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:

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Edit: Here's a very rough drawing of what I'm thinking:

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  • You have a gas furnace, right? Also, I take it there isn't a spare wire hiding in the cable between the central junction and the thermostat, correct? – ThreePhaseEel Nov 9 '19 at 22:32
  • Gas furnace, yes. and no, there's no extra wire hiding in there :( – mrhollywoodgates Nov 9 '19 at 22:40
  • Does the blower turn on with the system set for AC and the fan set to Auto? Also, what make and model is the existing 'stat? – ThreePhaseEel Nov 10 '19 at 2:59
  • Are you sure you have your colors right? I see where blue is being used for the Y wire, and yellow is unused. At the risk of stating the obvious, it makes more sense for Y to be yellow. – Harper - Reinstate Monica Nov 11 '19 at 0:15

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