Does your mini-split heat pump do this?

March 19, 2024 4:42 AM

We just got new mini splits installed in two upstairs rooms. The one in our bedroom is doing this thing when it's cold out and I'm not sure if it's normal operation or something we should call the installer about. If the latter, any info you might give about what could be happening would help prepare me for that conversation.

We have two upstairs rooms that are not well cooled by our A/C, so after a few years of dealing with window units (and getting new windows installed that we don't want to ruin) we got mini-splits installed. We chose to get ones that heat and cool, and opted for the "hyper heat" model intended for cold climates. It is a Mitsubishi system installed by a reputable company.

They are the less-typical "floor mount" units because our second floor is a converted attic with angled ceilings and knee walls (sort of like this image, although we don't have a baseboard radiator below it.) They are not centrally controlled, each has its own remote control.

The unit in the bedroom emits a low hum, although I have not noticed the one in the other room (which is my home office) doing so. It has not really been cold since they were installed about 3 weeks ago, but on the couple of nights that it has been in the 20s (including last night) I have woken up in the middle of the night to notice that the humming noise is cycling on and off.

Last night I paid attention and didn't notice a pattern, but it would go something like this: 10 seconds hum, 10 seconds off, 20 seconds hum, 10 seconds off, 1 minute hum, 10 seconds off. I can't remember if it ever cycled off for longer than 10 seconds but the periods it was running varied in duration. This morning it seemed like it was just constantly humming with no cycling.

Our only option for where to install it is not ideal for airflow but not terrible, it's in a bit of a recessed nook, so we have the fan set to run on a higher setting (rather than auto) to keep airflow moving more. My partner and I both have alarm clocks that display interior temperature. His is very close to the unit and usually within a degree or two of the set point. I have noticed on my side of the bed, farthest from the unit and closest to north-facing but brand-new windows, it's maybe 4-5 degrees colder than the set point. I don't know how accurate those thermometers are.

So, how much of this (the cycling and the temperatures) is normal for a mini split heat pump? What is the humming noise that cycles on and off, seemingly only in the middle of the night? Should I contact the installers to come check it out (even though I don't know if it will be cold enough to display this behavior), and if so what should I ask about? Thanks!

posted by misskaz to Home & Garden (7 answers total)

In a quiet room, my Mitsubishi mini-splits make a noticeable noise. It bugged me at first, but life got busy, and I didn't get around to fixing it. By the time I circled back, I'd gotten used to it. Searching for a fix, I found out the noise might be from the smart tech that checks if someone's in the room. Does your air handler have something that looks like a marker cap rotating from side to side? I think mine was called something like the 3D i-see Sensor. If so, does the does the nosie go away if you turn that feature off?
posted by phil at 5:47 AM on March 19

Response by poster: I can double check the manual when I'm back home but I don't believe there is anything like that on our floor mount units.
posted by misskaz at 7:02 AM on March 19

Generally, heat pump technology makes all sorts of inexplicable noises at various times, some of which is completely normal. Some noise patterns do mean bad things. I realize this by itself isn't terribly helpful, but I definitely had to recalibrate my noise expectations once I started living with heat pump-like devices.

More specifically, one thing that alters the noise patterns is defrost mode. Are you able to ascertain if when it is cycling this is related to defrost mode? If this happens every once in a while this is probably normal as well. If it is happening a lot, that probably isn't normal. (Also, is the cycling affected by adjusting the temperature target?) There might be issues simply with the external unit not being sufficiently vibration-isolated from indoors. (My downstairs neighbors had a broken / frequently cycling external unit audible through my window for a while, I can sympathize with how annoying this is. I did actually get used to it before it got fixed.)

Even more specifically, when I googled "mini split defrost mode noises" to try to calibrate my comment (I have more experience with heat pumps than mini splits per se), I got a lot of threads specifically about hyper heat mitsubishi models, including many where it sounds like there can be non-trivial install(/installer) issues. For example: 1, 2. It sounds from these like it could definitely be worth contacting the installers and seeing if they could do some noise mitigation, update your firmware, etc.
posted by advil at 8:45 AM on March 19

You said you had two mini-splits installed; do they each have their own outdoor compressor/condenser, or are both indoor air handlers connected to the same outdoor unit? If they're separate, it's possible, especially given the cold temperatures, that the noisy unit is going through defrost cycles.

If they're both connected to the same outdoor unit, are both of the indoor air handlers set to the same mode? We have two Mitsubishi mini-splits sharing an outdoor compressor/condenser; the advantage was lower cost and less space than two separate units, but the disadvantage is that both of our indoor units must be in the same mode (heating, or cooling). If one is in heating and the other is in cooling mode, they will "fight each other" over whether the outdoor unit should be running in heating or cooling mode. This can also happen if one or both units are left in "auto" (heat or cool as needed to maintain X temperature).
posted by xedrik at 9:12 AM on March 19

Response by poster: Just a quick response, not trying to threadsit:

- The sound is a very quiet, low hum, like from a fan or vibration, though I don't feel anything if I put my hand to the wall or unit. It doesn't seem to match the descriptions of the defrost cycle noise, and the cycling is much more frequent (on the order of seconds, not 15 minutes). It's so quiet that I'm not sure my partner even hears it. I'm not even annoyed by it, I'm just worried it's indicative of a problem we should get taken care of now, while the installation is new and before the weather warms up and it's 6 months before it's cold enough to happen again.
- The two indoor units are connected to the same outdoor unit and are both set to heat mode; we were cautioned about not having them in different modes.
posted by misskaz at 10:07 AM on March 19

The simplest answer might just be that it's firing up the fan to move enough room air over its internal thermostat to determine whether the unit needs to demand heating or cooling.
posted by Kyol at 10:47 AM on March 19 [1 favorite]

I'd probably call the installers and have them check it out, which really ought to be a free visit. In a system with two indoor units sharing one outdoor unit, and just one of the indoor units is making a humming noise that is cycling very fast... it's worth checking out, at least. When our units were installed, after it was all said and done, they scheduled a 2-week follow-up just to make sure everything was working, and if we had any questions. If that's part of your package as well, maybe notify them of the concern, and leave it up to them if they want to wait for the scheduled follow-up, or escalate it so they check it out sooner. But I think it's worth having them check it out, and again, something like this so soon after installation, that ought to be a courtesy visit. Maybe "sometime in the next few days" when they have a tech in your area, but that really ought to be a courtesy visit, even if they just listen to it and go "Oh, yeah, that's this thing here, and it's normal, and here's why the other one isn't doing it."
posted by xedrik at 1:04 PM on March 19 [2 favorites]