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Installed a new Nest thermostat yesterday. My setup is a 4 wire system. Green, Blue, White, and red. I have central air and gas heat. They had the Blue wire "C" into the Y for the AC. Since the Nest needs constant power, I had to use the "C" blue wire for constant power for the Nest. Now I have no wire for the central air. How difficult is it to install a "y" yellow wire so my AC will work?

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    Nest has good customer support. Have you tried that? You may just need to install a jumper. – isherwood Feb 11 at 16:22
  • Big thing to remember is colors are based on convention and whoever installed your A/C is not required to use the "correct" colors. If your old thermostat had a blue wire going to the A/C terminal, it's very unlikely that is the "C" wire. Probably more like "oh, blue is a cold color, I'll use that for A/C"... In other words, you're looking for a solution to the wrong problem. – JPhi1618 Feb 11 at 19:56
  • Is pulling a new thermostat cable an option? – ThreePhaseEel Feb 12 at 1:22

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