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Community and Q&A

Minisplit Noise

William Morse | Posted in General Questions on

I have several Mitsubishi 6k units wall mounted on an older ~1900 home. I have terrible vibration/noise from all of them. Even on low from the inside it sounds like a diesel truck idling outside. From the outside, no noise whatsoever, I have to look at the fan to see if it’s running.

I have asked my installer to move them down on stands; he wants another $2k for the move. The noise has been a problem from the beginning, and the installer has been unresponsive.

Has anyone had any luck either getting the installer to fix a problem like this, or by going to Mitsubishi getting any relief?

Thanks, Bill

PS, other than this, LOVE the minisplits!

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  1. Expert Member
    Zephyr7 | | #1

    Sometimes using vibration isolators in place of regular mounting bolts (McMaster Carr has these isolators) will make a big difference, but not always. Best is usually to use a stand. You can probably build a stand from strut to mount the units on the ground, and build the stand such that the units themselves don’t have to move much. I think we had a recent poster on here describe how he did just that.


  2. William Morse | | #2

    Yeah, they have to be moved enough to require new whips and the units will need to be reconnected after moving. I wish it were that simple, but they hung them up pretty high on the wall, which I think exacerbates the vibration problem. They also already added the isolators.

  3. Expert Member
    Akos | | #3

    No easy solution here. The stock rubber mounts do nothing for the low fequency noise from the compressor.

    One option you can try is to put the units on proper spring mounts such as these:

    You can try it on one unit and see if it is good enough. Note that you have to adjust the preload on these to match the weight sitting on it. Check the PDF from the manufacturer.

    The unit can move a lot more on these mounts so make sure there there is enough free length of refrigerant pipe to allow for this.

    Along with the isolators, adding some mass (~ 200lb) to your brackets, but I can't see an easy way of doing this and also not overloading them.

    Ground mount is always the best. Depending on how high they are you can mount it on a post structure and disguise the posts as a trellis.

    The easy way to support these is to lean a long ladder along the wall that goes over and past the unit without touching it and support it with a couple of ratchet straps off one of the rungs. You can then take out the wall bracket and build the ground mount under it. This is what I ended up doing with mine, but it was only 4' off the ground.

    1. William Morse | | #4

      Akos, thanks so much! Not sure I see how the spring mounts attach to the wall brackets, since they are "floor mounts". I will pursue this and let you know. Still hoping for experience with Mitsubishi helping out on problems like this. Seems like an installation fault.

    2. William Morse | | #7

      Those spring units will cost about half of moving the units down to stands on pads, so I guess I'm still stuck with that.

      1. Expert Member
        Zephyr7 | | #8

        A ground mount will give you the best sound isolation though, so that's a plus. If you want to keep the stand cost down, try using some heavy precast pavers or "deck blocks" and galvanized strut to assemble the stand. Everything you need is available at the box stores. You can have the stand assembled in a few hours this way for probably $100-200 in parts. Just try to keep things level.

        Note that if you use deck blocks, you can make things a lot more stable by partially burying the blocks in the ground. I've also used solid concrete blocks 8x8x16 inches buried until only about an inch is above grade. Anchor the strut to the top of the blocks using some tapcons or other type of anchor. Squirting a little silicone caulk into the hole before installing the anchor will help to prevent any freeze/thaw issues with water cracking the blocks.


  4. Expert Member
    Peter Engle | | #5

    I don't think Mitsubishi can help much. They can't anticipate the resonance frequencies of your wall system. Resonance is weird. Even the location on the wall can make a difference whether the resonance is amplified or dampened, and at what frequencies. Ground mounts are the only guaranteed fix.

    1. William Morse | | #6

      Peter, I agree, but think my installer should have "forcefully" steered me in that direction.

  5. James Howison | | #9

    I agree that this is something the installer should have been aware of and that Mitsu needs to build into their training. I suspect they don't because wall mounting is very common in Asia (where the construction is concrete).

    Similarly installers should be aware that the units make a loud noise when doing their defrost cycle, way above their listed "max noise" (even when ground mounted as mine were). But installers are either not aware or able to claim ignorance. Fixing the noise of the defrost cycle by moving the compressors also cost me $2k so perhaps that's the default rate for fixing these entirely foreseeable issues!

