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Community and Q&A

Noisy Minisplit

tech1234 | Posted in Mechanicals on

So before I call Mitsubishi warranty I just wanted to run this by you guys and see if any of you have had a similar experience and know what’s going on… I had 2 Mitsubishi Hyper Heat Mini Splits installed in my newly built “pretty good house” using the one mini-split per floor routine. This has worked out very well for me. The downstairs unit is a 12K and the upstairs is a 9k. Wall mount heads. The upstairs unit has been flawless since day one. This is not a multi-head system . They are both single heads on their own outdoor units. Downstairs unit works well but here’s the issue it does… very infrequently (but the exact amount of times is hard to pin down because I’m not always home of course. But I’d say I noticed it about twice a week) (and I have never noticed it on the upstairs head)  the downstairs head will do a weird water or refrigerant boiling sound with some weird creaking and hissing not real loud but if you’re sitting on the couch watching a movie it will make you look back and see what’s going on. The first time I ever noticed this noise was definitely the worst time. The first time it was slightly louder than when it does it now and the head stopped producing heat until I flipped the breaker and “reset” it. This resolved both issues. This first time I immediately called the contractor I had install them and he came right over and checked them out and verified everything was fine with them as far as refrigerant amounts, line pressure etc. Of course there’s a possibility those things are not actually correct because as we know contractors aren’t always the most honest especially when it comes to warranty repair work. However after he left it’s never been as bad as the first time again….??? So I don’t know if he actually changed something and just didn’t tell me. Anyways since then I’ve called him and even send him a video recording of the noise with his response being there’s nothing more he can do and it’s on me to call Mitsubishi factory warranty hotline. I just haven’t had the time to deal with what I know is going to be a long ordeal so I have not made the move to call Mitsubishi yet but I figured on the off chance that if this is some kind of a normal defrost routine or something like that I should probably ask those who have been living with them longer than me. Also as a side note the downstairs unit line set runs in an “open” space under the stairs which allows me to listen to the lines directly unlike if they were in a wall. When it does this noise situation I can go under the stairs and hear fluid moving/bubbling in the lines. Any ideas?

Southern NH

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  1. johns3km | | #1

    Sounds like normal defrost cycle. I wouldn’t reset the breaker let it finish the cycle. The heat always come back right?

  2. natesc | | #2

    That is the defrost cycle. Ice builds up on the outdoor unit, so it will reverse the refrigerant flow and actually heat up the outside coil to melt the frost. This also means it will actually be in a very mild 'cooling mode' on the inside for those 10 minute or so cycles. It is completely normal, mine will sometimes run once an hour or so when the weather is really nasty (snow coming down sideways)

  3. tech1234 | | #3

    Thanks for the responses guys... how loud should the defrost cycle be? my downstairs unit is about equal to a pot of water boiling on a stove... interestingly the upstairs one I have never heard and it is just outside my bedroom....???

  4. johns3km | | #4

    Outdoor unit should sound like this:

    Indoor unit should be silent until the cycle is over. You'll hear refrigerant sounds when it starts back up again. The upstairs unit doesn't work as hard as the downstairs unit in the winter so it won't defrost as frequently.

  5. walta100 | | #5

    Defrost should not be an uncommon event. My heat pump ran 3050 hours in 14 months with 265 defrost cycles. The first time I noticed one the plumber was there and we both were looking for the broken pipe as it sounded like running water later changing to the sound of boiling water. My defrost cycles last about 3 minutes or so.


  6. ElectricHoosier | | #6

    It's a shame heat pumps aren't also rated for peak defrost cycle sound levels. Our standard split unit makes a noticable, short squeel ocassionally at compressor during defrost cycle, although it doesn't wake me up like 15 Kw heat strips blasting at 1000 CFM+ in old ducts (previous electric furnace).

  7. cldlhd | | #7

    I've been hearing this sort of noise, sounds a good defrosting cycle on my inside Mitsubishi wall unit, and it's on air conditioning mode. Never had it before, I haven't had it looked at yet but was just curious

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