Hello Hive Mind, I'm installing a Mitsu MXZ-3C30NAHZ2(Hyper Heat) outdoor unit, with two heads--MSZ-FH15NA, and MSZ-FH18NA. Both indoor units arrived with 1/4" and 3/8" copper tubing factory installed. HOWEVER, the 3/8" lines have 1/2" connectors factory installed!!! Is this typical? What size Line Set do I use? Do I match the 1/2" connector, or do I match the 3/8" line? Where will I install the inevitable Line adapter--at the indoor unit, or at the outdoor unit?

This sounds like it's your first multi-split installation, possibly your first ductless(?). Do you have any experience &/or the equipment for installing and commissioning split-AC or heat pumps?

Installing a multi-zone ductless isn't a DIY project few newbies. While reasonably competent people can successfully mount the equipment, run the power etc. it's better to leave anything involving the refrigerant line connections, purging, & testing to a qualified pro. Most people don't have the necessary equipment and experience to get it right, and even if the thing seems to work initially, even minor screw ups can dramatically reduce the service life and/or efficiency of the system, and DIY installations generally are not covered by warranty.

If you ARE a qualified refrigeration tech the correct place to resolve the questions is Mitsubishi's technical support line (and they will talk to you.)

Thank you Dana. I am a highly competent contractor. I am not a licensed refrigeration tech. I am installing the mechanical and electrical aspects of the system and will have a licensed tech complete the testing and start up. I thought I might find some experienced voices here.

There are very few refrigeration techs posting these pages, and fewer still versed in what it takes to properly commission a multi-split.

BTW: The FH18 and FH15 are pretty hefty heads with a very substantial amount of capacity when married to a cold climate type compressor (like an --NAHZ). The FH18 is good for about 20,000 BTU/hr @ +5F, the FH15 about 18,000 BTU/hr. (That's about as much heat than my sub-code 2400' house + 1600' of insulated basement house needs at +5F.) Are you sure you have enough LOAD to run them efficiently?

The max output of the -3C30NAHZ2 @ +5F is ~28-29K, still a significant amount of heat, but the minimum modulated output of the -3C30NAHZ2 @ +47F is about 11,600 BTU/hr. That means any time the combined load of the 2 zones is less than 11-12K it will start cycling the compressor, which takes a toll on as-used efficiency if you don't have enough load. It tests at HSPF 11 with a pair of 3/4 ton heads plus a 1 ton, but with a 1.25 ton + 1.5 ton setup it'll probably be somewhere in the high-9s, even if there is sufficient load to not short-cycle the compressor (and less if the loads are too low.)


http://meus1.mylinkdrive.com/files/MXZ-3C30NAHZ2_ProductDataSheet.pdf

http://meus1.mylinkdrive.com/files/MSZ-FH18NA_For_MXZ_MULTI-ZONE_SYSTEMS_Submittal.pdf

http://meus1.mylinkdrive.com/files/MSZ-FH15NA_For_MXZ_MULTI-ZONE_SYSTEMS_ProductDataSheet.pdf


If separate compressors were used each would have more modulation range at the low end. Both the FH15NA (and -NAH) and FH18NA compressors can throttle back to about 5000 BTU/hr, and while they will still start cycling when the loads are low, it's spinning up a compressor only about half the size of a 3C30NAHZ, with less wasted energy. There is a strong tendency for people to oversize both single-zone mini-splits and multi-splits, but the turn down ratios of multi-splits are usually quite a bit lower, taking a bigger efficiency hit when cycling. Even when right-sized HSPF & SEER effiiciencies of single zone mini-spilts are usually ~10% higher than that of multi-splits using the same heads:

http://meus1.mylinkdrive.com/files/MSZ-FH18NA_MUZ-FH18NA_Submittal.pdf (
http://meus1.mylinkdrive.com/files/MSZ-FH15NA_MUZ-FH15NA_Submittal.pdf (HSPF 12, SEER 22 )

Was a zone by zone Manual-J load calculation performed by a competent third party used to spec the sizes?