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I have a rental apartment with two a/c units.

I installed magnetic contacts on the windows of each room and a small module (a sonoff basic flashed with tasmota firmware) that shuts off the a/c relay (25A) whenever a window is open.

It was a proof of concept project which turned out to be a success and it's working great. Now I'm thinking of installing similar modules to another apartments.

My concern is, what will be the effect of that (probably frequent) shut offs to my a/c unit. I understand that it's not good for any device to shut its power off while it's working but what's the case with a/c units. I ve seen several similar applications in hotel rooms, are they doing it differently? What would be the correct way to achieve this result if any?

Thanks in advance.

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    It is not harmful for an a/c unit to operate with an open window. I also think it is not harmful to the a/c unit to cut off its power while the compressor is on. What might be harmful is if the power is turned back on before the high side refrigerant pressure has equalized. When the compressor cycles off, a timer prevents the unit from coming on to allow the high side pressure to return to resting pressure. If you shut off the unit by interrupting the power, then this timer might not function. – Jim Stewart Aug 25 '18 at 13:26
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    That equalization needs electricity to happen or does it just happen naturally? Meaning if I added a delay to the power on after a power off, would that solve it? And what should that delay be? – krasatos Aug 25 '18 at 13:33
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    It might be better to have your sensor break the thermostat's call for AC rather than break the AC power directly, the AC may handle that interruption more gracefully. If you want to do it for heat as well, you could set it up with a relay with two pole contacts, one breaking the call for cooling, one breaking the call for heat. – batsplatsterson Aug 25 '18 at 14:55
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    The equalization of refrigerant pressures occurs even if the power is off. Three minutes should be enough; five minutes for sure. – Jim Stewart Aug 25 '18 at 15:09
  • Not only is it better to break the thermostat's call, it's a lot cheaper relay. – Jeff Learman Dec 10 '18 at 20:20
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You will need to protect against short cycling the air conditioner

When you turn an air conditioner compressor off, the pressure in the system does not equalize instantly -- instead, it takes a few minutes for the pressure built up on the high side of the refrigerant circuit to equalize with the pressure on the low side.

If you turn the compressor back on before then, this is a problem because now the compressor is trying to pump refrigerant into a zone that's already at high pressure, which puts quite a bit of stress on the compressor.

As a result, your device will need to delay a few minutes (3-5 is typical) before allowing the air conditioner to turn back on after it shuts it off.

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    Does the equalization of pressure happen while the power is disconnected or does the unit need to remain on standby? Is the compressor triggered when I connect power (standby) or when I turn the unit on by remote? Dont modern units have any protection for this built in? Will a 3 minute delay on the module before powering back the unit protect me from this? Or should I instruct my guests to allow the unit to be on standby for a couple of minutes before turning the a/c on again after all the windows are closed? Also do those apply to heating mode too (during the winter) or only to cooling mode? – krasatos Aug 25 '18 at 14:07
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    @krasatos -- equalization will happen fine with the power disconnected, and the whether the compressor starts or not when the unit is powered back up will depend on the room temp and setpoint, and if your unit is a heat pump, yes, these concerns apply to heating mode as well – ThreePhaseEel Aug 25 '18 at 14:09
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Most AC units today have a timer to prevent the compressor from turning on after a power cycle. I have not seen a larger unit that doesn't have this protection and most smaller units also have this but not all. Yes the fans turn on right away but the compressor may not start for 3 or more minutes. Very high efficiency units that I have installed really don't need more than a few seconds to equalize and only run the compressor based on tempatures and pressure levels. Killing power it tough on the electronics on these units but turning the thermostat down or off won't cause a problem on these units.

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As amplification to the correct answer of @ThreePhaseEel:

  1. Decades ago: if an A/C short cycled, the high load on restart would blow the fuse or circuit breaker for that branch circuit. One solution was a Slo-Blo fuse. It would blow at the same overcurrent level, but would tolerate that overload for a short time (seconds?) before blowing. So the A/C compressor could have enough time to get running against the back pressure and drop back to normal current draw.

  2. More recently: the A/C can sense that it's drawing too much current and stop the attempted compressor re-start for a pause of a minute or so. Eventually the back-pressure drops enough that the A/C allows the restart to continue, since the current overload is small enough/short enough to not damage anything (even with normal fuses or circuit breakers)

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