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Questions about minisplit distance from ceiling, vibration, and noise

snugglez | Posted in Mechanicals on

Hello! I have a multi-head mitsu hyper-heat system set up:

MXZ-3C30NAHZ2-U1 outdoor unit

indoor units

All three are connected to MHK1 remote thermostats, with the remote selected as the thermostat used.

Two questions:

1. msz-fh06na was installed 2 inches from the ceiling, meaning it is reallyclose to the ceiling. When they installed it, I marked the height where I wanted them to install all the units which they basically ignored and I didn’t catch until it was too late. The other units are at least 4 inches down, but this one concerns me:
a. is this going to be an efficiency issue? The installers (of course) said it’s not a problem. The manual says 3 1/8 inches minimum, but it doesn’t say why, so that could be just for convenience of access, which brings me to
b. Is this going to be a maintenance nightmare? These units seem like a total pain in the ass to remove the cover from. This just adds to that, but it’s unclear to me if it’s just a little bit of an extra pain, or if it will actually create real access issues down the line.

I will call them to move it if it’s going to be an efficiency or maintenance issue. But I’d rather not move it, as it will leave a hole in the house I’ll need to deal with.

2. the outdoor unit really vibrates a lot during cold weather. I’m not talking during the defrost cycle, just hours of loud humming and vibrating that is annoying to me and my downstairs neighbors. My understanding was that these ought to be virtually silent except during defrost. Do I need to have them look at this? I feel like “it’s noisy” is a pretty vague complaint. Is there an SPL or something I should measure off of it so I can tell them it’s X and it should be below Y, fix please”

The already replaced the outdoor unit once, and at that time I had them move the indoor unit that was in my bedroom to the room next door because the vibration from the outdoor unit was traveling up the lines and buzzing the evaporator coils. The replacement of the outdoor unit did not seem to effect the vibration much, but at least the buzzing is in a different room while I sleep. But it still is weird that the outdoor unit vibrates so much, no? Again, I know it’s a vague symptom.

Thank you for any/all advice. I’ve had some sleepless nights because of this, and I’m hoping to resolve before I just rip them off the wall and go back to gas, ha-ha.

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  1. bencarsan | | #1

    Mitsubishi describes minimum clearances in the install manual, page 3 here:

    Minimum ceiling clearance for the indoor unit is 2 3/16" measured from top of housing, but note clearances are greater for left side piping (in that case the unit needs additional clearance to be able to tip up for access to piping connections made behind the unit.)

    I've not experience the kind of noise you are describing. Hanging the outdoor unit on stud walls can be problematic, as you can get resonance between the unit and the structure. (The installation manual specifies a "rigid wall structure" for that reason.) On wall hung installations on stud walls I use rubber isolation pads between the feet of the unit and the bracket to reduce this and I've not had any complaints. Never heard of a buzzing lineset. Mitsubishi specs on noise can be found in the submittal here:

  2. snugglez | | #2

    Thank you, Bennett.

    Yeah, it's definitely left-side piped, which I forgot to mention. I'm baffled how they even got the pipes up in there without being able to tip the unit.

    I guess it really comes down to how annoyed I am about potential future hassle. vs having them move it which will be for-sure hassle. I guess what would push me to do it is if I was certain that the 2 3/16" minimum clearance was effecting efficiency as well... Are you are aware of that either way?

    Yeah, the vibration has been a huge issue. What I'm wondering is if the problem is that the outdoor unit is slightly too small, and thus is running for hours at max output just to keep up on cold nights. It still seems like the compressor at max should be quieter than this one is. I'll get a sound-meter on it to compare to the spec. They already swapped it out once, so it could just be me, but that's hard to believe.

    To be clear, the symptom is: very late at night, when the units are asking for the most from the outdoor unit, the outdoor unit will just start to vibrate. the copper lines vibrate, and the unit closest to the outdoor unit--the one that was in my bedroom--gets all that vibration. The evaporator fins start buzzing against something, and it's pretty horrible when you are trying to sleep. You can hear it cycle up-and-down with the compressor outside. It's not the fan because I can turn the unit totally off, and if one of the other units is calling for heat, the turned off unit will vibrate(!).

