Yes, there are valves to restrict the flow of refrigerant into each zone, but they will still allow a small amount of refrigerant to flow to zones that aren't calling for heat. It does this to prevent liquid refrigerant migrating to unused units. If this is enough to cause overheating of a zone, it means that indoor unit is grossly oversized for the zone it's serving. Sadly, there have been many complaints like yours, because making a zone consist of one room only (unless the room is very large or has an unusually high load) is a common misapplication of this system. I would bet any money your bedrooms with 6K units actually have a peak load less than 1/2 or even 1/3 that. So when they turn down to minimum heating, they're still putting out way more heat than the room needs much of the year.

Since ripping it all out and installing a properly designed system probably isn't in the cards at this point, the band-aid fix is to use a remote mounted sensor on the offending units, and then program the fan in the unit to cycle completely OFF when no heat is needed. This will at least stop the fans blowing hot air into rooms that have met setpoint. Your installer should do this for every zone that is overheating, hopefully this will make the situation better.

Here is a bulletin from Mitsubishi that covers this problem in more detail: https://mylinkdrive.com/viewPdf?src...009\Application Note 1036 ME - Applying MXZ-C Multi-Zone Systems - 20190110.pdf

 

Alas one of the complaints I've always had about Mitsubishi multiport units that they use branch boxes. It's a much cheaper way to make a multiport unit and ironically Mitsubishi then charges A LOT more for the product. Most manufacturers, even the cheap ones, use a series of solenoids that stay closed when a particular zone is not calling for heat and that way only the units calling for heat will have refrigerant pumped to them. It saves energy and works much better. Sorry OP but there's no way to resolve this

 

Not all MXZ use branch boxes, only Hyper Heat 3 tons and up and standard units 4 tons and up use them. The overheating complaint is not unique to the branch box models nor Mitsubishi, it has been observed with poorly applied systems on all manufacturers, branch box or no branch box. There is nothing inherently wrong with branch boxes (other than maybe making installation more complicated), they are standard for high end commercial VRF installations and they have the same components that would be inside the unit on the non branch box system. I don't think any of these systems use solenoid (on/off) valves, to my knowledge they all use modulating electronic expansion valves, which as I said do have a "minimum position" to control the migration of liquid refrigerant.

 

Because they are heat pumps, sizing of different zones may vary drastically from heat to cool mode. So, I can’t really say it was not sized properly. However, this is just how the Mitsubishi multi splits work. The fix was also stated, use the MHK2 and simply turn off the continuous fan when that zone’s unit is not calling.

 

Mitsubishi's bulletin linked above is clear, if the unit is more than 50% above the peak load for either heating or cooling, they consider it oversized and do not recommend it for that application. As soon as I saw that he had units serving individual bedrooms I pretty much knew they were oversized. Yes heating and cooling loads vary but there are few bedrooms that need anywhere near 6K for either. For single splits you can get away with that kind of oversizing because the tiny inverter compressor can ramp down super low, multi splits not so much.

The overheating problem is the most glaringly obvious symptom of the misapplication, but even if this is fixed by shutting the fan off, this system sadly will still fail to perform to the efficiency or comfort level it should, because it will have to cycle individual zones with tiny loads on that massive compressor. Salesmen are guilty of giving the consumers unrealistic expectations with these systems, they claim they'll do "room by room zoning" whereas in reality, they're barely more capable of this than a ducted system is (unless you shell out for actual separate units per room).

 

Thank you all for sharing your opinions and experience. I will be talking to the installer this week, and I will ask about these units being oversized, and if they feel the behavior is expected. I will also discuss the possibility of adding the MHK2 remote sensors in the smallest zones, to possibly mitigate the issue in this rooms.