Mitsubishi Hyper Heat Minisplit Noise, Defrost Cycle Issues
Since install of our dual-head Mitsubishi Hyper Heat Ductless Mini Split, we’ve noticed a couple issues when in heat mode and when outdoor temps are <36 degrees F.
The unit goes into defrost mode approximately every 15 minutes. Each defrost mode lasts for 10 minutes, blowing no air or cold air during this time. The unit only gets about 5 minutes of heating in before going back into defrost mode and blowing cold air again.
The worst part is that defrost mode is LOUD. I would liken it to the sound of a toilet flushing but amplified/echoing. Loud enough to drown out the television and too loud to sleep through.
We contacted the installer, a diamond contractor, who believed the length of the line was causing the loud noises. They moved the outdoor unit and added length to the line, but nothing has changed. They are at a loss and so are we.
1. Is such a loud noise normal in defrost mode?
2. What could potentially cause the unit to defrost so frequently? Is this ever normal?
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If they are at a loss, then they should contact their wholesaler. You always have the option of filing a warranty claim. I would at least call Mitsubishi directly and ask if there is any other option than going the warranty route.
Good point, we’ll give Mitsubishi a call ourselves if they don’t figure it out at the next tech appointment.
Noise when switching to defrost unfortunately is common. Some brands are better than others but never silent.
The bigger red flag here is the constant defrost cycle. Provided the base pan is draining and you are getting good airflow for the outdoor unit, this sounds like the refrigerant charge is off. You either have a leak or they did not top up the unit properly during install.
Part of installation of multi splits is calculating how much refrigerant the setup needs and a lot of times additional refrigerant needs to be added. Multi splits are pretty sensitive to refrigerant volume, if this is off there will be excess defrost and overall output and efficiency will be off.
Unfortunately you can't check refrigerant fill with a pressure gauge on a variable speed unit (this is most likely what the HVAC tech will do, but it is wrong). The refrigerant needs to be reclaimed, weighed and the correct weight added back in. HVAC tech won't want to do this as it is a lot of time, so it might need some prodding from Mitsubishi. Also make sure somebody calculates the proper fill and show you how they got the value.
Thanks for the advice. This is very helpful. We’ll be sure to mention this when the tech comes out on Monday.
Another thing to check is kinked line set. This is very easy to do where the line set comes through the wall and hard to see unless you open the insulation on the outside.
Seconding the verification of refrigerant calculation. You could find your model on mylinkdrive.com and double check the numbers yourself if you’re reasonably math savvy. There’s also a dip switch setting that forces the unit to defrost more often. It’s intended for ultra-cold climates - but I’d sure hope the diamond installer would know that. That info could also be found on mylinkdrive.com
I'd call Mitsubishi and keep pushing until you can talk to one of their tech consultants. Good luck.
I would not trust a diamond installer to know anything. Two different diamond installers who confirmed over the phone they are well versed in Mitsu ducted mini's sent techs to my house who took one look at the unit and stated "I've never seen one of these before; I thought all mini splits were wall mounted heads."
The Mitsu diamond program is all smoke and mirrors marketing. They may have good equipment but it is hell trying to find anyone who knows how to work on it, and Mitsu's only answer to everything is "call a diamond installer listed on our website." Useless.
We are having the exact same problem! Mitsubishi has admitted to a firmware mismatch on some models and have just revised the firmware in my condenser. This has corrected the 15 minute defrost cycle and significantly reduced the "whooshing" noise, but has not completely solved the problem. There was still unacceptable noise from the head unit (even when one was off) during the defrost cycle. The refrigerant has just been captured, measured and replaced, and now it almost seems to be operating correctly. There is still an occasional loud vibrating noise from at least one head that seems to coincide with the beginning and end of the defrost cycle. This is a Mitsubishi problem, but good luck contacting them directly. I have to go through my installer, and I think they are as frustrated as I am, bur we're making progress.
Hi Steve, I have two units outside, both multi zone, a 15,000 and 12,000 on one of them and on the other multi zone It’s two 9000 and a 6000, they are hyper heat, 3 series and I just had them installed, all five indoor wall units makes swishing noises at night time but it’s every 15 minutes, was your refrigerant low?
