Window unit AC compressor thumping - why?
June 11, 2021 2:53 PM Subscribe
I have a six year old window unit air conditioner which produces a single pronounced thump at intervals. Why?
I've been assuming that it's the compressor cycling on and off, but I'm not sure. When I try googling it I have trouble getting compressor-related results for window units and most of the window unit results are for rattling, loose parts, etc.
Basically, at intervals the air conditioner will make one and only one thump that sounds/feels a bit like something heavy being dropped. It shakes the window frame a bit. I believe but am not sure that it happens more often during the day. The air conditioner is cooling normally. There is no rattling or clanging that would suggest loose parts or something broken inside it.
What is this? Can I fix it? I've cleaned the air conditioner and vacuumed it out. I don't want to take it out of the window right now unless I have some idea what I'm looking for because the weather is extremely hot and the air conditioner is a pain to install correctly.
I've been assuming that it's the compressor cycling on and off, but I'm not sure. When I try googling it I have trouble getting compressor-related results for window units and most of the window unit results are for rattling, loose parts, etc.
Basically, at intervals the air conditioner will make one and only one thump that sounds/feels a bit like something heavy being dropped. It shakes the window frame a bit. I believe but am not sure that it happens more often during the day. The air conditioner is cooling normally. There is no rattling or clanging that would suggest loose parts or something broken inside it.
What is this? Can I fix it? I've cleaned the air conditioner and vacuumed it out. I don't want to take it out of the window right now unless I have some idea what I'm looking for because the weather is extremely hot and the air conditioner is a pain to install correctly.
Response by poster: I feel as though in past years it was not this loud. Also my partner was really surprised at how loud it was (we have separate rooms due to sleep schedules, snoring, etc and hence separate window units). It is around 90 today and has been over ninety every day this week, in case that means anything. I'm concerned that the loudness means there's something wrong with the compressor or some other aspect of the thing.
My point is that it is SUPER loud!
posted by Frowner at 2:58 PM on June 11, 2021
My point is that it is SUPER loud!
posted by Frowner at 2:58 PM on June 11, 2021
I wonder if there are window units higher up in your building that could be having periodic drips or leaks that are falling on your window unit? If not this, I wonder if a 6 year old unit might be having mechanical problems.
posted by citygirl at 3:10 PM on June 11, 2021
posted by citygirl at 3:10 PM on June 11, 2021
First, make sure it’s related to the compressor turning on or off. That will narrow things down. Do it with the fan on constantly (not on econ mode).
Check the filter. It could be blocked or not sitting right. Take it out for a few cycles to be sure.
posted by Jobst at 3:11 PM on June 11, 2021 [1 favorite]
Check the filter. It could be blocked or not sitting right. Take it out for a few cycles to be sure.
posted by Jobst at 3:11 PM on June 11, 2021 [1 favorite]
I’ve never taken apart a compressor (of this sort, anyhow) but I had the same symptoms in my last refrigerator. The very plausible-seeming explanation I dug up was that the compressor’s pump is mounted on springs inside its housing, and when one of those springs breaks the pump can move enough at startup to thunk against the interior wall of the housing. It doesn’t indicate immanent failure, but it’s not fixable.
https://m.youtube.com/watch?v=3Uwx-_VecHo
posted by jon1270 at 5:55 PM on June 11, 2021 [3 favorites]
https://m.youtube.com/watch?v=3Uwx-_VecHo
posted by jon1270 at 5:55 PM on June 11, 2021 [3 favorites]
I have ground several hermetic units open and jon1270's supposition is correct. The actual working part of the compressor is suspended on springs that can weaken or break allowing more movement of the sprung assembly than then case allows for. Usually it occurs on compressor shut down rather than start up but the latter isn't unheard of. Because it can happen when the springs merely stretch rather than break it is something that can get worse over the years.
The A/C could literally run for years with this condition; I've rarely seen a shutter stop directly lead to a failure. People fix it / replace it because the noise rather than system failure.
PS: this is one of the reasons to avoid where possible transporting and/or storing compressor equipment (A/Cs Fridges Freezers) on their side. Up/down motion becomes sideways movement that can cause this issue.
posted by Mitheral at 6:41 PM on June 11, 2021 [1 favorite]
The A/C could literally run for years with this condition; I've rarely seen a shutter stop directly lead to a failure. People fix it / replace it because the noise rather than system failure.
PS: this is one of the reasons to avoid where possible transporting and/or storing compressor equipment (A/Cs Fridges Freezers) on their side. Up/down motion becomes sideways movement that can cause this issue.
posted by Mitheral at 6:41 PM on June 11, 2021 [1 favorite]
Agree that this on compressor shutdown. We had a window unit that did this for years (and kept running) and I just got used to that thump meaning the thermostat had kicked it back off.
It wasn't that loud though, so if yours is thumping loudly then maybe consider spending $200-$300 on a new unit for purposes of your sanity.
posted by intermod at 8:48 PM on June 11, 2021
It wasn't that loud though, so if yours is thumping loudly then maybe consider spending $200-$300 on a new unit for purposes of your sanity.
posted by intermod at 8:48 PM on June 11, 2021
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Has it always done it?
posted by Jobst at 2:55 PM on June 11, 2021