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I'm trying to install the Amazon thermostat to replace a mercury switch thermostat. The existing wiring has yellow, white, green, red and once the wall plate was removed there is a blue wire untouched. I used multimeter to check the wires with around 27 readings. The blue wire has power but the green didn’t and when I go to the furnace as far as I can tell the green wire isn’t in use. I think the blue wire actually controls the fan because I hooked it to G and turned fan on and it started to run manually. Is it possible to use the green as the common without damaging anything? I don’t care to not control the fan. It’s old furnace and I’m not to sure of where to add the C if I do need to go that route. Some pictures included hopefully I can be directed where/how to add the C. I have more pics but I can’t seem to add them.

Wiring in furnace & what I’m guessing to be transformer:

Outdoor thermostat wires

Furnace board wiring

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  • What does "there is a blue wire untouched." mean? Is the blue wire not connected to anything? Also, "I used multimeter to check the wires with around 27 readings." which wires had 27 readings? Please feel free to edit your question to clarify. Have you looked at the 8 bajillion other "how do I add a C wire for my thermostat?" questions?
    – FreeMan
    Apr 18, 2022 at 18:11

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You'd be really foolish to use random colors for the key functions of your furnace. I don't care what color the C wire is, but the green is supposed to be fan control (fan override). Here are the standard wires:

  • R = power supply to the thermostat (red)
  • W = "Call for heat", furnace automatically controls fan.
  • G = "Call for fan", run fan even if furnace is not burning.
  • Y = "Call for A/C", air conditioning unit starts.

They are Red, White, Green and Yellow obviously. If you deviate from these colors, then "the next guy" working on your system will be very confused and likely to damage something.

Here's the important part. The A/C uses the furnace's fan to distribute cooling. But the furnace is not attached to the Y wire so it doesn't know when the A/C is running. That is why the thermostat needs the G wire -- so it can "call for fan" whenever it "calls for A/C". Without it - no A/C.

Yes, the furnace controls the fan automatically for calls for heat - but that only works for heat.

So, do not defeat the "G" wire/fan control.

The preferred color for the "C" wire is BLUE. So if a blue wire is available, use it for C. You will probably need to connect it at the furnace or move its connection.

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    The way I remember is C = Common = Cyan. Which isn't exactly the usual thermostat wire blue but close enough. Apr 18, 2022 at 23:04

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