My units only go into defrost mode once it hits 37° And it drives me nuts when I’m trying to sleep at night.
I spoke to my installer, and he said something with the new series 3 which just came out about six months ago are having issues, but he didn’t get back to me yet.
He said he’s waiting from Mitsubishi .
Everybody I talked to said my refrigerant is probably low and that’s what’s causing the defrost mode to go on every 15 minutes.
But now I hear a firmware mismatch, not sure if that’s on the new series 3 or not.
Your thoughts.
I think we have very similar systems - I have 2 24K HyperHeat 3 condensors with a total of 5 heads, and the firmware fix solved the reversing valve noise and the problem of going into defrost mode every 15 minutes. I had my units install in Sept, but it wasnt cold enough to notice the problem until late October. The problem, as I understand, is only with certain newer models, which mitsubishi can check by serial number. I'm not sure if it only applies to multihead units.
As Akos mentions, we still hear the reversing noise, but it is now almost as quiet as the fan.
I havent yet got a definitive answer about how much refrigerant was recaptured, but it seemed as though replacing the refrigerant helped even more. The technicians only did this on one condensor, so I'm going to compare how the two sides of the house are operating. They said it was hard to recapture all of the refrigerant on a cold day, but I got the impression that they were satisfied that the the refrigerant wasnt low and there we no leaks. I'll keep you posted.
Hi Steve - we've been having the same issue with our unit "defrosting" every 11 minutes once it hits 37*. It's reassuring to hear that you experienced something similar! We just had our installer reweigh refrigerant, but it didn't solve the problem (even though we were definitely low on refrigerant!) Our next step is to have the Mitsu guy come out for the firmware fix. We really hope that works for us. Sounds like that helped you, but wanted to know -- does your unit now go into defrost at a "regular" time interval now, at a lower temp (37 feels a bit warm to us, but maybe that's by design), and only when the coils frost over?
The reversing valve in a heat pump needs a pressure difference to actuate, that means there is no way to reverse flow without some noise. This noise will be heard in all indoor units whether they are on or off.
Besides the gross oversizing issue, this is why it is a bad idea for a wall/ceiling/floor mount in a bedroom. There is nothing your installer can do to reduce the noise.
Unfortunately there is no easy or cheap solution to this problem. The best way to deal with it is to take out the wall mounts in the bedroom and install a small ducted unit in a hallway ceiling or closet to feed all the bedrooms. This fixes one part of the oversizing issue plus that extra bit of separation means you won't hear the unit going into defrost.
Excessive defrost could be a sign of low refrigerant but could also be caused by outdoor unit installation if there is not enough free space around or it is not protected from snow or ice coming off the house roof.
In case a tech does come out and weighs out the refrigerant and finds it to be low, make sure they fix the leak. Common practice is for them to simply refill to spec but without fixing the leak, it will be low again in a year or two.
OK, what I don’t understand is that I have a 9000 single zone in my addition and that does not go into a defrost mode at all, day or night.
Does the swishing noise only happen when it’s on a multi zone?
And only when the sun goes down, I’m a little confused, because I have no problems with my single for the last seven years.
It's because your 9k unit can turn down to a crazy low minimum capacity compared to a multi split. Look at the electricity consumption of a single 9 k or 12k single zone Mitsubish at minimum capacity. Its using only 60, 80 or 110 watt!. I verified all these numbers with an Emporia Vue electricity monitor.
https://ashp.neep.org/#!/product/34427/7/25000///0
Even if a single zone is oversized your cycling on such low minimum capacity the electricity consumption is nothing compared to these multi splits that have minimum capacities of 24k btu/hr using 1800 kw.
Thanks for that info, I’m still confused on why my multi zones keep on going into defrost mode every 15 minutes.
But like I said the series 3, from what I heard have some issues and that’s what I’m waiting for from my installer.
There’s no reason it should go into defrost mode every 15 minutes for 10 hours straight.
Hoping they figure something out.
Maybe it is a firmware mismatch.
I do want my outside units drained, and re-weighed to make sure they’re not light on refrigerant. That way I can rule that out.
I reached out to other techs from different companies and they said it could be that
your refrigerant is low, and I said on both units outside, what are the chances of that, but like my tech said these are series 3, and they’ve been having problems with them.
