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Thread: Trying to find the Common wire for my Nest.

  1. #1
    Junior Member

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    Trying to find the Common wire for my Nest.

    White = Cooling
    Yellow = Heating
    Green = Fan
    Red = 24 V AC power

    I have 2 other wires Blue and Brown. Which one is common?

    When I measure from Brown to Red I get 24 V AC
    When I measure from Blue to Red I get only around 18 V AC

    Reason I need the common wire is to keep the nest charged or else wifi keeps dropping. Can i just connect that Brown wire to common since it's providing 24 V AC constantly?

    I mean I can go up to the attic and look but it's scary up there. . Is there any other way to find out maybe ohm Blue or Brown wire to the ground on the outlets?

  2. #2

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    it is imposable to go by colors alone. There are some standards for thermostat wire colors like yellow for cooling but as you can see your system is not wired that way. The best way to check the common wire would be for someone to check how the wires are terminated at the furnace. With the readings that measured it my be that the brown wire is the common but it would be best to confirm.

  3. #3
    Junior Member

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    Instead going up into the attic to check the common of the furnace can I not just go backyard to the ac unit and look at the color of the common there?

  4. #4

    User Info Menu

    Checking the common wire on the outdoor condensing unit will not confirm the common on the wiring going from the thermostat to the indoor air handling unit.

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