    Anyways, thank you for your comments, any other thoughts are very welcome.

  3. Expert Member
    Dana Dorsett | | #3

    The 2" ceilng clearance isn't a real issue for the FH06. Without the MHK1s being that close can sometimes cause excessive temperature offsets between the actual room temp and the programmed setpoint, but with the MHK1 that becomes a non-issue.

    How is the outdoor unit mounted? Wall brackets? Condenser pad?

  4. GBA Editor
    Martin Holladay | | #4

    One of the many reasons why you don't want the unit too close to the ceiling is that the temperature sensor (which senses the temperature of the incoming air) will think that the room is warmer than it really is, because the air near the ceiling is the warmest air in the room. Homeowners report that they have to dial up their thermostat to be comfortable in these situations -- although your remote thermostat probably solves that problem for you.

    In general, there are fewer homeowner complaints about vibration and noise when the outdoor unit is mounted on a concrete pad than when the outdoor unit is on wall brackets. Here's what Vermont builder Paul Biebel told me: "I've also learned that there is always some vibration from the outdoor units, so it's important to choose the location carefully. When things are quiet, you can hear a vibration or hum.”

  5. snugglez | | #5

    Awesome to know about the clearance. That sets my mind at ease for sure--thank you.

    Outdoor unit is on this metal... table? with ~12" legs bolted to a concrete pad they put down on the dirt yard. It's not touching the house, though of course the linesets are run along the house. I'm in boston, so the ground is frozen solid. Should I ask them to install rubber feet or padding under the unit or the mount?

  6. snugglez | | #6

    @ Martin Holladay.

    Yeah, I'm not worried about the temp sensing, since, as you say, the remote takes care of that. Mostly just that the unit would "choke" without more room to suck in air. I know that ducted units lose efficiency if you choke the return ducting, and I assume that these split units derive at least some of their efficiency from the fact that the fans don't have to work that hard to deliver the airflow. If you think 2" is enough to supply the unit, that's good. I just don't trust the guys who just ignored my big fat pencil marks on the wall indicating where to install the unit to tell me the truth, you know? "Oh yeah, uhhhh, 2" is TOTALLY enough." I just could not be certain they were being straight with me.

    Yeah, I know there will be some vibration, but this just seems like more than most people would put up with. It's hard to believe that most people would put up with this, but it's difficult to pin down if it's really an issue or just me.

  7. Chris_King | | #7

    Hey N W my mini split it also mounted on concrete pad right outside my bedroom window which rest on dirt. Anyway we have 4 rubber (washers) not sure the technical term that are about 1.5” thick that rest between the unit and the bolt on point for each leg. The stand itself has no rubber and we can barely hear the unit when it’s running with sloppily installed cheap windows.

  8. bencarsan | | #8

    As others have said I would not worry about clearance from an efficiency standpoint if you are that close to spec. And the question of whether you ask them to remount so that the unit is properly serviceable is a judgement call. Your ODU is properly sized for those three heads.

    If you have a ground mount I doubt rubber feet are going to help. Are you sure the noise you are hearing is not the defrost mode?--that can be noisy, but it's not fan noise, more like the hissing and popping of an electric kettle

  9. Expert Member
    Dana Dorsett | | #9

    Almost all outdoor units (even those on stands) get mounted with rubber vibe-dampers these days- I'm surprised that yours has none.

    There are resonant frequencies to the steel legs on the stand that may be a factor, and a 2" condenser pad can sometimes behave as a tympanic surface much like a speaker.

  10. snugglez | | #10

    Hi Bennett. Cool, I guess there's still a question in my mind of if there is an actual efficiency difference between "minimum allowable spacing" and "best possible spacing." Mitsubishi doesn't say anything about it, which *could* mean that they are the same number, but at this point, I think we are left to guess at something we are in no position to know.

    Out of curiosity, I'm going to follow up with Mitsubishi on both issues, just since: why not. If I get real confirmation from them, I'll repost.

