Mitsubishi heat pump issues

ksm

I had a 3 zone 30,000btu ductless Mitsubishi hyper heat pump installed in October 2023 in New England. 

Condenser: mxz-3c30nahz4
Heads are all msz gs series.    

The house was built in 1884 and is overall pretty well insulated. The first floor is open floor plan about 800 sq ft but one entire wall is a sliding glass door and floor to ceiling glass. There is an 18,000btu head for that area. The master bedroom is about 225 sq ft and has a 12,000 Btu head. The guest room is about 145 sq ft and has a 6,000btu head. The bedrooms are upstairs and seem to stay warm in winter only running the 18,000 downstairs. 

The first issue was when it started getting cold it was defrosting every 20 minutes then would go full blast to recover then defrost again.  It would do this even when the coils had no frost whatsoever. I finally was able to prove to my contractor and he replaced the circuit board to one with new firmware which solved the constant defrost issue.

Second issue is the upstairs heads randomly start buzzing very loud, even when they are off. It will do this for random amounts of time then stop. I have no condensate pumps so that is not the issue. 

Third issue is very large electric bills which I cannot figure out if this is normal or not.  I used to heat the house with a wood burning furnace that only used a blower to distribute heat and I averaged 800 kWh per month. With the mini split on at 72 degrees all the time I seem to average around 1900 kWh per month. This is mostly just running the 18,000btu downstairs for heating. I don’t have any experience with cooling yet and hope there are no more issues. 

Fourth issue is with the upstairs heads only. When the thermostat is satisfied the veins will just stay wide open and the fan will shut off completely. When my downstairs head thermostat is satisfied the veins will go horizontal and the fan will circulate air, then when it calls for heat again the vein will drop down and start blowing heat. As I understand the downstairs is working properly because it constantly runs the fan to get the air temp whereas the upstairs just go dead once their satisfied. 

Any help is greatly appreciated 

Hot_water_fan

Third issue is very large electric bills which I cannot figure out if this is normal or not.  I used to heat the house with a wood burning furnace that only used a blower to distribute heat and I averaged 800 kWh per month. With the mini split on at 72 degrees all the time I seem to average around 1900 kWh per month.

Not enough information to say if that’s high or not. You swapped fuels. If I went from oil to gas, it’d be silly to complain that my gas usage skyrocketing without mentioning my oil usage plummeting. 

That’s a gigantic system for a house that size. The installer should not have installed that: these systems perform poorly when 1 outdoor unit is tied to multiple indoor units that are oversized for the space. Mitsubishi explicitly tells installers NOT to do this but it’s easier than ductwork. 

GGross

according to mitsubishi website the msz gs series are all single zone only indoor units, maybe someone else here can confirm that? but they are showing on mitsubishi website they are not compatible with multi zone outdoor units, and they start at 30,000 BTU/hr so I'm thinking you have different heads than what you stated, or mitsubishi has incorrect info listed

The buzzing seems strange, when you say the units are "off" are you actually turning them off of heating mode entirely or are they just satisfied? Multi unit ODU need to have multiple heads functional at all times, but buzzing certainly seems out of the ordinary, I don't see this on the other brand units I sell and would definitely assume something was not right.


Your electric bills will obviously go up when you switch from solid fuel to heating with electricity. My math isn't the best so hopefully someone can check me on this but if you assume a COP of 3 (no data I can find on these, but they should have a COP of 3 or more even at low temps), 1100 x 3 = 3300 then convert to BTU 3300x3412= 11,259,600 BTU per month 30 days in the month, times 24 hours in a day thats 720 hours,
so 11,259,600 / 720 = 15,638.33 BTU/hr which is pretty low heat usage, though again i'm prone to math errors so maybe I messed this up .

your upstairs heads, make sure they are programmed to run the same way as the downstairs one. It is possible that these are to large for the space and shut off completely, they are almost definitely too large for the ODU minimum output which is likely why you see longer runtimes on your larger head. If the heat loss guesstimate based on your electric usage is correct, the 18k unit is capable of satisfying the entire load by itself.

your 18k unit is putting out 22 btu/square foot, which is a somewhat normal heat load
your 12k is putting out 53.33 BTU per square foot, that is an incredibly high heat load per square foot, it is almost certainly oversized
your 6k head is putting out 41 btu per square foot, again an incredibly high heat load and is probably oversized
these "per square foot" heat loads and whether they are close to what you need are dependent on the outdoor temp as well. A well insulated house in northern michigan, will typically have around
20-25/btu/hr per square foot, when it is -10f outside.





so I have some questions for you.