  6. Will R | | #10

    How does one balance line length and not putting one of these units right outside of a bedroom since from what I'm reading, there could be significant noise?

  7. William Morse | | #11

    Make sure you use the recommended line length. Once I moved the units off the house and onto stands there is barely any noise.

  8. Expert Member
    Zephyr7 | | #12

    You can also help to limit noise transmission by not running the line set straight from the outdoor unit to the wall. Form a good-size "U" bend as a sort of vibration isolating section. The "U" acts a little like a spring, limiting the transmision of vibration which is where the sound comes from. Limiting rigid couplings (such as straight pieces of pipe) between the unit and the wall is the best way to limit sound transmission.


    1. Aun Safe | | #13

      Zephry, where precisely does the "U" go? Between the flared connection at the outdoor unit and the line hide on the exterior wall?

      1. Expert Member
        Zephyr7 | | #14

        It depends on the installation, but basically, yes, you want it somewhere between the connection on the outdoor unit and the point at which the lineset enters the wall. The idea is to provide a curved length of tubing between the wall and the outdoor unit that can act a bit like a spring and absorb some of the vibration instead of transmitting it through to the wall. It is rigid connections between the outdoor unit and the wall that transmit vibration, and that vibration sets up resonances in the wall which is where the sound you hear is coming from. By limiting those rigid connections, you limit the amount of vibration transmitted which reduces noise levels.

        I would typically recommend the line set exit the outdoor unit, make a 90, go up maybe a foot or so, then "U" back down a foot, then 90 and into the wall.


        1. Will R | | #15


          Does this also help create a "drip spot" so water runs off there. Is that important?

          I'm finally moving forward with our ductless installation. Are you aware of a best practice installation guide? (Through wall PVC or sleeve, penetration boot, on stand etc.) I've been piecing it together from reading different articles but can't find a definitive one. Thanks.

          1. Expert Member
            Zephyr7 | | #20

            You don't water to run down the line and into the wall, so I suppose the loop could help with that. I'd seal the wall regardless of the loop though.

            I like to sleeve passages through the wall, since it makes future maintenance easier. I would use a boot on a roof. The old way was a pitch pot on a roof, but don't do that -- they aren't reliable, and they make maintenance much more difficult.


  9. Will R | | #16
  10. Trevor Lambert | | #17

    Are the "isolators" currently installed actually isolators? Both mini splits I got, one a DIY and one professionally installed just had rubber pads between the bracket and the unit mount, secured with a bolt. That bolt completely negates any damping the rubber pad might provide. If that's what you have now, it would be worth trying rubber isolators. It's a rubber block with threaded rod fused to either side. They are pretty cheap.

    1. William Morse | | #18

      Trevor, do you have a link for these?
      Thanks, Bill

    2. Expert Member
      Zephyr7 | | #21

      If the pads are the type I'm thinking of, the bolt is there to prevent tip over and things like that, it's not supposed to be tightened all the way. The bolt acts to stop excessive vertical travel, but should have a gap between the nut and the top of the foot over the pad. If you tighten that bolt, you clamp the unit and cancel most of the vibration damping ability of the mount.

      Better rubbed vibration isolators have threaded studs on either end of a rubber cylinder or block. The rubber can go into either compression or tension with these and still function to damp vibration. These are better than the pads with stop bolts.

      I prefer spring isolators though, but they're more expensive and less flexible in terms of how you can install them. The best option is to install the noisy unit on a mass block (usually a concrete pad), with that block suspended or supported by spring vibration isolators. This is how large pumps are typically installed in commercial buildings to prevent vibration from getting into the floor and travelling through the building.


  11. Nick Defabrizio | | #22

    Funny, but from your post I thought you were saying that the inside units are making noise. We had this problem on some of our units and it was solved by putting a small wedge behind them so they were tight against the mounting bracket (we actually used butter knives on both corners of the wall unit until I could find wedges). You could use wood shims.

    If the noise is from the outside units then I agree mounting on a stand works best. In coastal flood zones, we typically mount them on platforms made of pressure treated 4x4's. I have seen them up as high as 10 feet to avoid mean flood level. If you attach the platforms to the house for stability, use rubber grommets to isolate any vibration and you only need one or two attachment points if your platform is sturdy to begin with.

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