I wanted to get a Nest Thermostat E, but my current thermostat doesn't have a C wire. From what I read online, it's better to use a C wire, so I was considering adding a Venstar Acc0410 adapter.
However, when I looked at my wiring (I rent a host but got approval to install this), it didn't match what I expected.
The furnace has 5 wires: Green in the G terminal Red in the R terminal White in the W terminal Blue in the C terminal Yellow in the Y terminal
This runs up and out, but then gets split in the rafters above the furnace. Green goes to the G on the thermostat Red goes to the R on the thermostat White goes to the W on the thermostat Blue goes to a white wire on the AC unit Yellow is Unused(?!?)
So that's 3 to the thermostat (G,R,W), one to the AC. The Thermostat also has another wire connected to Y going directly to a red wire on the AC.
It's weird to me that the C terminal on the furnace is going to the AC unit, and that the Y terminal is unused.
I could try to make the Vernstar adapter work, or I could run a new 5-wire from the thermostat, but I'd still have a weird situation with the C wire and that unused Yellow wire.
Could I, with a new 5-wire, connect the thermostat C wire to that unused yellow, and move it from the Y terminal to the C terminal on the furnace? Then connect everything else the same as it is now.
Here are some pictures I took:
Edit: Here's a very rough drawing of what I'm thinking: