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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.

krasatos
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3 Answers3

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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.

ThreePhaseEel
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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)

DJohnM
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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.

Ed Beal
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