, one down
  • 2 AC units outside, one for each floor (obv)
  • 1 Heating/Fan unit in the attic.
  • Downstairs stat appears to be the primary one given how many wires are actually connected
  • There is a weird water valve sensor that is throwing me off
  • I am not sure which wires go to the outside units, so I didn’t mark that on my diagram

My questions:

  • Following the app walkthrough it suggested that I need to install the C-wire adapter, the forum tells me otherwise. I have free wires for each stat, do I need it or not?
  • When I went through the app and follow the directions for the downstairs stat I got no power. I tried a couple of times one evening and gave up. Been doing research ever since. Hoping for some assistance on where to go from here.
  • I’ve seen some complex stuff on the forum when searching for similar people in my situation. I don’t believe there is a damper involved or any other complex wiring that I have missed. Am I wrong?

Let me know if any more information is needed. Thanks!!!

Frame 1

The diagram is great, but do you have photos? For optimal troubleshooting, I’d like pictures of each stat , the zone boards, whatever’s going on with the float switches, and the furnace/fan controller board. Thanks in advance.


@speadie See this Google folder for some pics.

Also an UPDATE: I have added the black wires from each of the thermostats wires to the C on the board. I don’t have an updated pic, but I can get one if it is crucial. I tried to hook everything up last week in my downstairs stat and all it does is click. I’ve read up on it and it sounds like it is related to the voltage. I believe I was getting the standard 24V when I hooked up multimeter to it.

Thermostat (Up)
Thermostat (Down)

Switch Board - hopefully you can see enough. I have since added the black wire from each thermostat to test it as a C, to no avail (see update above for more details)
Water Leak Switch - The two larger gauge yellow wires from the furnace go to this.
Safety Valve - There are two black wires in the furnace that hook to this.

Let me know if you need any other angles or info. Thanks in advance for your help. If you can ask the mods for greater access then uploading more pics would be nice here. :slight_smile:

I don’t see a link to the google folder/ images.

@speadie Sorry, here is the link.


Ok, I see the confusion,… that second set of wires you have going to the top thermostat in your schematic does not in fact go there, instead it goes to one of the AC units through the float switch to kill the power to the compressor if there is a condensate drain blockage. You might have a zone control somewhere. if you follow these wires, do they lead to another control panel in the crawl space, or do they go down into a wall?

Hi there. Thanks again for your help. I have followed those wires a couple of times and it does go straight out of the blower unit through the attic then down into the wall. Let me know if you need any more pics. Not really in a position to open everything up, but can a little later.

@speadie Added a couple photos to the same Google Drive folder for reference.



  • 8-15_StatWire - This is showing that the brown wire for stat1 is going out the blower.
  • 8-15_StatWireCloseup - Shows the stat wire coming straight out of the blower.

There has to be a zone control somewhere. not sure where it is in your case, maybe a utility closet? Usually it would be mounted on the ductwork to control dampers as well.

Or, I suppose you might have a second air handler somewhere for the other thermostat (crawl space under the house?)… if that’s the case, try uncoiling the blue wire that is behind the upstairs thermostat and connecting it to C on the wyze.

As for uploading the images here, you should get access to do that automatically after a few days here, or you can flag a mod who can give you access.

You are definitely correct that there is something missing. Did some thinking about it today while I worked and I decided to go outside. It looks like one of my units is actually for heating & cooling while the other is just for cooling. I believe that the upstairs uses the cooling only one and the main floor uses the combo unit. Not sure if that makes sense, but it does in my head. :slight_smile:

Unfortunately, where the unit is happens to be the opposite side of the house of my crawl space door. I am going to try to get under there tonight and see what I can find. I believe you are right though. If I can’t get under there tonight, I may just attempt to install the upstairs stat just to prove my concept. It is all making a little more sense now. Thanks again. Stay tuned.

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Well @speadie (and my assumptions above) was right, there was a whole Heating/Cooling unit for the main floor. I definitely wondered why one of the HVAC units was so huge outside, and that is because it is both heat/cool. All I had to do was turn off the power, open up one of the side panels, make sure the black wire was connected to C (it was labeled B in my case). Once I put everything back together and installed the other stat, all worked like a charm.

Not sure if this will help anyone in the future, but I am definitely new to the Southeast where it is common to have an HVAC unit for each floor. I definitely learned a lot along the way that should help in the future. If anyone has two units and finds this thread feel free tag me with questions. It was pretty quick once I figured it out with Speadie’s help. Thank you!!!

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