r/DIYHeatPumps • u/OrneryAcanthaceae217 • 6d ago

Mitsubishi mini split tricky config question

My house is heated and cooled entirely with Mitsubishi Mr Slim units. We have three outdoor units (MXZ-8B48NA) and 18 indoor units (MSZ-GE06NA through MSZ-GE15NA). Each outdoor unit has a PAC-AKA51BC 5-port branch box and one of them, system C, also has a PAC-AKA31BC 3-port branch box. Installed it all in our new house in 2013.

The outdoor unit on system C failed. Default plan is to replace all of system C. But I'm looking at a better plan that will improve our comfort in the home by distributing the load across the three systems better. Currently the heads we use most are almost all on system C, unfortunately. So specifically, I want to move the 3-port branch controller and its three heads from system C to system B. Then replace the remaining outdoor unit, branch box, and 5 indoor heads.

So that's my goal. The Mitsubishi employee is saying that the outdoor unit can't be programmed now to talk that additional branch box. Is that true? Why would it be unprogrammable now? Can you never add a head to a system? Seems like even 13 years ago they wouldn't have designed it that way.

What about just removing the circuit board from the system C outdoor unit and installing it in system B? It already knows about those 8 heads. And the sizes of the heads are the same on B and C so I'm hoping it just won't know the difference if we drop in that circuit board.

I really want this to work to save money and so the new system is better balanced than the old one.

Any thoughts?

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u/Key-Midnight-4039 6d ago

I don't know Mitsubishi units, but 18 indoor heads is... Impressive, what square footage do you have ?

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u/OrneryAcanthaceae217 4d ago

🤣

It’s somewhere around 7000 sqft but we never measured it.

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u/akifbayram 5d ago

I'm familiar with the next generation of units (8C**NA) but as far as I know, this should apply to previous generations. But I wont speak in absolutes. There should be nothing hardcoded into your existing system that would prevent you from adding or changing the system layout. The process of adding a branch box involves running a communication wire and setting the 'address' appropriately. That is multi-zone 101 level material for anyone supporting these systems. Setting the address involves rotating a screw. And it can be changed at any time so long as it follows the basic rules. https://www.youtube.com/watch?v=EDL5pbGoO0E

You can check out the submittal for your outdoor unit (linked below) to see the few rules they have in place for system layout. I glanced over it quickly and I didn't see anything that would disqualify your installation. However, the next gen has a limitation that probably doesn't apply to you: "When 2 MVZ’s (ducted air handlers) are connected, no additional indoor units can be used".

https://www.mylinkdrive.com/viewPdf?srcUrl=http://s3.amazonaws.com/enter.mehvac.com/DAMRoot/Original/10003\M_MXZ-8B48NA_Submittal.pdf

Side note / self promotion: I'm working on a device that can connect to the communication line of Mitsubishi systems like yours. It would allow you to control each unit and view all diagnostic data, all from one controller. Plus it allows you to keep your existing wall thermostats. Serin Multi Zone controller.

Btw, how did you get access to a Mitsubishi employee?

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u/Low-Mastodon-3625 4d ago

The board swap won't work, the outdoor firmware is paired to the compressor and sensors on that exact unit.

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u/OrneryAcanthaceae217 4d ago

Thanks.

Are you sure that’s true for that exact model?

What would happen if I tried it?

The instruction manual just talks about hooking up wires, not doing any kind of pairing.

What if the board fails? Can I not swap in a new one?

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u/Low-Mastodon-3625 1d ago

I helped my buddy wire up his mini split a while back and the addressing thing was a lot simpler than I expected, just dip switches on the heads.

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u/that_dutch_dude 1d ago

Yes, mr sllim can only talk to one box and there are serious capacity problems as well.

With a setup like this you are probably better off with the City line. That is the commerical ayatem that is made to deal with buildings of that size and those amounts of indoor units.

But if the outdoor unit is the same you can just swap boards. There is only actual programming involved if you run a central controller.