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I would like to buy a smart thermostat and I am confused about the C wire. I went to check my furnace in the attic and I saw a white wire connected to the C, I was excited as I thought that that was the C wire, but then behind the thermostat, I have only 4 wires connected, a white one connected to W, a yellow connected to Y and green connected to G. There is another cut blue wire that is not connected. Is that the C wire? If the blue wire behind the thermostat is the C wire, why I don't see any blue wire connected to the furnace? I am attaching a picture in the hope that someone can help me. Thank you in advanceenter image description here

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  • Welcome to DIY! The photo of the terminals is just a little too close -- it would be helpful to see where those colors are bundled into cables. (Take a close look at the cables; is the cut-off blue wire just barely visible?) A photo of the wiring at the thermostat could be helpful too. – Greg Hill Oct 20 '20 at 22:57
  • Yes, can we have a zoomed-out shot that shows where those wires join into cables, please? That'd help us make sure of what's going on here – ThreePhaseEel Oct 21 '20 at 1:04
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Somebody was thoughtful

The good news is that somebody was thoughtful and ran a thermostat cable with extra wires in it, so you have the spare wire you now need. So, take that blue wire, land it on the C terminal on your furnace board alongside the existing C connection from your air conditioner (if you need to extend it, just get some blue 18AWG TFFN and nut it to the existing blue wire, then land this pigtail on the C terminal screw, being careful not to undo the existing connection of course), hook the blue wire up to the C terminal on your new thermostat, turn the furnace back on, and enjoy!

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  • Thank you so much for your reply! This still sounds very complicated to me, as there is no blue wire in the attic where my furnace is, as you can see from the picture, there is only a white one. – Monica Blade Oct 23 '20 at 0:29
  • @MonicaBlade -- can you post a zoomed-out shot of your furnace's wiring compartment then, please? I don't have remote X-ray color vision :) – ThreePhaseEel Oct 23 '20 at 0:32
  • Thank you. I just added it below – Monica Blade Oct 26 '20 at 2:18
  • @MonicaBlade -- your photo didn't post – ThreePhaseEel Oct 26 '20 at 2:35

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