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Community and Q&A

Noise from Single-Zone Mitsubishi Heat Pump

tanzerson | Posted in Mechanicals on

I have a newly installed Mits H2i (FS18) single zone heat pump.   From startup and now continuing, there is an audible tone coming from the unit when operating.   I haven’t measured the frequency, but it’s probably few hundred Hz.

The installer said it may be because of the line length (<28′).  The vertical lift is not much more than a foot.

The tone does not seem to change in volume or frequency.   I can clearly hear it outside from 50′ away and around a corner.

I have a second FS18 that does not make this noise.

I will be in contact with the installer, but am looking for opinions to counter a possible “that’s normal” response from them.

I’m disappointed as one reason I went Mits was for low noise.  Any suggestions on how to resolve?


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  1. gusfhb | | #1

    inside unit or outside unit making the noise?

  2. tanzerson | | #2

    It is the outside unit, installed on a stand. One is fine, the other "sings".

    Thanks, I realize I wasn't clear about that.

  3. Expert Member
    AKOS TOTH | | #3

    What you are most likely hearing is the switching noise from the inverter. Most mini splits put power through the compressor windings even when off to keep it warm (this avoid having an extra dedicated compressor heater), thus the noise you are hearing. If this noise carries into the house, you might want to add a couple of bends to the refrigerant line, you never want a straight run from the outdoor unit into your wall, it should have at least two 90 deg bends to minimize noise transfer.

    How much switching noise a unit makes depends on a lot of things, it could just be that somebody skimped a bit on the encapsulation on your unit's compressor. Annoying, but I wouldn't call it a defect.

    Sometimes it could also be that a lines or one of the metal panels has a resonance at the same frequency and is amplifying the sound. You can try pushing with your hand on the outside panels/lines to see if it makes it quieter. If it does adding on some extra mass such as Dynamat to the metal should help.

    1. PBP1 | | #4

      Sometimes the fan is a source of unbalance, as these things aren't like turbochargers spinning at 200,000 rpm, individually balancing each unit may be a cost too great. The fan being large in diameter and seemingly cheaply made of polymer/plastic, as well as carrying the load of moving air, can be a source of unbalance and hence various modes/resonant frequencies. My Mitsubishi is wall mounted and I added vibration damping material. From my observations, things are worse when the fan is spinning. To balance, really need a balancing rig with read out as to where to put the weights/cut mass - something that should be done at the factory (maybe they do it, not sure). Just went outside, 15 F, and the fan is spinning with wobble at the axis. Not well balanced and hence vibrating.

  4. gusfhb | | #5

    When it is running lay your hands on various parts of it. It is possible a cover or something is loose and vibrating with the compressor

  5. onslow | | #6


    In addition to the good suggestions made so far, I would add looking at the unit's vibration mounts. The installation sheet I looked at doesn't cover what form of isolation mounts are used or provided for the outside unit. I know from other fan/compressor dealings that I have had that off-center, torn or over compressed isolation mounts can produce the sound you are hearing. The fact that you can hear it 50 feet away suggests at least one has completely failed or possibly a preparation step was not completed. Sometimes units are shipped with the compressor or other vibration isolated components "locked" down to prevent the mass from thrashing about during transport. Sometimes the restraint is as simple as zip ties at the mount points. Sometimes through bolts need to be backed off until the pad material is uncompressed.

    For warranty reasons it is probably not a good idea to dive into the unit for a look-see. Asking the tech to check all the points of isolation would be reasonable in my opinion.

  6. norm_farwell | | #7

    Hmmm. I have not encountered what you are describing. I’d say anything that loud means either an equipment problem or an install problem. Lineset length is probably not relevant here. Under 12’ it can help to pull out a bit of the factory charge, something I don’t think I’ve seen in the install manual, but worst case there I’d expect a bit of intermittent harmonic vibration, nothing like what you describe.

    If the sound is the same for heating and cooling and regardless of load (I’d test that) then I think it’s likely electrical. Your installer might be able to pin point the source by pulling the cover and looking for the power supply board.

    I have a db meter app I downloaded for my phone in order to document noise levels when it might be relevant for tech support. You’ll be off the charts with anything you can hear from 50’. For comparison you can find factory spec noise levels in the submittal sheet for that model: 52-55 dB\M_SUBMITTAL_MSZ-FS18NA_MUZ-FS18NA_en.pdf

    Good luck, let us know if you figure out the mystery.

    1. PBP1 | | #8

      iPhone microphone record will give a visual of the waveform(s), I did this for the annoying condensate pump in one of my Mitsubishi air handlers, with gravity drain that noise is eliminated when in cool. You might be able to replicate the time series data and run a Fourier transform to go from time domain to frequency domain to see what frequency(ies).

  7. tanzerson | | #9

    Thanks for great input. Sorry for the slow reply, I've been out of town. I took an FFT and matched it with a tone generator. The tone from the compressor is pretty near 400Hz and about 25 dB above nearby frequencies and noise of the other compressor at that frequency. It's constant when the fan is running and only occurs on one of the two identical units. Any thoughts?

    I have a call in to the installer, if they don't come up with anything I will call Mits as this doesn't seem right.

    On the plus side, we've had a few nights below -5f and the units keep the house warm.

    Thanks again

    1. PBP1 | | #10

      So it's singing a G4, about 392 Hz? Akos might reply, seems like Akos has a lot of experience.

      For heat pump dryers:
      "Experimental characterization of a household heat pump tumble dryer noise".

      Never thought of wrapping the heat pump, I might check out a local auto wrap company to see if they can do it, make it blend in more or look cool (example in the link at p. 16, vinyl art)

    2. Expert Member
      AKOS TOTH | | #11

      I don't know if these have a test mode, you can try searching for the service manual for your specific model. This is a great resource for these:

      You want to figure out which part is making the noise. If the noise doesn't change with load, my first guess it is the fan, but those tend to be relatively queit on these even if out of balance.

      If the fan can be disabled or unplugged, the tech can run the unit to see if the noise goes away, if it does it is a simple fan swap. If that doesn't solve the noise than the problem is most likely with the compressor. No easy way to fix that without replacing the outdoor unit.

      400hz is a pretty annoying sound and hard to ignore, it should not be making that.

  8. norm_farwell | | #12

    One other thought—if it’s clearly audible indoors as well, you might look for a kink or constriction in the copper lineset. I had an FH15 that made a somewhat intermittent high pitched singing sound that had me baffled— very audible both indoors and out. Turned out the tubing on the low side had a bad bend that was constricting refrigerant flow.

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