Suggestions for reducing noise from Mitsubishi Heat Pump?

AndrewR

Neighbors in my small residential building recently renovated their unit and scrapped the hydronic heat for a Mitsubishi heat pump (MXZ-SM42NAMHZ).

It is a relatively quiet machine compared to other, older units in the neighborhood, and by all accounts was installed properly.

In the summer, in air conditioning mode, it is relatively inoffensive, in part because windows are open and there is a lot of general background noise in the neighborhood (traffic, planes, window AC units, other mini splits).

In heating mode, which is three-quarters of the year here in the northeast, it's clearly having to work a lot harder, and it is destroying our sleep.  The unit is mounted on a concrete pad about thirty feet from our second-floor bedroom, and the compressor emits a low-frequency pulse around 240Hz that is impossible to ignore, particularly from about 8pm to 7am.  The noise is just over the threshold of audibility, but that is at a frequency that is hard to stop.  It's similar to what you hear from idling jet engines when your plane is at the gate.  There is also an annoying higher-frequency whine that I assume is coming from the fans.

I know from searching the web that others have encountered this problem with other Mitsubishi units, but I have yet to discover a solution.

Would anyone have any suggestions? Thank you!

Jamie Hall

I doubt there is one. And it's not just Mitsubishi units — I have heard of that sort of problem with lots of different makes.

AndrewR

😓

bburd

I would try a white noise machine by the bed; they are inexpensive and surprisingly effective at masking background sounds like that. I have an old one from Marsona that works well.

Various things can be done to improve the sound isolation of windows, like adding inner panels; but this is quite a bit more expensive.

Kaos

There are some things that could be done to reduce noise. The issue is that it is not your unit. Is your neighbor willing to work with you on it?

What is the construction of your condo? That concrete pad, what is it sitting on?

The main issue is that most rules have a dB limit not the quality of the noise, so unless they are willing to work with you, not much can be done.

Jamie Hall

The problem is that 240 hz hum (the middle B natural on a piano) (I'm a little surprised by it, actually — 60 or 120 hz or even 180 hz would be more common but it may be from a four bladed fan). That is a low enough frequency that it is remarkably difficult to screen or shield.

AndrewR

Thank you all for weighing in.

The condo is early 1900s wood frame, clapboard siding, and vintage aluminum over that. There is no insulation in the walls, but the walls are dominated by double-hung replacement windows. The bedroom is on the second floor and features a three-window bay that hangs out over the driveway by a bit. I'm sure none of this helps, and there may be some reflection from the house across the driveway and the garage at the end.

The concrete pad is sitting on dirt, as far as I know. It is not touching the foundation. There is a vestibule on the other side of the wall behind the unit, and in mid-winter the sound is quite noticeable.

The neighbors are willing to work with us on this. It's a two-unit condo, and as a lawyer once said to me, people in these units tend to have to find a way to get along.

Jamie is right: the frequency is the problem. The worst of the noise seems to be coming from the top right - it's quite clear if you put your ear to the side of the case, just to the right of the Mitsubishi label. Apparently I'm not alone: https://hvac-talk.com/vbb/threads/2244557-240-Hz-hum-from-Mitsubishi-heat-pump-in-heating-mode

Unfortunately, I haven't found anything on the web that indicates someone has found a solution.

I'm going to look into some rubber isolation mounts and some Roxul products that might help reduce any reflection off the pad and also be effective in a barrier (observing all required clearances).

I'm wondering if something like Dynamat could be applied to the exterior sheet metal, and whether it would help. I suppose a roof over the thing might help, but the clearance required may be prohibitive.

Kaos

Good start that is not mounted to the building. In some ways it is good that it is higher frequency as the usual 30-60hz stuff is much harder to deal with.

Yes, those dynamat type of vibration pads work especially if one of the metal panels is amplifying the sound. I've used them before for equipment that had a loud fan inside. Try first going outside when the unit is making the hum and put your hand firmly against some of the metal panels. If this reduces the sound add the pad there.

You can search for generator soundproofing for ideas. There are also pre-made kits you can buy. The things that work is loose hung MVL sheet and rigid mineral wool panels on something solid (metal or cement board).

The units also have some jumpers that lets you select max output and quiet mode (see link). Playing with them might also help.

https://mylinkdrive.com/viewPdf?srcUrl=http://s3.amazonaws.com/enter.mehvac.com/DAMRoot/Original/10006/M_MXZ-SM-NAM-NAMHZ_INSTALL_.pdf

Intplm.

@AndrewR

Your idea to use noise-deadening isolation mounting material is a good start. Using this on any area that comes in contact with something attached to the building or anything else for that matter should help.

Please post your results when you use it.

bburd

Keep in mind that whatever is done to reduce the noise must not interfere with the free flow of air in and out of that outdoor unit.

AndrewR

Thank you all. Yes, I have downloaded the manual for the unit and will observe all the required clearances.

Kaos

If you really want to nerd out on sound, since you know the frequency you need to deal with, you can design a helmholtz panel absorber tuned for it. They are simply a perforated sheet over shallow cavity filled with fluffy insulation. There are many on-line calculators for this ie:

http://whealy.com/acoustics/PA_Calculator/index.html

These are very effective and often used inside duct silencers.

rynoheat

Adhesive butyl rubber(like dynamat) works really well to dampen higher frequency noises. It doesn't even need to fully encapsulate what's generating the noise. Car manufacturers will use fairly small squares of it on the middle of body panels that are prone to vibration noises.

hot_rod

Hotels along the freeways sometimes have sound damping, acoustic glass, it makes a huge difference. Same with truck windshields.

I have sound screen glass in my truck and can notice the difference when riding in same trucks without it. I don't know if that technology can be added to glass. It's probably laminated in?

AndrewR

Thank you - really appreciate these last three ideas. I have some experience sound proofing a TV room / music practice room, and it worked, but dealing with exterior noise is a lot harder. I know some buildings near the flight paths in and out of the airport were fitted with special windows. I'll have to investigate.