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Community and Q&A

Mitsubishi Hyper Heat Minisplit Noise, Defrost Cycle Issues

Maris016 | Posted in General Questions on

Since install of our dual-head Mitsubishi Hyper Heat Ductless Mini Split, we’ve noticed a couple issues when in heat mode and when outdoor temps are <36 degrees F. 

The unit goes into defrost mode approximately every 15 minutes.  Each defrost mode lasts for 10 minutes, blowing no air or cold air during this time.  The unit only gets about 5 minutes of heating in before going back into defrost mode and blowing cold air again. 

The worst part is that defrost mode is LOUD. I would liken it to the sound of a toilet flushing but amplified/echoing. Loud enough to drown out the television and too loud to sleep through. 

We contacted the installer, a diamond contractor, who believed the length of the line was causing the loud noises.  They moved the outdoor unit and added length to the line, but nothing has changed.   They are at a loss and so are we.  

1.  Is such a loud noise normal in defrost mode?

2.  What could potentially cause the unit to defrost so frequently?  Is this ever normal?

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  1. mr_reference_Hugh | | #1

    If they are at a loss, then they should contact their wholesaler. You always have the option of filing a warranty claim. I would at least call Mitsubishi directly and ask if there is any other option than going the warranty route.

    1. Maris016 | | #4

      Good point, we’ll give Mitsubishi a call ourselves if they don’t figure it out at the next tech appointment.

  2. Expert Member
    Akos | | #2

    Noise when switching to defrost unfortunately is common. Some brands are better than others but never silent.

    The bigger red flag here is the constant defrost cycle. Provided the base pan is draining and you are getting good airflow for the outdoor unit, this sounds like the refrigerant charge is off. You either have a leak or they did not top up the unit properly during install.

    Part of installation of multi splits is calculating how much refrigerant the setup needs and a lot of times additional refrigerant needs to be added. Multi splits are pretty sensitive to refrigerant volume, if this is off there will be excess defrost and overall output and efficiency will be off.

    Unfortunately you can't check refrigerant fill with a pressure gauge on a variable speed unit (this is most likely what the HVAC tech will do, but it is wrong). The refrigerant needs to be reclaimed, weighed and the correct weight added back in. HVAC tech won't want to do this as it is a lot of time, so it might need some prodding from Mitsubishi. Also make sure somebody calculates the proper fill and show you how they got the value.

    1. Maris016 | | #3

      Thanks for the advice. This is very helpful. We’ll be sure to mention this when the tech comes out on Monday.

      1. Expert Member
        Akos | | #5

        Another thing to check is kinked line set. This is very easy to do where the line set comes through the wall and hard to see unless you open the insulation on the outside.

  3. 1869farmhouse | | #6

    Seconding the verification of refrigerant calculation. You could find your model on and double check the numbers yourself if you’re reasonably math savvy. There’s also a dip switch setting that forces the unit to defrost more often. It’s intended for ultra-cold climates - but I’d sure hope the diamond installer would know that. That info could also be found on

  4. user-5946022 | | #7

    I'd call Mitsubishi and keep pushing until you can talk to one of their tech consultants. Good luck.

    I would not trust a diamond installer to know anything. Two different diamond installers who confirmed over the phone they are well versed in Mitsu ducted mini's sent techs to my house who took one look at the unit and stated "I've never seen one of these before; I thought all mini splits were wall mounted heads."
    The Mitsu diamond program is all smoke and mirrors marketing. They may have good equipment but it is hell trying to find anyone who knows how to work on it, and Mitsu's only answer to everything is "call a diamond installer listed on our website." Useless.

  5. SteveL139 | | #8

    We are having the exact same problem! Mitsubishi has admitted to a firmware mismatch on some models and have just revised the firmware in my condenser. This has corrected the 15 minute defrost cycle and significantly reduced the "whooshing" noise, but has not completely solved the problem. There was still unacceptable noise from the head unit (even when one was off) during the defrost cycle. The refrigerant has just been captured, measured and replaced, and now it almost seems to be operating correctly. There is still an occasional loud vibrating noise from at least one head that seems to coincide with the beginning and end of the defrost cycle. This is a Mitsubishi problem, but good luck contacting them directly. I have to go through my installer, and I think they are as frustrated as I am, bur we're making progress.