I’m thinking maybe I should just have all the units single with their individual outside unit.
The unit low on refrigerant is not your problem unless somebody put a nail through the line, it is an install error. It common for installer to blame end user for this, I've been told by one that the reason the unit is not running properly is the air filter on indoor unit was not clean (the issue was of course fixed by adding refrigerant).
I would start by getting the existing setup running properly (proper charge and firmware) and go from there if needed.
As for sizing issues, I would start with figuring out what your heat losses are first before looking too much for new equipment. This is a simple tool to get you into the ballpark:
https://betterbuiltnw.com/hvac-sizing-tool
Mitsubishi does have their newer SM series multi splits which are semi commercial. These, at least according to the NEEP site, have much better turndown:
https://ashp.neep.org/#!/product/57008/7/25000///0
They also have setting for the LEV to reduce noise in defrost and when the indoor units are off. I don't know if it will fix your noise issue but would help. These are slightly more complicated install as they use a branch box but it also makes them much more flexible as you can get all your heads onto a single outdoor unit.
There are so may oversized units being installed. What's the secret to trusting your HVAC suppliers to get it right?
These multi split units were originally designed for high load mostly commercial places in Japan. They are being poorly installed with their minimum capacity not being fully understood.
These large multi splits just cant run low at low modulation and reliably circulate refrigerant oil. So a 48k btu Mitsubishi multi split needs to run at extremely high low minimum capacity like 24k btu/hrto ensure oil circulation.
A single zone 12k Mitsubishi will use close to 10 times less power at minimum capacity and will deliver a way better COP.
The secret is not to trust them. I've had good luck with almost all other trades but a good residential HVAC tech seems to be impossible to find. Even getting the right part number seems to be hard, a while back I asked for a 9k unit and the installer showed up with a 12k, by the time I noticed it was already installed.
With a bit of work, it is possible to check any proposed design which at least will avoid major issues.
Your thoughts?
It’s funny, as soon as the outside temperature is 37° my units stop going into defrost mode
I agree with Akos assessment that you most likely have a refrigerant leak. A 4 zone multi split has 16 flare fittings. Thats a lot of fittings that can leak and one smaller leak will absolutely destroy the efficiency. These units need to be critically charged to the exact refrigerant amount to run efficiently. The first sign of a low charge is ice on the coils in both heating and cooling mode. Your coil is icing up and defrosting most likely because it's low on charge.
Yes, but what are the chances that both units outside have a refrigerant leak , because they both do the same thing. I have a multi 3 zone and a multi 2 zone and they are both doing it.
My tech installer was out to check that a week ago and he found no leaks, I just had these installed January 6 of this year.
Are the outdoor units actually frosting up and producing a lot of water?
The one problem with oversized multi splits is the refrigerant that is being bypassed when only a small zone is calling for heat. This extra refrigerant needs to be evaporated so no liquid gets into the compressor which will make the outdoor condenser run colder thus more chance of frost buildup.
Are your outdoor units the 3C24 or 3C30 ? In either case the min on them is above 1 ton, so you need at least one big head running pretty much at full tilt for it not to bypass refrigerant.
I just want to thank everybody for all the feedback.
Greatly appreciated.
I haven’t been outside to look at them at night to see if there’s any frost buildup but I know my one unit is a 24,000 which runs the two 9000’s and one 6000.
And the other unit outside runs a 12,000 and 15,000 unit . Not quite sure what that one is, this is all new to me.
I was told by a lot of people that these wall units are nice and quiet inside and that’s not the case on my end.
Hopefully my installer gets back to me soon with a response from Mitsubishi.
If he tells me that this is normal, I will be very upset.
Spent a lot of money having this work done to my home.
Trying to get away from oil plus my boiler is 35 years old.
Did you ever get your Whooshing issue resolved? I have the same issue on mine...
