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I just got a Nest E thermostat and I'm trying to hook it up. There is a blue wire that I had assumed was the "C" wire, but looking at the furnace, it doesn't look like that's the case, as the blue wire looks like its connected to the W2 terminal.

So my question is, is there something I can plug this blue wire into that will make it "live" so it will power my Nest? I think I can plug it into the B/C one, but can I just hook it in together with that existing white wire? The white wire goes with one of the yellow wires off to something other than my thermostat.

How I have the Nest hooked up

The furnace with the terminals

  • Do you care about the second stage of heat on your furnace? – ThreePhaseEel Apr 20 '18 at 22:21
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The wire currently hooked to Y and C will be going off to your air conditioner condenser unit. Y is cooling.

C is common, and yes, you can connect another wire to that terminal.

You appear to have a two-stage furnace however, with W1 and W2. That means you have a couple options:

1 - Install another wire

To take full advantage of two-stage, you need to connect all 6 wires (R, C, Y, G, W1, W2).

You can run another physical wire, or buy 3rd party accessories that add a unit at each end that allows doubling up of some of the wires.

2 - Run as single-stage

You can possibly run your furnace with only W1. Many (all?) furnaces will automatically go to the second stage after they're run in single stage for a few minutes. This isn't as efficient as if your thermostat tells it to immediately go to second stage when needed, but is still usable.

3 - Run without fan control

You can potentially skip hooking up G (fan) but this means you can't run the fan in 'on' mode (or use periodic air circulation features) -- it'll only run when the furnace or A/C is on. (I'm not entirely certain about air conditioning with this setup though -- I am fairly sure A/C call will turn on the fan even without G connected, but not 100%, and it may vary by furnace model -- be sure to test if you are going to do this).

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