    1. JayMac312 | | #9

      Hi Steve, I have two units outside, both multi zone, a 15,000 and 12,000 on one of them and on the other multi zone It’s two 9000 and a 6000, they are hyper heat, 3 series and I just had them installed, all five indoor wall units makes swishing noises at night time but it’s every 15 minutes, was your refrigerant low?
      My units only go into defrost mode once it hits 37° And it drives me nuts when I’m trying to sleep at night.
      I spoke to my installer, and he said something with the new series 3 which just came out about six months ago are having issues, but he didn’t get back to me yet.
      He said he’s waiting from Mitsubishi .
      Everybody I talked to said my refrigerant is probably low and that’s what’s causing the defrost mode to go on every 15 minutes.
      But now I hear a firmware mismatch, not sure if that’s on the new series 3 or not.
      Your thoughts.

      1. SteveL139 | | #13

        I think we have very similar systems - I have 2 24K HyperHeat 3 condensors with a total of 5 heads, and the firmware fix solved the reversing valve noise and the problem of going into defrost mode every 15 minutes. I had my units install in Sept, but it wasnt cold enough to notice the problem until late October. The problem, as I understand, is only with certain newer models, which mitsubishi can check by serial number. I'm not sure if it only applies to multihead units.

        As Akos mentions, we still hear the reversing noise, but it is now almost as quiet as the fan.

        I havent yet got a definitive answer about how much refrigerant was recaptured, but it seemed as though replacing the refrigerant helped even more. The technicians only did this on one condensor, so I'm going to compare how the two sides of the house are operating. They said it was hard to recapture all of the refrigerant on a cold day, but I got the impression that they were satisfied that the the refrigerant wasnt low and there we no leaks. I'll keep you posted.

        1. lpj23 | | #26

          Hi Steve - we've been having the same issue with our unit "defrosting" every 11 minutes once it hits 37*. It's reassuring to hear that you experienced something similar! We just had our installer reweigh refrigerant, but it didn't solve the problem (even though we were definitely low on refrigerant!) Our next step is to have the Mitsu guy come out for the firmware fix. We really hope that works for us. Sounds like that helped you, but wanted to know -- does your unit now go into defrost at a "regular" time interval now, at a lower temp (37 feels a bit warm to us, but maybe that's by design), and only when the coils frost over?

  6. Expert Member
    Akos | | #10

    The reversing valve in a heat pump needs a pressure difference to actuate, that means there is no way to reverse flow without some noise. This noise will be heard in all indoor units whether they are on or off.

    Besides the gross oversizing issue, this is why it is a bad idea for a wall/ceiling/floor mount in a bedroom. There is nothing your installer can do to reduce the noise.

    Unfortunately there is no easy or cheap solution to this problem. The best way to deal with it is to take out the wall mounts in the bedroom and install a small ducted unit in a hallway ceiling or closet to feed all the bedrooms. This fixes one part of the oversizing issue plus that extra bit of separation means you won't hear the unit going into defrost.

    Excessive defrost could be a sign of low refrigerant but could also be caused by outdoor unit installation if there is not enough free space around or it is not protected from snow or ice coming off the house roof.

    In case a tech does come out and weighs out the refrigerant and finds it to be low, make sure they fix the leak. Common practice is for them to simply refill to spec but without fixing the leak, it will be low again in a year or two.

  7. JayMac312 | | #11

    OK, what I don’t understand is that I have a 9000 single zone in my addition and that does not go into a defrost mode at all, day or night.
    Does the swishing noise only happen when it’s on a multi zone?
    And only when the sun goes down, I’m a little confused, because I have no problems with my single for the last seven years.

    1. bfw577 | | #12

      It's because your 9k unit can turn down to a crazy low minimum capacity compared to a multi split. Look at the electricity consumption of a single 9 k or 12k single zone Mitsubish at minimum capacity. Its using only 60, 80 or 110 watt!. I verified all these numbers with an Emporia Vue electricity monitor.!/product/34427/7/25000///0

      Even if a single zone is oversized your cycling on such low minimum capacity the electricity consumption is nothing compared to these multi splits that have minimum capacities of 24k btu/hr using 1800 kw.

    2. JayMac312 | | #14

      Thanks for that info, I’m still confused on why my multi zones keep on going into defrost mode every 15 minutes.
      But like I said the series 3, from what I heard have some issues and that’s what I’m waiting for from my installer.
      There’s no reason it should go into defrost mode every 15 minutes for 10 hours straight.
      Hoping they figure something out.
      Maybe it is a firmware mismatch.

