Adding Unused C-Wire for Nest Thermostat

furnacehvacnestthermostat-c-wire

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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.

Thanks

ETA: Picture of terminal block

Left to right: R C W Y G

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Best Answer

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.

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