The first thing you need to understand is what actually triggers the system to go into defrost mode. The system has no idea if there’s actually frost on the coil. Evaporator coil temp (outdoor coil during heating) and compressor runtime are the primary inputs. The cycle itself is not timed, it looks at time to hit target defrost termination temp at the coil from last cycle and adjusts accordingly. A properly functioning system will NOT defrost every 15 minutes. Disturbing to hear of the firmware issue with the HZ3 I haven’t seen that one yet and hope not to. If weighing in a new charge resolved the issue and now it’s recurring then yes by far most likely (like 99.9%) there’s a leak. The problem is the techs are not locating and repairing the leak. This can be done a few ways, a high quality infrared leak detector is great for checking connections. Leaky flares at install are common because some installers don’t properly leak test the system prior to commissioning it. If they can’t find the leak with leak detector, they should pump the system down (capture existing charge back in the system), and put 600psi nitrogen in the lines and monitor pressure. It should hold steady like a rock for….ever (minor fluctuations with outdoor temp changes). Minimum 4hrs but best to do overnight. New installs should only need connection leak although in the thousands of units we’ve installed I’ve had ONE indoor unit (was a floor model) leak when new. These systems have a target evap temp, that doesn’t change under low load, excess refrigerant is stored in the accumulator which is quite large in these multi-zone systems. A 9,9,6 on a 24 is fine. During those times when the combined output of all three zones on the system is less than the minimum compressor output, you lose efficiency but it will not affect how it performs otherwise nor will it affect defrost cycle. They’re not new to there systems they have an intricate understanding of how they work. We’ve been installing hyper heating multi zones since they were first produced. Zero defrost issues. The only thing we’ve seen were defective reversing valves. We’ve changed probably 7 or 8 total and a couple of those were for other companies that did not want to attempt the repair. Have a competent tech find and repair the leak(s), weigh out the charge to be thorough, then weigh in a new charge. If no leak can be located, weigh the charge out of the system. IF they charged the correct amount last time then weighing the charge out will tell the story. This is is a very very basic repair that’s all too often necessary because system was not properly installed.
You are low on gas. Heavy wooshing sounds during defrost is a dead giveaway. Also above 37 there are no defrosts so this is not an indication of anything.
Yes, they came out and evacuated the lines and refilled, but now it is doing the same thing again!
There's gotta be a leak somewhere. I requested that a Mitsubishi tech come out to my home to try and get to the bottom of this. The installer checked all of the connections on the outside of the house for discoloring at the joints and found none (discoloring means leaks is what I was told)
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During install. They should also use a micron gauge at the outdoor unit then valve it off when target (less than 500, we use less than 300) vac is reached. Pressure rise, or lack thereof, on the micron gauge tells the story. It should be steady. This is the second part of the leak test which also verifies there is no moisture present in the system.
I can confirm this firmware issue is a "thing". I had a 30k btu H2i multi-unit system installed in summer of 2022 by a Diamond installer. Had all kinds of issues both heating and cooling, irregular temps from different heads, etc... It took them a year and around a half-dozen trips for pumpdowns, new parts, etc. to finally replace the outdoor unit. Cooling was great after that, but once it got cold I started seeing exactly what is being described. For me it's like clockwork, 25 minutes of runtime and then a 15 minute defrost cycle. Something new is also happening in terms of how loud that cycle is, to me it sounds like the heat pump fan spins on turbo for about 2 seconds and then stops. A bit like a hurricane out your bedroom window. My 900 sq ft space is only able to get to around 58 degrees when it's 12 degrees outside. What happened to the 100% capacity down to 5f and "it works down to -13" speak?! Laughable... I started logging the temp rise on output air and frequency of the defrost cycle. What I see is after about 10 minutes it's only producing 70f degree air, at 20 minutes it's approaching 80f(yay!) and at 25m it shuts off and starts the whole process over again. I'm probably getting 10-15m of "warm" air per hour. After some prodding, my installer reached out to Mitsubishi and they confirmed there is an "issue" but that it's "early" and the only fix is to send my board to them to reflash the firmware. They won't allow the installer or local rep to flash it for some reason. Unfortunately that means someone cracking in to my new unit and doing major surgery (again). It also means me having to otherwise heat the space some other way. Luckily it's a small cabin, so doable. I feel for someone who has this as an only source of heat in a primary home.
So another confirmation this is a real issue…
Here in Quebec, my installer confirmed that he had a similar case and managed to get a replacement board with new firmware.