      I do want my outside units drained, and re-weighed to make sure they’re not light on refrigerant. That way I can rule that out.
      I reached out to other techs from different companies and they said it could be that
      your refrigerant is low, and I said on both units outside, what are the chances of that, but like my tech said these are series 3, and they’ve been having problems with them.
      I’m thinking maybe I should just have all the units single with their individual outside unit.

      1. Expert Member
        Akos | | #16

        The unit low on refrigerant is not your problem unless somebody put a nail through the line, it is an install error. It common for installer to blame end user for this, I've been told by one that the reason the unit is not running properly is the air filter on indoor unit was not clean (the issue was of course fixed by adding refrigerant).

        I would start by getting the existing setup running properly (proper charge and firmware) and go from there if needed.

        As for sizing issues, I would start with figuring out what your heat losses are first before looking too much for new equipment. This is a simple tool to get you into the ballpark:

        Mitsubishi does have their newer SM series multi splits which are semi commercial. These, at least according to the NEEP site, have much better turndown:!/product/57008/7/25000///0

        They also have setting for the LEV to reduce noise in defrost and when the indoor units are off. I don't know if it will fix your noise issue but would help. These are slightly more complicated install as they use a branch box but it also makes them much more flexible as you can get all your heads onto a single outdoor unit.

        1. nynick | | #21

          There are so may oversized units being installed. What's the secret to trusting your HVAC suppliers to get it right?

          1. bfw577 | | #23

            These multi split units were originally designed for high load mostly commercial places in Japan. They are being poorly installed with their minimum capacity not being fully understood.

            These large multi splits just cant run low at low modulation and reliably circulate refrigerant oil. So a 48k btu Mitsubishi multi split needs to run at extremely high low minimum capacity like 24k btu/hrto ensure oil circulation.

            A single zone 12k Mitsubishi will use close to 10 times less power at minimum capacity and will deliver a way better COP.

          2. Expert Member
            Akos | | #25

            The secret is not to trust them. I've had good luck with almost all other trades but a good residential HVAC tech seems to be impossible to find. Even getting the right part number seems to be hard, a while back I asked for a 9k unit and the installer showed up with a 12k, by the time I noticed it was already installed.

            With a bit of work, it is possible to check any proposed design which at least will avoid major issues.

  8. JayMac312 | | #15

    Your thoughts?

  9. JayMac312 | | #17

    It’s funny, as soon as the outside temperature is 37° my units stop going into defrost mode

    1. bfw577 | | #18

      I agree with Akos assessment that you most likely have a refrigerant leak. A 4 zone multi split has 16 flare fittings. Thats a lot of fittings that can leak and one smaller leak will absolutely destroy the efficiency. These units need to be critically charged to the exact refrigerant amount to run efficiently. The first sign of a low charge is ice on the coils in both heating and cooling mode. Your coil is icing up and defrosting most likely because it's low on charge.

  10. JayMac312 | | #19

    Yes, but what are the chances that both units outside have a refrigerant leak , because they both do the same thing. I have a multi 3 zone and a multi 2 zone and they are both doing it.
    My tech installer was out to check that a week ago and he found no leaks, I just had these installed January 6 of this year.

    1. Expert Member
      Akos | | #22

      Are the outdoor units actually frosting up and producing a lot of water?

      The one problem with oversized multi splits is the refrigerant that is being bypassed when only a small zone is calling for heat. This extra refrigerant needs to be evaporated so no liquid gets into the compressor which will make the outdoor condenser run colder thus more chance of frost buildup.

      Are your outdoor units the 3C24 or 3C30 ? In either case the min on them is above 1 ton, so you need at least one big head running pretty much at full tilt for it not to bypass refrigerant.

  11. JayMac312 | | #20

    I just want to thank everybody for all the feedback.
    Greatly appreciated.

  12. JayMac312 | | #24

    I haven’t been outside to look at them at night to see if there’s any frost buildup but I know my one unit is a 24,000 which runs the two 9000’s and one 6000.
    And the other unit outside runs a 12,000 and 15,000 unit . Not quite sure what that one is, this is all new to me.
    I was told by a lot of people that these wall units are nice and quiet inside and that’s not the case on my end.
    Hopefully my installer gets back to me soon with a response from Mitsubishi.
    If he tells me that this is normal, I will be very upset.
    Spent a lot of money having this work done to my home.
    Trying to get away from oil plus my boiler is 35 years old.

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