Could l wire a thermostat outside and and set is to 68 degrees and use it as a switch to prevent the air conditioner condenser from running when below 68 degrees. And use in-line with the yellow wire.

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    I'm sure you could... Is there a problem with the indoor stat that's causing the AC to run even though the house is cool enough inside? Seems like solving that problem would make more sense.
    – FreeMan
    Oct 1, 2021 at 12:55
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    @FreeMan I think the OP is part way to "if the outside temperature gets low, I want to turn off the A/C and automatically open windows instead. I agree it's silly. Oct 1, 2021 at 14:22
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    Patrick, it's trivial to wire two thermostats in parallel, thus requiring both of them to read their local temperature as greater than your setpoint. However, as the comments point out, it's the wrong solution to your problem Oct 1, 2021 at 14:24
  • There are plenty of home automation setups, @CarlWitthoft, that would very easily handle this. Of course it requires a computer (like a RaspberryPi for less than the cost of a new stat), and some other bits of hardware and free software. But it could, actually, turn off the AC and open a window (or two).
    – FreeMan
    Oct 1, 2021 at 14:26
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    @carl witthoft thermostats in parallel create an or circuit if either A or B stat is on the system turns on.
    – Ed Beal
    Oct 3, 2021 at 0:37

1 Answer 1


Regardless of how good of an idea this is, what you want is possible and is actually common for heat pump systems (albeit at freezing temperatures). Your outdoor unit just has a pair of wires going to it that allow the indoor thermostat to switch it on. You would just need to find an outdoor thermostat that has the range you want and wire it in to the control wire. Its low voltage, so it's a simple and safe DIY project as long as you disconnect the power to the outside unit before opening the panel.

One issue is that the indoor thermostat would still turn on the blower inside, but it would just circulate air in the house and not cool. The blower could come on for a long time if circulating the air doesn't cool the area the thermostat is in.

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