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Community and Q&A

Intolerable Vibrations From Minisplit Outdoor Unit

frasca | Posted in Mechanicals on

Hey GBA – I’m hoping that this community has enough experience on minisplits to help me salvage this situation:

I’m in Zone 4C, and I just replaced the furnace that heated my downstairs with a Mitsubishi MZ-GL15NA. To be efficient with outdoor space, I ordered a QuickSling so that my contractor could position the outdoor unit above an existing one that serves my upstairs. The old one was previously mounted to the wall on brackets, and I could hear it through the wall when it ran. I thought that by getting it off of the wall and mounted on the ground I’d be greatly minimizing the noise transmission.

We seem to have mostly solved that problem, but at the expense of worse vibrations from the new (upper) outdoor unit. The issue this go-round isn’t so much sound, but vibrations (lower frequency than sound?) that I can feel whenever I enter the room on the inside of where the units are. Sadly, that unit happens to be our bedroom. At different places in the bedroom it can be felt more or less strongly; ironically the worst place is NOT immediately adjacent to where the outdoor units are, but on the diametrically opposite side of the room. If I stand there – in my closet door – I feel like I am vibrating at the first fundamental frequency of the whole rectangular room.

The vibrations are coming almost entirely from the top unit, as the bottom unit doesn’t run as much in the winter since it serves the upstairs. The level of rumble that you feel in the house seems to be dependent on the speed that the unit is running… sometimes it seems to go away if the unit is running hard enough. I can’t tell if it’s being generated by the fan or the pump; my contractor thinks the pump and I’m inclined to agree. When I feel different parts of the with my hands, the vibrations seem to be getting transmitted via either the refrigerant lines or the electric whip. My contractor thinks the lines have too much foam around them to be the issue but I’m not so sure.

Things we have tried/are considering trying:
1. We hung a resilient (hat) channel on clips before we put the drywall back up, and used two 5/8″ sheets with green glue in the middle, and it dampened the vibrations somewhat but not enough that we’ll be able to sleep in the room without developing insomnia.
2. I ordered some rubber isolator mounts ( that we’re going to put in this week, but my previous contractor used those on the bottom unit previously when it was on brackets and it didn’t make a ton of difference, so I am not optimistic.
3. We could move the subpanel/disconnect off the wall a little bit, or put some foam behind it.
4. We ran the refrigerant lines almost straight horizontally through the siding, then turned them downward into the crawlspace inside the 2×4″ cavity of the wall. If we have to, I suppose we could run them down the exterior into the crawlspace directly (not sure how hard it is to disconnect and reconnect refrigerant lines without donating R410A to the atmosphere)
5. Nuclear option – I could find somewhere else to mount our exterior units.

Does anyone have experience with this? Anything intuition on which of the above is likely to work, or anything else that I haven’t thought of?

Many thanks!


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  1. Expert Member
    Akos | | #1

    Most likely the problem is the line to the house. The short straight run is coupling any vibration from the unit straight into the structure, this is probably what you are hearing. I've seen this happen with outdoor units where the pipe will practically sing inside the house.

    You want the pipe to come out of the unit than do at least a 90 before going through the structure (ie up/down/sideways from the outdoor unit, 90 than into the wall). This will allow a bit of flex there and significantly less sound transfer. If you have extra length in the crawlspace, pull some of it back to add a bit of slack.

  2. walta100 | | #2

    I tend to think the vibration is being transferred thru the piping. A360° loop may be over kill for this application but you can see how it would absorb any vibrations. This YouTube video has some good images.

    Adding some mass to your stand could not hurt. As a test, try laying 2 sand bags on top of each unit and see if it sounds better.


  3. charlie_sullivan | | #3

    In addition to the good suggestions already made, I'd add some diagonal bracing to the stand to stiffen it. Maybe just some 2x4s attached with U-bolts as a quick and easy thing to try.

    If the loop in the refrigerant line combined with diagonal bracing doesn't do it, and you suspect the electrical connections, there are more flexible conduit and conductor options: LFNC-A conduit with fine-stranded conductor like welding cable inside.

  4. gusfhb | | #4

    I agree with the pipe theory.
    Perhaps the copper pipe is physically hard against the wall feedthrough

    Perhaps just a little tweek and some foam....thinking like medium density foam rubber.....would shut it up

    I will say, my Mitsu minis are totally silent. The Pioneer I just installed in the garage thrums pretty loudly

    The difference?
    All my mitsus are mounted on concrete, the Pioneer I bought one of those plastic bases.

