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Wall hung mini-split low-frequency vibration – what vibration isolators are most effective for the lower frequencies?

rossn1 | Posted in Mechanicals on

I have a large wall-mounted mini-split condenser, installed by a contractor, with low frequency vibration issues. Seems to be related to compressor, not fan.  More background, below, including why it must be wall mounted.

What type of vibration isolator and material works best for low frequency vibrations? Seems like silicone would be better than rubber, but I haven’t found any yet that support at least 70lbs ea (condenser is 278 lbs).

Safety over time is also a legitimate concern, with a 280lb (tall… i.e. needs to be stable in the wind) weight over a walkway.

Pics of various vibration isolator and the install, below.


Additional Background
I have a Mitsubishi MXZ-8C48NAHZ2 mini-split condenser that is wall hung, and installed by a local reputable contractor. We have some low frequency vibration noise issues within the home. At the right frequency, it can sound like a helicopter is flying overhead (actually, the first time my wife said she went outside to look for a helicopter. It only presents itself at certain frequencies.

The contractor says they’ve never had issues, but some internet and trade forum searches disclose this can be a real problem with mini-split wall-hung condensers.

In our case, between setbacks, total system line set length, and needing to maintain enough clearance next to the house so the tractor can drive by, it has to be wall mounted in the location it is. Wall is real brick, with brick ties to studs.

It seems the only practical option when remaining wall-hung is to use better mounts between the wall bracket and the condenser. The tried some common AC vibration pads, but they didn’t work.

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  1. Expert Member
    Akos | | #1

    Thats a big one. Are those brackets even rated for that weight?

    This has come up before, to isolate low frequency you need mounts with a lot of deflection. This means spring mounts. They are not cheap but the right ones should do it.

    Since you are on masonry, you could try a set of cheaper soft rubber mounts first. It might be good enough. This is a good source of options:

    1. rossn1 | | #2

      Thanks, Akos.

      I had seen a few other threads, but not the two you pointed out. Your commentary on the first thread about 1/2" deflection needed to over the resonant low frequency concerns me a bit, mainly due to the height to depth ratio of the unit, and that it's 275 lbs mounted 6' up.

      Under the scenario of the said spring mounts, installed, what can I expect when I get a 80 MPH gust of wind in a storm? Again, that height to depth ratio is pretty bad.

      I asked the installer about the wall mount, originally and he said it was spec'd for the unit. I'll have to check tomorrow if it matches the one shown in the Mitsubishi link at the top of the post. Thinking about this more, and given how 'featured' the brick is. I would be willing to bet those arms are not entirely parallel, and if not, that could be a contributor.

      I see a lot of issues on this, but Mitsubishi specs out wall mounts. While I understand the enormity of installation scenarios, the mfg should have some knowledge and provide some installation information about harmonic issues that can exist in such installs. At the back of my head, I also wonder if the compressor is a bit out of balance. I don't think it is the fan... when the hum is heard, I can see the fans turning rather slow.

      Given the number of issues with this, I'm very surprised someone isn't selling a fully dampened wall mount bracket.

      1. Expert Member
        Akos | | #3

        Because of the tall unit and how high it is, best to go with through bolt style mounts such as these:

        This way no matter what, you have a metal screw keeping the unit in place.

        One quick improvement you can try is put a piece of rubber between the bottom portion of the bracket and the brick where the weight of the unit is causing it to push against the brick. This should help decouple some of the vibration from the unit especially since it in the right direction with respect to the compressor.

        Your compressor is not out of balance, they all vibrate a bit like this. Generally not an issue with masonry unless that noise couples into the structure. When this will happen is hard to predict, unfortunately this is nothing your installer or Mitsubishi would be able to tell. Even if the bracket is from Mitsubishi, there is no guarantee that somebody won't have issues.

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