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I had a really weird issue yesterday where my upstairs A/C unit quit cooling.

Now I've seen this kind of thing before. Usually it means the unit has frozen up. So I checked the line coming out of the upstairs fan unit. No ice. Then I went outside and checked the line around the compressor. No ice. Seemed perfectly fine. The two units' fans are next to each other, and the working unit was blowing out a lot of hot air, while the upstairs unit's fan was blowing out a lot of air-temperature air (kind of refreshing).

So I figure its still most likely ice frozen somewhere I can't get at (or at least I don't know what else to do), so I put in a fresh filter and turn the unit off for 3 hours. When I turned it back on, its been working fine since.

My wife joked about "rebooting" it, but that got me thinking... We actually had two brownouts yesterday. I was here for the second one, and it was so subtle I wouldn't have known it happened except my computer rebooted.

I just checked with my son (who is up there all day), and he told me the same thing happened the day before. We had a very similar brownout that day at 3PM. He checked the airflow at the vent, and the volume was fine (unlike when it freezes), just the air wasn't cool.

So I'm wondering if a really short power dip like this might actually cause a compressor to need to "reboot". I always thought they were entirely mechanical parts, so that doesn't make sense, but I don't know what kind of electronics are in a modern compressor.

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  • Probably not a reboot as in computers, but a reboot allowing it to cool down/ice to melt. Many year ago put gas in my car(before electronics), started losing power till it just quit. Had it towed home, and it started right up with power to spare. Think ice in carb.
    – crip659
    Aug 26, 2021 at 15:23
  • Did you reset the breaker for the compressor?
    – JACK
    Aug 26, 2021 at 15:25
  • @JACK - No, I just turned the A/C off via the thermostat. But I it seems possible there may be some electronics in there that gets de-powered when the compressor isn't running?
    – T.E.D.
    Aug 26, 2021 at 15:27
  • @T.E.D. Not with just the compressor not running but a full shutdown from the breaker could. When powered by two single breakers and hand tied, I've seen one breaker trip but still appeared to be closed, hence my comment about resetting the breaker.
    – JACK
    Aug 26, 2021 at 15:46
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    I'm not sure of all the mechanics and physics involved, but a brief brown/blackout could cause it to short-cycle which could cause it issues. There are a number of questions here about short-cycling which would explain it better. Turning it off for 5-10 minutes (even just at the thermostat) after a brief power blip would prevent that from happening and the ill effects that result, as I understand it.
    – FreeMan
    Aug 26, 2021 at 16:01

2 Answers 2

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Considering that the unit has a microcontroller-based control board (unless it's really old or really old-tech) then yes, things like brownouts or other transients can cause it to get into an odd state and a "reboot" may help.

It's not the compressor itself but the electronics that are controlling the compressor.

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  • I think its either 1 or 2 years old, fwiw. The relative newness of the unit was a large part of why I was wondering this. Assuming this were happening, would just turning the A/C off upstairs be enough to cut the power to it, or would I need to flip a breaker? It seemed like the former worked.
    – T.E.D.
    Aug 26, 2021 at 15:23
  • I doubt that turning off the A/C at the thermostat would actually reset the logic board but it may have allowed it to regain its sanity and start working properly again. A full power off at the breaker would ensure that it's completely shut off and restarted cleanly.
    – jwh20
    Aug 26, 2021 at 15:33
  • Well...easy enough to go do right now I guess...
    – T.E.D.
    Aug 26, 2021 at 15:34
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    I wouldn't say "really old". Many A/C systems are very basic and have little to no computer control. Mine is about 10 years old and the A/C is really just a couple of relays and contactors when you get down to it. The furnace has a logic board for all the gas valves and safety switches, but the A/C is dumb and simple.
    – JPhi1618
    Aug 26, 2021 at 16:03
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    @T.E.D. One failure mode of a compressor is for the windings to short to ground, which would trip a breaker or stop it from staying on. Good luck, and feel free to post another question about troubleshooting a compressor failure or other AC problem.
    – JPhi1618
    Aug 26, 2021 at 19:02
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After the circuit breaker failed too, I was forced to call out my HVAC guy who installed it last year, so I suppose I have a definitive verdict now.

It was in fact a bad capacitor on the compressor, just like in this question with very similar symptoms to mine. It was still under warranty, so no problem there.

Of course the circuit breaker was probably vintage 1976, and very much out of warranty. Also out of stock nearly everywhere (thanks COVID!), but it certainly could have been much worse.

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  • Not changing my checkmark, because it was not only the right answer to my question, but ended up leading me to a relatively speedy resolution. Wouldn't have been fun to limp along the rest of the summer and part of next on a failing compressor capacitor.
    – T.E.D.
    Aug 27, 2021 at 15:29

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