Replies

1. 
   4khz | Jul 24, 2023 07:14pm | #1

   I'm not able to add a lot of insight to this (though eager to learn), but on the neep.org website this unit is listed as min. 22,500 @ 47 F. All MXZ-SM36NAMHZ ducted, ductless, and mixed ducts were the same @ 22,500. I checked out similar sized multi's from other manufacturers and found some with much lower min. @ 47 F. FYI - the links you provided wouldn't open, for me anyway

1. 
   gbryan | Jul 24, 2023 07:30pm | #2

   Thanks, good idea to check the neep site!

   Re links: it’s annoying that the full link isn’t clickable, but you can copy and paste the text of the link, and that will work. I’ve also attached PDFs of the content from those links as a backup option.

2. 
   Mitsuman | Jul 24, 2023 08:32pm | #3

   I came across this old thread and thought I would help you understand this issue. Excellent question and yes there are some seemingly contradictory specifications but in actuality they’re not at all. The short answer is 22,500 is the correct minimum capacity. 12,000 is minimum connectable capacity. Just because the system can MAKE a minimum of 22,500 doesn’t mean it can’t DELIVER less than that. If you had access to the old submittals you would see these used to have a rated minimum of below 8,000 btu which implied a very good turn down. However, that number was misleading as it really was referencing the minimum DELIVERABLE btu…not the minimum actually being generated by the system. That begs the question…what happens to the BTU’s being created but not being delivered? Basically they are circulated throughout the piping system and are not used. This of course has a negative impact on efficiency. The branch box systems are excellent systems, but are not necessarily good solutions for net zero type builds…due to efficiency loss under low very low load conditions which are more frequent in an advanced envelope. This is actually the reason why the systems minimum output were later re-rated to show their true minimum output. Engineers were using them for net zero type builds based on minimum capacity of under 8kbtu and they were not meeting efficiency targets. They then investigated the situation and Mitsubishi made the ratings adjustments to be sure all had clear/accurate data. The branch box systems can certainly deliver those low amounts of heating and cooling and do an excellent job of maintaining comfort and dehumidifying, but they are not as efficient when the building loads are extremely low. Many manufacturers don’t post their true minimum output. Mitsubishi is extremely conservative in their ratings, if you notice another manufacturer has a similar system and it appears it has a much better turn down then it’s most likely they’re posting the minimum deliverable capacity not true minimum output. The minimum turn down capacity in a multi-zone system is limited due to the necessity for the compressor to move enough refrigerant and maintain enough velocity to ensure proper oil delivery back to the compressor. This ensures compressor longevity and is one of the many reasons Mitsubishi is renowned for their reliability. Branch box systems are excellent whole-house solutions for both retrofit and new construction, but they must be properly applied and should not be used in ultra load situations. Those are by far best handled by single zone 1:1 style systems that have excellent turn down ratios. Hope this helps.

1. 
   maine_tyler | Jul 24, 2023 09:40pm | #4

   really great info

2. 
   gbryan | Jul 25, 2023 12:31am | #5

   Fantastic detail! Thank you for sharing!

   It sounds like these units are good in terms of staying on the set point and dehumidifying but that the problem is energy efficiency in low-load situations. If using them under low load—say 6k BTUs load because of mild weather—they'd still have to produce the minimum of 22,500 BTUs in order to deliver only ~6k BTUs. That would mean a lot of wasted energy.

   So what I'm wondering now is how much energy do they use in order to produce that minimum capacity. If looking at the part-load capacity chart on the 1st attachment above, the minimum deliverable capacity at 47 degrees (70 degrees indoor) is ~12k BTUs at 572 watts. Does that mean it'll really use only 572 watts to produce the 22,500 BTUs but delivery only the ~12k BTUs?

1. 
   pnw_guy | Jul 25, 2023 10:35am | #6

   Actually, if you search this forum, you will find that dehumidification and maintaining precise setpoints are significant challenges for these multi-split style units, and that many owners have complaints about exactly those two things.

2. Expert Member
   Akos | Jul 25, 2023 11:34am | #7

   When in doubt check out the data book here:

   https://mylinkdrive.com/USA/M_Series/R410A_Systems-3/Outdoor_Equipment/R410A_Outdoor/MXZ_SM36NAMHZ_U1?product&categoryName=R410A_Outdoor

   Looks like the min is 12k and uses 570W which is not too bad.

   If you want decent efficiency, what you have to watch with these is a only a small zone calling for heat, that is when there is a lot of refrigerant bypass happening and terrible COP.

   Generally, if you stick to larger zones that are above the min output, you should be mostly OK.

   There are also SM specific indoor units that have a built in EEV that don't require a branch box. These might run better but refrigerant bypass could still be an issue at low load.