A set of three air-conditioners cool a storage building in an American sub-tropical environment. There is no heat: AC only. Cooling is signaled by switching the RED (24V) to:

  1. Yellow wire (cooling relay) AND
  2. Green wire (Fan relay)

When cooling is engaged the same 24V from the red wire is connected to both the Yellow (cooling relay) and Green wire (Fan relay).

For a given AC unit, is there any reason to not short the yellow wire to the green wire? This would simplify the controller design and reduce wiring for a set of three air conditioners.

  • I take it you're OK with not having the ability to ever turn the fan on by itself? Feb 1, 2020 at 18:42
  • @ThreePhaseEel Yes. Is there any reason to run the fan without cooling?
    – gatorback
    Feb 1, 2020 at 18:44
  • I could see where this'd be done under mild conditions (I have to do it in my own house under part-load to get anywhere close to comfortable) Feb 1, 2020 at 18:45

1 Answer 1


Measure the voltage between the yellow and green wires, if it is zero volts there's no reason not to connect them. It most-probably is zero volts.

  • I added additional commentary to the OP to reflect that YEL and GRN are shorted together through RED when the fan and cooling relays are engaged
    – gatorback
    Feb 1, 2020 at 4:01

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