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Mitsubishi Mini Split Hyper Heat Pump Loud Flushing Noise

Zoffyman | Posted in General Questions on

I’m experiencing this problem that seems to be similar to an older post but the symptom is a little difference.

I’ve 3 outdoor units each served 2 to 3 indoor units. Whenever the temperature dropped to 37, the loud toilet flushing sound came out from the outdoor units. Before the flushing noise, some of the indoor units would also sound like they were sucking in a lot of air and 30 seconds later, a flushing noise also came out from the indoor units like they were releasing the air. About a minute later, the flap would come down and blow hot air like normal.

My installer came a few times, performed the pressure test, increased the length of some coils, add refrigerant, etc. But still couldn’t fix the problem. They just said everything was fine, it was just in defrost mode. When I said defrost wouldn’t occur every time the unit kicked in for a heating cycle (especially the outdoor temperature was just ~37), he started to use the term “preheat” instead of “defrost”.

I’m very disappointed with that because I spent a lot of money to get rid of my old and noisy steam boiler in Spring 2023 hoping to have a quiet and nice winter this year. But now figured out this monster even made more noise! I just can’t believe it’s normal as this heat pump should be capable to handle really low temperature! 37F is no way to be low temperature.

The installer said they talked to Mitsubishi tech support directly and they replied the noise was normal for each heating cycle. I’m curious to know the entire discussion between the two parties. Unfortunately I couldn’t reach the tech support directly.

Appreciate if anyone can advise on this 🙏

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  1. dcooke570 | | #1

    I have the same issue! Mine will go through the defrost cycle 5 or 6 times per hour when it drops between 32F and 37F. The installers have been out to the house twice now to recharge the refrigerant lines. This time I asked they send out a Mitsubishi rep to get to the bottom of it.
    There is also mention of a firmware issue where the repeated defrost cycles goes away with a software update. Hoping the Mitsubishi tech and figure this out soon!

    1. Zoffyman | | #4

      Your installer is very helpful! I hope my installer will do the same! Wish me luck and good luck to you too!

      1. dcooke570 | | #5

        Just got word that the installer needs to come out to the house and call the factory rep once on site... the installer is coming Friday so I'll let you know what happens, if anything.

        1. Zoffyman | | #9

          Good luck! And yes, please let me know how it goes at your side. I’ll keep you update too! Thanks!

  2. greenright | | #2

    Defrost should run at most every 45 minutes or so and not always at the beginning of heating cycle... so that might indicate a problem...

    And yes - you can get defrost mode up to around 37f or even higher as the outside coil runs about 10-20f colder than ambient so it can still freeze especially in humid weather. So that is not a problem.

    In defrost you might get some wooshing or gurgling noises from the inside units, but they should be fairly unobtrusive and low-key.

    Do all outdoor units do the same? There is something going on for sure- without looking at pressures my guess is your refrigerant is low on the offending units. This is common install mistake especially if extra lineset is installed.

    1. Zoffyman | | #3

      All indoor units will make the noise, but not at the same time. Usually the one that demand for heat will make the noise. I’m still not sure if it’s a defrost noise because it happens at the start of every heating cycle when the outside temperature drops to around 37F. The time between each heating cycle is definitely much shorter than 45 minutes. The outdoor unit also makes a loud noise at the same time so it’s very annoying!

      The installer came in twice. Performed the pressure test, added the refrigerant, etc. but still no luck. I’ve tried to contact Mitsubishi tech support but they needed a contractor to talk to them or write a diagnosis report. This is the biggest problem that I am facing now!

      1. walta100 | | #10


        “The installer came in twice. Performed the pressure test, added the refrigerant,”

        If the quote is accurate, it is a huge red flag for incompetent installers.

        The only accurate way to charge an miny split is to recover the refrigerant from the system. Weigh what was recover and compare it with the calculated weight based on the length of the line set installed in your home.

        Assuming the correct amount of refringent was install originally any and all losses must be due to leakage.

        If they added gas twice already the install is very leaky or the unit is now overcharged.

        This screams lazy and incompetent.

        My wild guess is they don’t install many miny splits and run their business on the gas and go model with conventional systems.


        1. Zoffyman | | #12

          Sorry, I just used some layman terms to explain it. They probably emptied it before refilled.

          As for your wild guess, you could be right🤦🏻

  3. markem7 | | #6


    I have similar issues with a recently installed 3C30NAHZ4 with three heads.

    Below -15c (5F), the unit cycles very often into defrost. At -18C the other day, I was getting 15 minutes of “heating” (actually 10 minutes of cold / ambient air and 5 minutes of starting to heat) followed by 10 minutes of defrost; rinse and repeat …

    The house temp goes down drastically and raising set point does nothing for this.

    Installer came over, does not suspect refrigerant charge as when it DOES heat, temps are very good.

