Wyze Thermostat: Converting from unsupported 2-wire to 3-wire

I posted this as a reply to a reply in another thread and realized I probably should make this a separate thread.

My present furnace / thermostat / transformer arrangement consists two wires: Rh and W. Since this apparently isn’t supported, I assume I need to install a 3rd wire and re-configure the wiring as follows. Can someone validate this approach for me, or correct it if I am wrong?

24 VAC xformer lead 1 of 2: to Wyse “C” and to furnace lead 1 of 2
24 VAC xformer lead 2 of 2: to Wysie “RH”
Wyze W1: to furnace lead 2 of 2

I haven’t tried this myself but I would think you would use the 2 leads from the 24v transformer to connect to the Wyze thermostat Rc and C just to power the thermostat, then use only W1 from your furnace, but I’m 100% sure.

I think you’re right about connecting to Rc instead of Rh like I’ve diagramed. I think they don’t use Rh (which is supposed to be 24VAC for the heating circuit) at all, and just want you to connect to Rc (usually for the cooling circuit.)

I think using Wi to the furnace is right, but that’s only 50% of the circuit, which is why I think COM from the xformer must be connected to both the furnace and C on the thermostat. WIll be giving this a go tomorrow, after I run a third wire.

I can say though that by connected the white wire to C and the red to Rc, the thermostat powers up.

I don’t know if mine is technically correct, but it works. I originally had a two wire system as you do RH and W that were in powered and went to an old mercury thermostat. A few years ago I added the wires C for power, G (for fan) and R for power so I could use a dumb programmable thermostat. That worked fine.
Anyway, now with the newly installed Wyze thermostat I connected RH to RH, C to C, W to W1, G to G and R to RC. I did get an error during setup about missing wires that I don’t need, but I somehow skipped it and it all works fine.
The reason I connected my RH to RH is because it contains no power.

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I’m not a HVAC tech so I’m not sure, but my theory is that if you wire a external 24v transformer (separate from the furnace) directly to the Wyze thermostat then connect the W from the furnace to the thermostat, when the Wyze thermostat calls for heat it will send the 24v down the W wire and the control board should fire up. I’m just not sure if the control board requires a 24v completed circuit from the R on the board to the W. It may be satisfied just seeing 24v on W.

@WyzeChuan can you clarify?

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are you using the transformer in the furnace?
Currently your thermostat is an automatic switch that closes a between RH and W1 to turn on the heat.
RH is 24vac
For wyze power you also need a common (from the same 24v souce).
If you can add a wire from the thermostat to the furnace. That would be ideal.

If you use an external transformer:
you would connect 24v on “RC” and “C”
You would put your existing heating wires on “RH” and “W1”

If you use the furnace transformer:
you would connect your existing “RH” to “RC” and common from furnace to “C”
You would put your existing heating wire “W1” on “W1”

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I finally got mine to work, I had a two wire system. I bought a 24vac transformer to add a c-wire.
from the furnace my white went to w1, my red went to g, then from the transformer one went to c then the other to Rc. Seems to be working good now.

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