Old Gas Furnace with Wall Adapter for C Wire

[speadie]

Wyze thermostat simplified block diagrams

I made a couple of simplified block diagrams to show why running the wyze thermostat requires a C wire and why if you use an external transformer, you cannot use the Cooling aspects of the wyze. Not sure if this will help anyone, but it’s worth trying. Current needs to flow through the R and C terminals of the transformer, through a load in order for that load to get powered properly. Without a complete loop back to the transformer, you run the risk of the current finding its way back through…

You could use an isolation relay on the furnace side to switch the fan separately, or you could just connect the G and W wires together and have the fan always run when the furnace is running. It’s not ideal, but it would work. You should have a fan control if you do this though.