    Apparently before Christmas they were made aware of a required electric control board replacement (I guess for firmware / software update). Some other clients had it replaced as they were getting 20 minutes of heads producing cold air before heating up!!

    Should have it replaced within the next month… will keep you posted if this helps

    I get a similar “flushing” noise for the defrost, I believe it’s the reversing valve actuating. I do not find it bothersome, however the defrost frequency and lack or heat is !!

    1. markem7 | | #7

      Forgot to add, that often when it goes to defrost under -15C (5F), the coils are not iced up at all!! Whereas at higher temps when it defrosts, the coils are white with ice / frost.

      This is in Quebec by the way, if the info helps.

      1. Zoffyman | | #8

        Wow…5F! It’s the limit of H2i that can provide 100% efficiency as mentioned in the specification! In New York City, we may just have a couple days with that cold temperature for the entire winter.

        I guess my case is more serious than yours because I have such flushing noise at the start of every single heating cycle when the temperature goes below 37F. That just doesn’t make sense to me.

        1. markem7 | | #11

          Hmm I get that flushing sound too .. it’s at the end of the defrost before it kicks into heating. The flushing sound actually gets less loud the colder it gets , I don’t know why.

          And yes it does seem to defrost between each heating cycle form what I can see. Never for instance just stops. And then later on starts off directly. Maybe to ensure there is never any ice or snow on coils when it starts

          Hmm maybe in your case it’s really loud ?? Hard to say without a decibel reading or a video or something, in my case you can hear it but it’s not that bad.

          1. dcooke570 | | #13

            here is a YT link to my Whooshing/Flushing sound (skip ahead to around 1:50 mark)

  4. Deleted | | #14


  5. Zoffyman | | #15

    @dcooke570 Seems like I can’t further reply to your post #13 so here we go!

    My indoor unit is even louder than the noise in your video. What is the outside temperature when you recorded the video? I totally agree there will be flushing noise even when the temperature is above freezing. But it is the frequency at a “not so cold” temperature that most of the experts I discussed with didn’t feel comfortable with.

    By the way, what is the continuous clicking sound in your video?

    1. dcooke570 | | #20

      37 degrees F (shows at the beginning of the video). Ignore the clicking sound as my woodstove heat activated fan was making that noise. :-)

  6. Mitsuman | | #16

    There is no problem. It’s defrost cycle. The “flushing noise” is the sound of instant depressurization when the reversing valve actuates and changes the direction of the refrigerant while the system is under load. The system is taking the hot gas that it’s making for heat and redirecting it to the outdoor unit to melt frost/ice/snow accumulation off the coil. It’s amazing to me the number of responses indicating they have the same “problem”. This is a basic and expected function of the system that the installing company should have educated you on. No need for a service call any decent tech should be able to explain this to you over the phone. The noise from the indoor unit during this process can vary but you may here whooshing noises, boiling noises, if you have ductless you may hear some clicking as the plastic contracts. Go to the outdoor unit when it’s under heavy heat load and pull the disconnect. You’ll hear the same noise as the pressure equalizes when the compressor stops instantly. The defrost cycle is variable, it looks at the last cycle and how long it took to reach coil target temp and makes adjustment accordingly. It may go in 45 minutes it may be two hours. It lasts a few minutes, again, not on a timer. You can tell with wall units because the horizontal vanes will point straight out during defrost. When it’s done, they will point down at the preset angle you’ve selected (should be about 45 deg angle for heat). When you hear it, take comfort in knowing that your system is doing what it should.

    1. Zoffyman | | #18

      I’m talking about 5-10 minutes, every heating cycle, 7x24 as long as the outdoor temperature stays at about 37F or below.

    2. dcooke570 | | #21

      you must be hard of hearing (no disrespect)... but if it is loud enough to wake a person in their bedroom 4 or 5 times an hour in the middle of the night that is a problem! Don't tell me they should have installed it elsewhere as that is where I need the heat at night... there is no wall in the hallway that has exterior access for the lines.

      1. jameshowison | | #26

        Agreed. Whether or not they are expected or normal, there is zero chance that these noises are under the specified noise level of the units documented on the datasheets.

        When we located the outdoor units we paid attention to those specifications. Then we ended up paying $2k to have our outdoor units moved precisely because of these noises waking people up at night.

  7. Mitsuman | | #17

    Keep in mind the horizontal vanes will also point straight out when the unit is satisfied…so just because they’re point straight in heat mode doesn’t mean it’s defrosting.

    Disclaimer- I also recently have seen a few wall units where the horizontal vanes do NOT point straight out in heat mode when unit is satisfied, and the indoor fan stops completely (which is not supposed to occur unless a jumper is cut on the control board). But this is probably a fraction of a percent and I’ve been meaning to discuss with Mitsubishi.