    It is a half price unit, so it may just be louder, but you never know

  5. Expert Member
    BILL WICHERS | | #5

    It's definetely the refrigerant lines coupling the sound/vibration into the structure. The top unit will be worst since it's at the top of the stand and higher on the wall, where everything is "springier", so less damping of the vibrations.

    The easiest way to deal with this is to take all your refrigerant lines out of the units, make a

    90, then go down to the concrete base pad. RIGIDLY ANCHOR the lines to the concrete pad. I like to use pipe straps with a wrap of 1/8" neoprene around the pipe itself under the clamp. Bring the lines into the house down low, near the foundation, where the structure is stiffest. If you do it this way, you should have very little vibration or sound coupling into the structure from the outdoor units. If the stand itself is vibrating, add some mass. An easy way to do this is to attach some solid concrete 1/2 blocks (about $1 each) to the underside of the shelves where the units are.

    Note that when you put those rubber vibration isolators in, they are most effect when operating in shear. This means you want them to wiggle with the vibration sideways (if mounted vertically with the threaded studs going up and down), not up and down. I've used these rubber isolators many times to quiet down small air compressors that are used with dry pipe preaction fire suppression systems so that the pump noise doesn't carry on the spinkler line and annoy people all over the building.


  6. frasca | | #6

    Thanks everyone for the advice!!

    Potentially silly question, but before I ask my contractor... there’s a way to detach the lines from the unit without releasing all that R410A, and he should have the equipment to do so?

    Second potentially silly question, there’s a way to potentially ‘splice’ a few feet of extra length into those lines if needed for the new route?


  7. XenOILphober | | #7

    I have three LG - 12k Cooling + Heating - 25.5 SEER & two ThermoCore 9K 26 SEER condensers mounted on stainless wall mounts that weigh about 94 lbs, but most of the weight is biased on the compressor verses the fan. I used 27+ lbs rated gel mounts under the compressor vibration mount # V10Z61MTHTW ($80 each) and I used 18+ lb rated gel mounts for the fan side of the condenser vibration mount # V10Z61MMN10 ($61 each!), each unit will need four vibration mounts (~$300 per condenser!!!), but it works and the wife stopped complaining!!! If I did it again, I would just get all V10Z61MTHTW's with the higher weight rating.. I did this install during peak COVID when it was hard to get anyone to come out and it was hard to get any equipment...

    It cost me some money, but no more vibration or sound, just the quiet whisper of the inside unit.
    Phone: 516.328.3662
    Fax: 516.328.3365

    1. Expert Member
      MALCOLM TAYLOR | | #8


  8. XenOILphober | | #9

    I'm not sure why you think this is SPAM, I just got my account today. It took me a couple months to solve a vibration issue on a 1920's farmhouse I am redoing, and this resolved the wall vibration on a unit mounted on a kitchen wall, dishes / hood vent / wall were all vibrating. The other room was the master bedroom, was getting a very low (sounded like the low rumble of a diesel engine), at certain condenser RPM's... tried rubber mounts and even another product called Sorbothane (which broke down in sunlight and wasn't strong enough to hold the load) they transmitted the vibrations, tried a couple of these on one unit and the resonating is gone.... you can see the original condenser that I will pull after all the fuel oil is used up this season. BTW I am a homeowner, and did the complete install with a little help at the end for the triple vac and freon balance for two of the longer runs to the upstairs portion of the house (one run was 50 feet the other was 70 feet). Other LG condenser is on the other end of the farmhouse on the kitchen wall.

    1. PAHighEffBldg | | #10

      Wow, great job and thanks for the tip on those mounts!

    2. Expert Member
      BILL WICHERS | | #12

      You posted across seveal older threads in one shot, at approximately the same time. Usually that's been spam stuff as of late. We've been seeing more resourceful spammers over the past year or so, so people are probably more suspicious than they once were.

      If you heard the sound at certain fan speeds, then you were hitting a resonance somewhere at those speeds. You can chance resonant frequencies with dampers, mass, or stiffeners. Dampeners work by absorbing vibration, mass works to damp vibration, and those two work well together. Stiffeners tend to push resonant frequencies higher, so still helpful, but working in a different way. Ideally, you want to try to mount all your outdoor mechanicals on ground mounts to avoid vibration transfer to your walls, and if you can't do that, mount them low, near the foundation, not near the middle of a wall. You need some loops in the refrigerant lines too, to help minimize vibration transfer. Straight lines with rigid connections are the worst way to go in terms of sound transfer.