    It's definitely not the defrost mode, this is sustained for long periods of time and the units are definitely putting out heat at the time. In defrost mode they are taking heat from the rooms. This sounds and acts exactly like a compressor that is just working very very hard.

    Yep, those heads (6/12/12) can be matched to my ODU:

    On the other hand, if you attached those exact same heads to the larger unit:

    They can output very significantly more heat. Meaning for the same load inside, the larger outdoor unit will have to work a lot less hard. What I'm saying is that since this vibration only occurs on the coldest days, it's possible that a better selection of outdoor unit would eliminate the vibration because the compressor would be working at 80% not 100%

    It seems to me from what I've read (after I had the job done, of course) is that oversizing the outdoor unit is just a smart way to go. Since the lower range of the capacity of both ODU is the same, what that means is that there is no risk of oversizing like there is with the regular ACs that don't scale as well to the load. I have a feeling a lot of mini split installers are old-school AC installers that are concerned that oversizing means cooling without removing humidity. But old ACs can't adjust the way these do, so oversizing is (almost) a non-issue unless you oversize a particular head in a confined space...

    Since I'm basically stuck with this ODU, I need to find a way to reduce the vibration when it's working at it's max output--which will be quite often, it seems. If the rubber pads will help, I'll ask them to put them in. But the more I think about it, the more annoying the whole situation is. I hired these guys based on them being diamond dealers from Mitsu, maybe that is just some BS Mitsu set up to reward dealers that buy a lot of equipment. I have to say, in almost every respect I've been surprised and disappointed in the choices and advice I got from the installer. I guess that is a common lament on this site, though. Oh well. I'll be better informed next time, I guess.

    1. donin13 | | #14

      I am desperate for help with this same problem My unit (there are 5 heads) is outside of my bedroom window. At first it was attached to the side of the house. I thought that was the cause of the vibration. It was moved to a pad. The vibration is not as bad, but it is still terrible. When the until shuts off it's like the feeling when loud machinery out side of your window stops and you realize how tense it was making you.

      My unit does have the rubber vibration pads.

      1. jweggy | | #15


        Did you ever find a solution to the noise/vibration problem? I recently had the MXZ-4C36NAHZ-U1 installed and am having the same problem. It is mounted into the studs on the side of the house using the quick sling QSWB4000 ( ) with rubber anti-vibe washers.



  11. snugglez | | #11

    @ Dana

    That is a very good point about the hard surface exacerbating the issue. I'll experiment with some sound-absorbing material underneath. Though to be clear: there is definite vibration in the lines themselves (I physically touched them as the noise was coming from the indoor unit). But at least some annoying noise is emanating directly out of the ODU itself, and maybe reducing the hard surfaces under and behind it would resolve that issue, at least. Thanks for pointing that out.

    1. jweggy | | #16


      Did you ever find a solution to the noise/vibration problem? I recently had the MXZ-4C36NAHZ-U1 installed and am having the same problem. It is mounted into the studs on the side of the house using the quick sling QSWB4000 ( ) with rubber anti-vibe washers.



  12. wmf | | #12

    N W, we also have some issues with noise from a Mitsubishi HyperHeat multisplit system. We are in Colorado, CZ 5 and the system is a retrofit. During colder weather, below 30F, there is a low drumming through the outside wall from the fans (single 48K unit) as well as a low hum through the linesets. This ODU is probably oversize for the load except in very low temperatures, and the drumming only is noticeable when the fans are starting to work harder. However, the oversize ODU is definitely a problem in the shoulder seasons because the system can't modulate down enough and cycles. But it kept us warm down to -17F. The system actually behaved better when I started heating the basement full time, and I haven't noticed much of an increase in electric usage. The ODU is about 12 in from a blank outdoor wall (2X6 studs with FG insulation and half log siding over OSB sheathing). The ODU is bolted to an angle iron stand through antivibration pads, sitting on concrete pavers on the ground. It doesn't seem that fan vibration is the issue so much as airflow vibration. We are considering turning the unit 90 degrees so that the fans face the garage wall, but wonder if the drumming will still be transmitted into the house structure. The main lineset comes into the basement to a branch box, and then the individual head linesets go out the rim joist and up the outside of the walls before coming in to the heads. The linesets are centered in the holes in the walls and foam sealed. It is the head that is turned off that hums. You can hear refrigerant flow in the linesets in the basement as a low whoosh. When I spec'd the system I got evaluations from two HVAC companies, one was a Diamond dealer. Neither impressed me much, but were the only companies who would come. The Diamond dealer wanted to install an even bigger system, and their ODU mounting was poor.. Any comments are welcome.