Apparently mine is on order now too and should come in soon…. Sucks that you have to send in yours ?? Maybe they are back ordered …
Will keep you posted !! I hope this “early” fix makes everything run OK and doesn’t bring up other issues ..
The installer wasn't over descriptive on the the "why". I asked that he press Mitsu to send a functional part or have a factory rep come and do it on-site if he was incapable. He replied that he tried and they wouldn't budge. Also that any board they could get would have the same (mismatched) firmware. Whether or not that is true I don't know. I'm going to setup some resistance garage heaters this weekend and will post once I get the re-flashed board installed, hopefully next week.
Just wanted to thank you folks for alerting me to the firmware issue. This is definitely the problem the OP is experiencing. I was checking a system over the other day and experienced EXACTLY these symptoms. Defrost every 15 minutes. I have contacts at Mitsubishi. They know of the problem and actually have a few programming tools in the field. Headed out tomorrow with them to erase the bad firmware and upload new program. Yes board change is another solution if you can get the board. Thanks again for the info on this. We have MANY of these out there, I can alert our customers and stay ahead of this as much as possible.
Nice that you have the ability to flash in field, boy would I have loved to have our local crew do that instead of pulling the board in the middle of winter. Our local rep couldn't get them(mitsu) to do it. Let us know if it fixes the issue for your customer, I'd be curious if all is well after the flash.
Ok I am triple posting in three threads, I just want to make sure if anybody searches for this problem and finds only one thread they will find this info :
Ok so this night we hit a low point of -18c (0F), temps at which the system was problematic.
(I honestly haven’t had time to check the system’s functioning now in detail, but I definitely saw differences last evening when it was warmer (Around 23f). System no longer shoots cold ish air for 10 minutes after defrost but seems to wait till coils are hot enough ..)
Anyways, normally at -18c ambient (0F) I would have to program set point to 22 or 24c (71 to 75f) and hope for a house temp at 20c (68f) in the house, but would usually wake up to 18-19c (64-66f) and a defrosting mini split.
Well, I decided to go all in and leave the setpoint at 20c (68f).
Go downstairs and …. Ambient -18c (0F) and house temp… 21c!!! (70f). TOASTY
I don’t want to celebrate too early but…. The fix seems to have worked!!!!!
ANYBODY DEALING WITH EXCESSIVE DEFROST CYCLING AND UNDERPERFORMING MITS MULTI HEAD HYPER HEAT UNITS IN HEATING : GET YOUR FIRMWARE CHECKED!!!!!!
I created an account just to thank you for this! I just had two MXZ-2C20NAHZ4 units put in and each was cycling into defrost every 15 minutes. I found this thread and shared it with the installer, but they had to go through a bunch of steps before they believed me. Checked every connection, vacuumed system, pressurized with nitrogen to check for leaks, weighed the charge, even came back and put in a whole new charge after flushing the lines in case the original charge was contaminated.
Finally, on the fifth visit, they swapped the board out after contacting their distributor who said this was a known problem. They're reflashing boards as they come in from the field and sending out reflashed ones as needed.
The behavior change with the new board is incredible. It went from sounding like an airplane toilet every 15 minutes to behaving quietly and consistently.
Thank you!!
Ha! Airplane toilet exactly!! Glad you finally got it resolved. My experience was much the same... checking, checking, checking... OK, we'll call Mitsubishi!
Sorry, I forgot to post back after my firmware update. It did fix the issue, and for the first time it seems like everything is functioning as expected. I'm able to maintain 70f inside when 0 outside, that was all I was after... Now I can get back to griping about the constant clicking noises in my bedroom from the wall unit when heating up/cooling down between cycles. Different problem for a different day...
that can be fixed with some teflon spray (ask me how I know...). If you are willing to take the cover off yourself you will see on the left-side end a bunch of copper coils held in place with plastic. Turn off the unit, spray them real good (don't over do it so its dripping or anything). Turn unit back on and see if the clicking from expansion goes away. Spray again if it still persists. Then put the cover back on when satified.
I've seen this, have yet to try. Next on my list... I was going to try a blaster brand dry lube so they don't gather dirt. Pretty sure it's teflon based. Let me know if you had a specific product that worked? Thanks for the reminder, I've been turning the bedroom unit off at night.