      If you're stuck, and the install is already done, then you get into the tricky stuff to try to quiet things down. You got lucky with the vibration isolators alone solving your problem -- sometimes it takes a lot more than that.


      1. XenOILphober | | #14

        I did post on several old threads, I was overjoyed that I solved the problem and wanted to make sure that others were aware of a possible solution. (and I wanted bragging rights for figuring this out ;)
        If I could redo this project, I would definitely have mounted the condensers on the ground on a raised stand. My refrigerate lines have not been resonating, but I am sure with temperature change and different loads, they could.

        I also like your idea of adding mass, this could definitely help with the vibration absorption, and I agree I got lucky after trying several different options and should have ground mounted these units, but had already made the investment in the wall brackets.

        BTW I love this forum and wish I had found it before I started this project.

        1. Expert Member
          BILL WICHERS | | #15

          Precast pavers usually used for things like patios and outdoor walk ways are great ways to easily add mass to vibrating things. You would typicall add mass to your stand in this case, with the vibration isolator between the stand and the vibrating gizmo. The nice thing about the precast pavers is you can drill anchors into them and then bolt them in place. Quick and easy.


    3. Expert Member
      MALCOLM TAYLOR | | #16


      I'm sorry, I assumed it was spam because you have a new account, added the same advice to every thread on mini-split vibration whether it was directly relevant or not, and gave specific information on how to purchase the vibration dampers. You post passed the sniff test for spam. I apologize if that wasn't the case.

      1. XenOILphober | | #19

        Thanks, It was pretty late when I posted, but glad to hear that I didn't get censored off the site. I appreciate that!

        1. Expert Member
          MALCOLM TAYLOR | | #22


          As Bill said, GBA has been inundated with some pretty sophisticated spamming recently. Sorry I lumped you in with them, and thanks for the link to the dampeners.

  9. frasca | | #11

    What’s the grey unit on the wooden stand?

    1. XenOILphober | | #13

      It's the original 4 ton, Rheem A/C condenser circa 2004. Once I use up all my fuel oil, I will pull this unit and reuse the stand and the abandoned circuit for a generator. I also installed a 200AMP sub panel on the wall behind the 2 right condensers, so my circuit runs were pretty easy... except for crawling 80 feet to the other end of the house with 6/3 wire to the main panel. I gained a new found respect for the HVAC guys... you have to be good with electrical, mechanical, and interior climate management.... it was a rewarding project, but I would do things alittle differently on my next project. Next project is how to quiet down my AO Smith Heat Pump Water Heater....

      1. Expert Member
        BILL WICHERS | | #18

        You'll probably be short a wire if you want to repurpose that old cable for a generator. Chances are it's actually 6/2 WITH GROUND (the ground wire doesn't "count" as a "wire" here), and you need an extra wire for a generator. The reason is that a typical air conditioner compressor runs on 240v, with two hots and a ground. The generator needs two hots, a neutral, and a ground. You CANNOT go without that extra wire!


        1. XenOILphober | | #20

          I will make sure it check the wire, I have tons of wire in my shop from numerous projects, but will make sure I do both a disconnect and double check the specs before I do anything stupid.

  10. Expert Member
    Akos | | #17

    " should have ground mounted these units,"

    That is pretty much the solution to 99% of noise issues. There should be a big warning in the mini split installation manuals not to mount them onto stud walls.

    1. XenOILphober | | #21

      This would have solved several issues I ran into:

      Routing of refrigerant lines
      Noise, Vibration & Sound standoff from house

      Once you punch holes through the house, it's kinda hard to rethink where you mounted the brackets.... There should be a tradecraft section in the manual...... it would have saved me some $$$ and time.

  11. stic | | #23

    Mini splits if not installed properly you might wind up with non condensable in the line or other foreign matter . Mini splits have 3 Mufflers 1 is right off the compressor if this is clogged with foreign matter this could cause vibration. Then there are two other mufflers with screens inside they are used just before the EEV to protect it. Again this could also cause problems if clogged. These mufflers all three are used to reduce noise and vibrations . The 2 with screens before the EEV is to protect the EEV from shards of copper. Then you have the compressor and finally the board which can send electrical . pulses again can also cause vibration problems

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