  13. snugglez | | #13


    great details--thank you.

    So I think in my case, the upgrade from 30k to 36k would have been seamless, as the 36k has identical lower output--meaning in the shoulder season it would be no worse than the 30k, while being able to maintain heat in the house during the coldest months more easily. Feels like a missed opportunity for me. Oh well, had I known...

    It does seem like there is some odd issue with your ODU. Am I understanding correctly that the ODU is basically in a "corner"--where there is a wall behind it and also to one side of it? I wonder if having two hard vertical surfaces is causing the air hitting the fans to "echo" or reverberate like in a canyon. Mine doesn't exhibit this type of sound at all--it is mounted to the ground with a wall about 8" behind it, but no other walls around it.

    Like I said, when it gets really cold, the unit is obviously at it's max output, and vibrates like hell. This means that the ODU itself makes a buzzing/humming like an AC would AND some of that is transmitted up the lines and to the indoor units. The closest one gets the bulk of the vibration, and some part of the evaporator is vibrating up against the housing of the unit, making a buzz. I know this is happening because it happens when the indoor unit is on OR off, and if I physically push the evaporator fins, and can alter and sometimes eliminate the sound. I know that someone could possibly come and figure out the exact point at which the metal was hitting plastic and insert a rubber pad there, but it still leaves me wondering just how much vibration is ok?

    The install manual of these things recommends a "horizontal loop" to reduce the vibration transmission to the indoor units. Whatever that is--I will follow up if I figure that one out.

    Yes I noticed that the head that is turned off still has refrigerant cycling through it, and I even used my IR camera to note just how much heat was radiating off the supposedly "off" indoor cassette. The installer said this is normal. I would think that the ODU would have valves on each lineset so that it can turn off the lines when they are not needed, but whatever, I'm no engineer. I'm sure there's a reason for it to cycle through all the units, even the one's that are off?

    In fairness to my installer, I've had much much much bigger idiots do work for me. At some point, I just have to accept that the blame must partly be my own, though I can't think of what I'm doing wrong, here.

    For your problem, where it's really a sound issue *in the air*, have you considered something like this:

    It's ugly as hell, but might resolve some of your problem. I don't think it would do much for me, since the problem is vibrations coming inside and creating noise in the room, but might work for you... maybe.

    GL please post if you solve it and how!

    1. jweggy | | #17


      Can you please share the location where the install manual mentions "horizontal loop" to reduce the vibration transmission to the indoor units?


  14. Seredick | | #18

    Hi, all. Discovered this thread out of desperation and sleep deprivation. We have the Mitsubishi mini split with the ODU on the roof of our townhome. Had these vibration issues previously and called builder (we have a 1 yr builder warranty). HVAC installers came out and determined the condenser was fried and replaced the unit. Noise gone. Now it has come back. They sent out someone again who told us about defrost mode. But like you, it's CONSTANT with the vibration, revving up, revving down. Sounds like a car with a really bad transmission. I haven't slept really in weeks. I fear that they don't want to deal with the hassle and are trying to hide time until the builder warranty runs out in a few months. Any insight or solutions work for any of you folks?

    1. Expert Member
      BILL WICHERS | | #19

      You may have some failed vibration isolators. Depends on the unit. Vibration isolators are like rubber blocks with a bolt embedded in either end, and are often used to mount units when you need to reduce sound transmission.

      You may have some issue with the condenser again, or you may just need a better mounting. I’d try mounting the unit on vibration isolators. Another option that sometimes helps is to mount the unit through some precast concrete pavers. The pavers add mass and help to damp any vibration.


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