Pros and Cons of Mitsubishi Multi-Zone Air-Source Heat Pump
Hi All- I’m part of a community program in Tompkins County NY advocating for both ground sourced and air-sourced heatpumps. I am still a little uncertain what to tell people when the participating installers recommend the multizone units. In particular, I am concerned about the MXZ series. They have good heating performance down to low temperatures, but, as pointed out in a previous blog here, they have the very high minimum heating capacity suggesting a lot of needed on/off cycling during the shoulder heating seasons. It seems like both the high minimum and the fact that it is likely the whole house on one compressor might make this activity more dominant. For instance, I myself am, getting a few single head units with better low minimums and also the option of not even turning them all on if weather is mild.
But I have no sense of how large this effect will really be for the MXZ. Does anyone have any more quantitative boundaries to put the energy efficiency loss in perspective? I could also imagine that the fact that the whole house is on just one compressor, so only one compressor is cycling on and off instead of several compressors for single head units might be a good thing. Does that make it less of an issue? Any help with this will help me advise other people participating in this program.
I’m also wondering if the larger size of the multizone units (quite a bit taller) could be a little bit of an advantage during high-snow conditions. Harder to bury it because they are much taller. I’m sure they still need to be kept clear at the base for the long run, but is there less liklihood of being completely buried during one storm and is that much of an advantage for immediate reliability and performance during storms?
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Jonathan,
I don't really have any data or direct experience with multi-zone minisplits manufactured by Mitsubishi. Perhaps another GBA reader with experience will chime in here.
I wouldn't put much faith in the fact that the outdoor units are taller than those of a single-zone minisplit system if you are hoping that fact will keep you out of trouble in a snowstorm. When ice accumulates on the drain pans of these units, you can have problems, regardless of how tall the units are.
The short version is that single-zone ductless minsiplits have higher efficiency, and a greater modulating range, than multi-zone units. That doesn't mean, however, that you should never specify a multi-zone system. In many cases, such a system is the best solution -- and may still save money compared to a heating system that burns propane or fuel oil.
The MXZ-C outdoor units can be used with both ductless and ducted units. The low end heating capacity at 47F outdoor and 70F indoor is (interestingly) the same - 7,200Btu/hr - for all of the sizes: 24, 30, 36, 42, 48, except the 20, which is very slightly higher at 7,400Btu/hr.
Whether or not the 7,200Btu/hr minimum is going to be a "big" efficiency problem during milder weather depends on the heating loads of the houses and what the applications are...supplementing existing HVAC systems will typically yield lower real world efficiency compared to a direct replacements.
In almost all cases, multiple, right-sized single-split systems will give you better efficiency than a mutli-split...both in milder conditions and closer to design temperature. Unfortunately, no one offers a "hyperheat" ducted model in a small capacity (less than 20kBtu/hr nominal)
In any snowy winter climate, the outdoor units should be placed on wall mount brackets or on a well-drained platform...your call on how high to elevate.
Thanks for the input so far! To add information, we are trying to help people completely replace fossil-fuel heating systems with heat pump technologies. these are not new homes built with all the excellent knowledge available on this site, but are mostly older retrofits. Even after efforts at improving insulation and sealing air drafts, the heating load may be 4 tons or more in many cases. For some the choice of the multizone units will be clear from the desire to interface with some kind of ducted system. However, in other cases it is simply a question of the pros and cons of perhaps 4 single head units or one larger multizone unit of ASHP. (Some will choose ground sourced but that is not relevant here). People might choose the multizone with multiple ductless heads because it is cheaper to install or because they prefer only one compressor outside their house or ......likely other reasons too. The multizone units will be less energy efficient, but by how large a penalty? How big a deal is the cycling on and off during shoulder seasons for these units? compared to an equivalent allocation of multiple single head units?
I'm curious what your experience has been on this over the past 5 years.