Mitsubishi Mini Split Condenser Sizing

Discussion in 'HVAC Heating & Cooling' started by Rossn, Sep 17, 2020.

  1. Rossn

    Rossn Member

    Dec 12, 2017
    Denver, CO
    As part of my remodel, I am installing a Mitsubishi 5 head mini-split system. I was going to rough it in, but ultimately, it only saved me a few thousand over a full install, so I am going that route.

    The contractor who I would like to use is sizing the system with a 5Ton condenser, and per the Diamond System Builder, I can actually be fine with the 4-ton. He is not charging me more for the upgrade to 5T.

    My head units come up to 60K BTUs, and 62,400 are allowed. Only 3 of the heads will run regularly, and they total 42k BTUs. The other 2 heads I would call very light duty cycle (4' below grade area).

    So, the question (for someone who has specific knowledge on these, please) is if I will be less efficient, short cycle, or have shorter equipment life with the 5T unit, versus the 4T? Or... does the modulating nature of these units make this relationship OK.

    From what I've read, oversizing with a mini-split condenser is not the best idea, and that in-fact, they run a bit better when they are operating above that 100% mark, due to the fact the head units modulate.

  2. breplum

    breplum Member

    Oct 2, 2004
    Plumbing and heating contractor
    San Francisco Bay Area
    What you need to start with is "Load Calcs".
    Title 24 actually requires room and system load sizing.
    The HVAC/energy industry have software that makes it easy.
    It is irresponsible and illegal to not properly size a home or office before installing equipment.
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  4. Rossn

    Rossn Member

    Dec 12, 2017
    Denver, CO
    I don't think that the legality applies to remodel, but regardless, I designed the radiant heating system using Uponor's ADS software, which also provided cooling BTUs, so that has been done to a T. The sizing for the head units is appropriate and takes into account elevation deration, so it is less about sizing the needs of the home and more about if a 60k unit is oversized, given the above BTU usage... or if it is similarly efficient, based on 42K regular cooling usage (modulated, of course), and 15-18K of occasional usage. If so, I'd certainly take the larger unit with more head room for potential future heads. If it will be too underutilized and lead to shorter life or inefficiencies, then I don't want to do it. I just don't know which way it falls.
  5. Dana

    Dana In the trades

    Jan 14, 2009
    Typical cooling design loads are about a ton per 1000-1500' for single family homes. Even when derated for altitude I suspect even the 4 tonner is overkill, maybe even a 2.5 tonner might do just fine, though that would limit you to 3 zones. Five tons is usually extreme overkill for most normal sized houses. This graphic of square feet per ton vs. total house size was from Manual-J loads calculated for homes mostly in the Gulf Coast states, which have far more latent load than you do in Colorado:


    Even the worst 3000' house in that sample set would have a design load (latent included) of less than 3.5 tons, and a sensible-only (more appropriate for CO) of less than 3 tons. Three tons derated for 6000' of altitude is still only calls for 3/0.95= 3.16 tons of compressor, and 3/0.8= 3.75 tons of head/cassette. I suspect your actual design loads are lower than that.

    IIRC you have a full conditioned basement, which would be a good way to do with mini-duct cassettes serving so 2-4 rooms each. Many mini-split installers shy away from any ducted solutions, and prefer the "ductless head in every room" approach, since that makes the ski-condo payments a bit easier to manage, and they don't have to spend time calculating loads or Manual-D duct designs. :)

    Most multi-zone unit compressors have a minimum modulated output of roughly 100% the output of a half-ton ductless head. If all heads are oversized by the same amounts the zones with half-ton heads will almost never modulate, but room to room comfort will be reasonable. But if all other heads are reasonably sized for their loads, whenever any head is calling for coolant there may be enough bypass flow in a half-ton head (usually way oversized for a single bedroom) to overcool the room even when the blower isn't running and the head is nominally "off".

    A room by room, zone by zone CoolCalc or LoadCalc cooling load estimate would at least give you some handle on the relative oversizing of the heads & whole system, and whether consolidating rooms into a zone served by a mini-duct cassette would be both more efficient and comfortable.
    breplum likes this.
  6. Rossn

    Rossn Member

    Dec 12, 2017
    Denver, CO
    Thanks, Dana.

    By the time this came through I had already settled on the 4T unit, with 82% efficiency is more like 3.25T. I could have scrapped one head downstairs for an 'occasional use' area and gotten away with the 3.5 for about 10% efficiency gain (mainly due lower minimum modulation BTUs), though the summers are getting hotter and longer here. It used to be we stopped using AC by the end of the first week of September, and this year we ran up until the start of October.

    Relying on the ADS calculated BTU needs, I'm right sized upstairs, and oversized slightly for one room downstairs. The other room, I've 'oversized', but practically speaking with a lunos pair of ERVs pushing into another room, it is probably only slightly large.
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