Mitsubishi Minisplit with 7 zones design question

RomanGK_26986764589

Hello everyone
We are in the process of getting quotes to install a 7 zone Mitsubishi mini split non hyper heat.
I have narrowed down to 2 contractors. One contractor recommends installing 1 "double decker" Mitsubishi condenser to feed all 7 zones and run the lines through the basement to the opposite end of the house. This set up would use branch boxes.
Another contractor suggests we use 3 condensers, two 20k btus and one 30k btu outdoor unit instead, and this way we won't have to run the lines through the basement and won't have to use branch boxes. He would install two condensers to feed one side of the house and one 20k BTU unit to feed the other side of the house. He states this set up would be more efficient and simple. He also mentioned if something happens to the outdoor unit I won't loose the cooling/heating in all the indoor zones.
Which method would you guys go with?
Thanks in advance for your comments.

GW

Either option is good, but if you ever have a leaking wall unit or leak in the line set, you are going to dread the labor cost to deal the big double Dekker. 

Also look at the specs, the minimum capacity is 30,000 BTUs for the big 8 zone non-hyper double Dekker. That’s not good.  The smaller units will have better minimum AC capacity numbers, much better for the dehumidification.

If your landscape allows, going with the three outdoor units suits me better.

Somehow my phone likes to put two “k”s in the word Dekker, I’m just gonna go with it

EdTheHeaterMan

I would also use 3 separate outdoor units. Even if it is slightly higher in cost. Easier to fix in the future, and with 7 heads, there will be at least one problem in the next 10 years. Also, think about surge suppression devices. Those circuit boards are just like computers. One little power surge and you will be without AC for a few days until the PC Board can be replaced.

RomanGK_26986764589

You guys helped me made up my mind. We'll go with the three separate units option. Thanks!

TAG

Do yourself a favor and make sure you get the tables from Mitsubishi --- look at the head units you are planning and the compressors. The tables will show you the outputs of the heads -- min and the max. You can also see what those heads do with different compressors.

You don't want to use the tables printed for the single setups .. the same head will not have the same outputs when linked to a multi compressor. The branch box setup work more like the single setups.

It's really important to get a proper load done ..... each room and overall house. You don't want to oversize the units. And you don't pick the compressor just on the heads .... IE 3 x12k heads =36k compressor. I have 3 x 12k heads on a 30k compressor.

Another thing to remember: The units are never off ... the units where all the lines go back to the compressor always have to have some refrigerant going to all the heads .... think about what head is going back to what compressor.

My new house is a large old stone building from 1870's -- it's a gut rehab with a new addition on the back. The stone building has a ducted HVAC system that takes care of most of it (except a loft 24' up) ... the new addition is a kitchen open on three sides w/ bathroom, mudroom, laundry under it (walk out) . It's in a very tight lot .. so I did not have room to build wider higher for ductwork. Mini-splits are the only good solution. 3 heads -- kitchen (15 or 18) / 9k Laundry/ 12k loft. All the contractors specified two units to my builder ...one for the laundry and a twin for the kitchen / loft. Being a kitchen -- I did want to err on the side of extra capacity if close. It's the only spot you don't want to get stuck w/o enough.

When I looked at the duel compressor tables for the 15k or 18k head for the kitchen ... I was concerned about the outputs and turndown in the kitchen head. The min and max was tight .... and not as wide. I also wondered how wise/ logical to link it to a loft head that was not going to be used all the time. So I have the kitchen unit running and I'm pumping refrigerant all the way up to a 12k loft head that I'm not using.

When we got further along on the project and it came time for the builder to decide on the contractor -- I raised my concerns. I got the deer in headlight look. Anyway -- to their credit the one contractor want back to Mitsubishi corporate ..came back with tables in hand. Mitsubishi agreed that the kitchen/loft was a bad fix/mix. For proper performance in the kitchen a single 15k was the unit. The loft and laundry were too hard to link .... So we went with three units. The other solution was the branch box .. but --hiding the duel fan unit required would have required another trip to the historical commission