  8. HeatPumpExpert | | #19

    Heat pumps typically go into defrost mode to address frost or ice buildup on the outdoor unit's coils. However, it's not common for a heat pump to go into defrost mode in every heating cycle, especially when the outdoor temperature is relatively mild, around 37°F.

    Defrost cycles are triggered by outdoor temperature and the accumulated frost on the coils. In milder conditions, the need for defrosting is usually less frequent. If your heat pump is entering defrost mode excessively, it could be indicative of an issue such as:
    1. Faulty Temperature Sensor
    2. Defrost Control Board Issues
    3. Refrigerant Issues
    4. Incorrect Installation

    I highly recommend you to request your installer to report the issue to Mitsubishi in details.

  9. dcooke570 | | #22

    OK, my installer just got off the phone with Mitsubishi. They had pressure gauges all hooked up and ready to debug with Mitsubishi, and Mitsubishi asked how often it was cycling. They told them 4 or 5 times per hour. Mitsubishi actually said it wasn't defrost cycling but something else and that there is indeed a firmware issue. The commercial condensers have the correct firmware but the residential condensers were shipped out with a bad batch of firmware. Mitsubishi told the installer to contact their supplier and get the firmware fix and it will all be fixed. So my installer will contact me when they have it from the local supplier.

    1. Zoffyman | | #23

      Congratulations! You finally figured out how to fix the issue! Wish me luck, my installer is STILL doing nothing to resolve the issue for me!!!

      1. dcooke570 | | #24

        Keep at them. Mine was a little low on refrigerant one time so they actually had to address that but when the cycling continued and I gave them some printouts of these types of discussions, they actually began to listen.

        1. Zoffyman | | #25

          I did the same! I also shared this link to them! At least your installer was willing to listen!

  10. dcooke570 | | #27

    They are coming Friday with the new part (I'll update on what exactly they replace and any improvements).

    1. markem7 | | #28

      Hey dcooke how did it go??

      On my end I got a call Friday that the card came in.

      They are supposed to come tomorrow (Monday).

      Good timing, we are hitting -20c ish nights Monday and Tuesday … I am crossing my fingers!!!!!!!!

      1. dcooke570 | | #29

        The new circuit board seems to have done the trick! So far no more anoying sounds. I just need another cold spell to test the heat; when it got down to 5 degrees F a couple weeks ago the heat was non existent. Hopefully it solves that problem as well. Again they addedd more refridgerant so we shall see.

  11. markem7 | | #30

    So the tech came by and changed my card.

    Oddly enough, I took a picture of the box and it seems to be card for the 3C30NAHZ2 whereas mine is Z4.

    Physically they seemed identical, and maybe the writing on the box is not the right one… anyways found it weird that they “retrograde” firmware if it’s the case.

    See attached pic, weird….

    Maybe Mitsuman can chime in?

    Anyways, as long as this works …. tonight cold night and tomorrow too.

    Crossing my fingers ..

    1. Zoffyman | | #31

      Mine is Z2. Instead of replacing the board, Mitsubishi rep came today. They erased the data from the board and then loaded the latest set of data back. As far as I know, the coil for the Mitsubishi Hyper Heat pump is different from the older version so the original logic in the board doesn’t recognize it and cause some kind of weird behavior.

      If that circuit board of Z2 and Z4 are physically the same, maybe the Z2 and Z4 board will become the same after the upgrade as it’s just a reload of latest data and logic to the board.

      Tonight will be cold enough to see if the fix really works.

  12. markem7 | | #32

    Ok I am triple posting in three threads, I just want to make sure if anybody searches for this problem and finds only one thread they will find this info :

    Ok so this night we hit a low point of -18c (0F), temps at which the system was problematic.

    (I honestly haven’t had time to check the system’s functioning now in detail, but I definitely saw differences last evening when it was warmer (Around 23f). System no longer shoots cold ish air for 10 minutes after defrost but seems to wait till coils are hot enough ..)

    Anyways, normally at -18c ambient (0F) I would have to program set point to 22 or 24c (71 to 75f) and hope for a house temp at 20c (68f) in the house, but would usually wake up to 18-19c (64-66f) and a defrosting mini split.

    Well, I decided to go all in and leave the setpoint at 20c (68f).

    Go downstairs and …. Ambient -18c (0F) and house temp… 21c!!! (70f). TOASTY

    I don’t want to celebrate too early but…. The fix seems to have worked!!!!!


    1. dcooke570 | | #33

      Awesome! Glad to hear some people getting some results finally.

    2. Zoffyman | | #34

      Just to give an update on my case. The coldest temperature I have experienced after reloading the circuit board data was 28F. No more defrosting noise since then. I think the problem has been fixed. Now I’m just curious what a “real” defrosting noise sounds like and when I will hear it😂

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