Models of each indoor head.
What is the actual heat loss of each area serviced by each size head, calculated hopefully by the installer prior to installation.
confirm correct wiring of each indoor unit
confirm that each unit is running in the same mode

what is the length and size of each lineset, and what refrigerant charge was used on installation. this is crucial that the refrigerant charge is correct, your contractor should be able to confirm this, there are no leaks, wiring correct, minimum and maximum lineset lengths were followed, and appropriate charge was added if needed.

I will add, troubleshooting inverter mini splits is not a DIY friendly affair, hyper heat are high tech units and generally require specialized training to work on.

ksm

Indoor unit models:
msz-gs18na   15 ft line set    8105btu/hr
msz-gs12na.  23 ft line set    3135btu/hr
msz-gs06na.  20 ft line set.   2405btu/hr

I can’t confirm wiring but each one seems to respond and deliver heat when turned on.
I can confirm they are all in the same mode which has been heat.
I cannot confirm refrigerant charge but all units do put out very hot air. 

I appreciate the responses. I do feel like this unit is oversized and wish I went with one to ones. I received quotes from 5 different contractors with good reviews and nobody offered that option and they all designed systems similar to what I have so I figured this was the norm. One contractor even wanted to put a 45,000btu Fujitsu condenser which I figured was way oversized so didn’t take that route. The only difference between the other quotes is in the master bedroom other companies suggested a 9,000 where my contractor said the 12,000 is the same unit as the 9,000 and basically same price so might as well use it.   
As far as the buzzing whether the units are turned on or turned off they will do the buzzing. A lot of time I just have the downstairs 18,000 on and we will hear the upstairs units buzzing. I can try to upload a video to YouTube and attach. 

ksm

https://youtu.be/ylVJ9UdjgVw?si=zAW_2jx3k5sJt26H

https://youtu.be/7zCjbmOiwUg?si=Bu13dXokIyy123VS

Here are the videos of the buzzing. In these videos the head units that are buzzing are in the off position and just the 18,000 downstairs is on. 

pecmsg

If the unit is oversized it can only turn down so much. At that point it starts allowing the extra load bleed to unused heads. 

That is why sizing is so critical. 

Is that head calling?

ksm

The head that is buzzing is shut off. The downstairs 18,000 is running 

Jamie Hall

Looks to me that @GGross 's math is fine. I'd take minor issue with the COP -- I doubt it's as high as 3 in colder weather -- but that's it.

You are in New England. Electricity rates are high. You will NOT save money with a heat pump over any other possible fuel -- oil, gas, LP, wood, you name it. In fact, you will probably spend more than you would have with oil or gas. Can't help that now...

GW

oh my--- with the greatest of respect, if you don't know the ins and outs of minis, please don't comment. The homeowner is already feeling stressed, and some of the comments don't help at all.

The GS series units are fine for M-series Multis. That's what we install 90% of the time when we do Multis

"Gigantic system"----what in the world.... This is a 3-zone system, how small do you think they make them? The 24,000 BTU has the same turndown ratio.

NOTE- your total line set is fine, no added Ref needed. (it's in 'meters', but it is about 98 feet before we need to add more juice). It's indicated on the label, on the outdoor unit.

NOTE- your Issue 1, your unit may need a "firmware" update. This is a known issue
https://www.greenbuildingadvisor.com/question/mitsubishi-hyper-heat-minisplit-noise-defrost-cycle-issues

We had one doing the same thing, and the Mitsu tech had to come and update the firmware (took 10 minutes or so)

Buzzing- that's weird! That's all I can say without being there and taking things apart to find the culprit.