enter image description here

I'm having an issue with my NEST thermostat short cycling my air handler fan while in running AC. This is a known issue, and the fix seems to be adding a common wire to the NEST.

My thermostat is wired with 18-5, and currently the yellow wire is unused. I want to use this for the common. Pic of the board is attached. Humidifier and AC compressor are wired to board.

Can I just add the Yellow wire to the C terminal without changing anything else? I should probably pigtail the 3rd wire, correct?

Sorry for the dumb questions, I just want to make sure I've got it right.


ETA: Picture of terminal block

Left to right: R C W Y G

enter image description here

  • Please tell us what are the letter codes below the screw terminals.
    – Stavr00
    Commented Jun 18, 2018 at 13:20
  • Sorry, didnt realize they weren't visable when I downsized the pic. They are, from left to right: R, C, W, Y, G. I'll put a pic below.
    – Troebs00
    Commented Jun 18, 2018 at 14:21

1 Answer 1


It would be preferable to use the blue wire instead. The conventions for colors assign Yellow to cooling. The other colors also have standard assignments.

The Blue wire from a 18-5 bundle is what would be best for the C common.

If following recommendations to the letter, then the three wires connecting to C should be pigtailed.

  • 1
    Thanks. Right now the blue wire is on the Y terminal on the board, and Y1 at the NEST. I'll swap the yellow to Y on the board, Y1 on the nest, and Blue to Common.
    – Troebs00
    Commented Jun 18, 2018 at 14:26

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