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Community and Q&A

Do ductless mini splits make sense for a large house?

Cherylann Schieber | Posted in Green Products and Materials on

Hi Guys.

We’re in the middle of designing a new house to be built at the Jersey Shore and would like to keep our carbon footprint as small as we can for a rather large house (about 3600 sq ft). We’re interested in ductless mini splits so we don’t have ductwork in the attic and crawlspace and for their superior efficiency. Can this system work with so many rooms? The first floor living space will be a fairly open floor plan, but the second/third floors will have 6 bedrooms/6 bathrooms, so lots of closed doors.

Also, being at the beach, we really need to be able to control humidity. Would a ductless system with the right house ventilation be up to the task?

I’ve read many of the articles here and on Gardenweb but can’t find any that address using these systems in large, many roomed homes. Thanks in advance for your help!

Cherylann

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Replies

  1. User avatar GBA Editor
    Martin Holladay | | #1

    Cherylann,
    One or more ducted minisplits make more sense than ductless minsiplits for the type of house you are describing.

    Here is a link to an article that describes several houses measuring more than 3,000 square feet each that are heated and cooled with ducted minisplits: These Superinsulated Homes Were Delivered By Truck.

  2. User avatar
    Dana Dorsett | | #2

    Last fall I got involved in specifying a ductless heating & cooling solution for a ~3200' house on Martha's Vineyard MA. It ended up being $10K cheaper to install than the next cheapest proposal, (a ducted system with a propane fired gas furnace with a 3-ton cooling coil in the air handler ) and cheaper to run too, despite being not fully optimal. It's more than just a bit oversized for the loads, but not super-egregiously oversized. If given more control over the process it could have been better, but the homeowner was running out of time. It will heat and cool the place just fine, just not as efficiently as if it had been right-sized. (This is a consequence of waiting until the very END of the project to specify the HVAC.)

    Until your house design is dialed in sufficiently to run a competent room-by-room Manual-J load calculation there's no way to know what's optimal or acceptable in YOUR house, but unless you're building way below code-min there are ductless solutions that won't break the bank.

    The high latent cooling loads are handles somewhat better by mini-splits than by the typical 1 & 2 stage ducted systems. If you want complete control over that you can install at least one Daikin Quaternity, which has the ability to dehumidify to a relative humidity setpoint without sensible cooling for days when it's just warmish sticky sea-fog and not much of sensible cooling load.

  3. Edward Cambridge | | #3

    Dana,
    I'm looking at an equally complex problem for the Cambridge (MA) house we're working on. This is a VERY large house, ~6200sqft above grade. I have the same question with regard to ducted mini-splits.
    The challenge with the older homes is the multitude of rooms and wide varations in fenestration, quality of windows and exposure,

    I LOVE the idea of a mini-split only house but wondering if it's pushing the boat out too far for a restoration this size. The good news is that the project may become a DER (full 4" on exterior) which would certainly lessen the load and reduce the swings.

    Your counsel here is much appreciated!
    Thoughts?

  4. User avatar
    Dana Dorsett | | #4

    With large houses in historical districts the size & locations of the outdoor units can be problematic, but with a DER the loads can be pretty reasonable even in big houses. (The simpler the shape, the better.) When you have the load numbers it will be easier to figure out, but it's probably in the 4 ton range, which CAN be done with a single compressor, but it'll stand up pretty tall and in Cambridge you'd want at least 2' between the bottom & grade (3' is better) to deal with snow accumulation during nor'easters.

    To get a perspective on how big they are check this video (though this one is hooked up to an air handler, not Medusa.)

    https://www.youtube.com/watch?v=PsQ0Zq-uqrY

    You can hang up to 8 heads on a 4 ton Mitsubishi and it can crank out over 50K BTU/hr @ +5F, (and still 42K @ -5F)., Cambridge's outside design temp is something like +10F. You may run into refrigerant line length issues with the installation in a house that large, and need to break it up. (The house in Martha's Vineyard ended up with a pair of 2 ton 3 zone Fujitsus.)

    http://www.ecomfort.com/manuals/MXZ-8C48NAHZ_Submittal.pdf

  5. Cherylann Schieber | | #5

    Thank you Martin and Dana. I read the article and watched the video and also found this link to pictures of an installed ducted Mitsubishi system.

    http://homeenergypros.lbl.gov/profiles/blogs/what-do-ducted-mini-splits-look-like

    Is a ducted mini split system still more efficient than a traditional one? I'll still have to find a way to keep duct work out of the crawlspace and attic. Also, because this is a vacation home, cooling and dehumidifying are more important than heating (though we do use it periodically in the winter). We only occasionally fill the house; most of the time it's just the two of us, so maybe we could do a ductless system that cools/heats the first floor and 2nd floor master bedroom/bath and a ducted system for the rest of the bedrooms.

    My goal is to understand these systems a little better before I talk HVAC with the builder. I feel like it's the most important part of the build and I want to get it right.

  6. Steve Knapp CZ 3A Georgia | | #6

    Cherylann. If you build a really tight and well insulated house, you may be surprised by how easy it is to heat and cool. My house is about 3200 conditioned squared feet, and I am able to meet my hvac needs with a 2 ton air source heat pump. I was going to install split minis, but my floor plan was not open enough for optimal air distribution. I also looked at ducted split minis, but the systems were much less efficient than the inducted or convential heat pump alternatives. (Or at least that is how I am remembering those particular details)

  7. Cherylann Schieber | | #7

    Thanks, Steve. That's really helpful information. There's so much discussion about super insulated homes and heat but not much about cooling and dehumidification. I've had two bad experiences with duct work in the crawl space here at the beach. Flood vents are required so conditioning the crawlspace isn't really possible. However, my architect is all about unvented crawls and seems to know how to handle it. Even so, I'll never put duct work in the crawl again. When I have the design I'll be back for more advice.

    Thanks again to everyone who contributes here. You're a truly wonderful resource!

  8. D Dorsett | | #8

    Ducted mini-splits usually test at a somewhat lower HSPF than wall-blob types, but that's only part of the story. Sizing a wall-coil type properly so that it modulates most of the season is difficult or impossible for individual room loads, and if it's not modulating it's not hitting it's HSPF numbers. It's often more efficient to right-size a ducted unit that distributes it's output to cover the loads of few room.

    The modulation range of ducted minisplits are generally MUCH higher than typical 1-2 stage ducted heat pumps or even modulated bigger-deal heat pumps, which gives it an edge even when it's bench tested HSPF can be lower.

    eg: The Carrier GreenSpeed can be in the HSPF 13 range depending on which air handler/compressor combination is being used, but it's turn down ratio is only 2.5:1, that is, it's minimum speed output only drops to 40% of it's maximum. A pretty-good 1.5 ton ducted mini-split like the Fujitsu -18RLFCD can drop to 3100BTU/hr out @ +47F, with a maximum output of 25,600 BTU/hr @ 47F, a turn down ratio of over 8:1, but tests at "only" ~HSPF 11.

    http://www.fujitsugeneral.com/PDF_06/Submittals/18RLFCD%20Submittal.pdf

    If your heat load is say, 20,000 BTU/hr @ +15F (a typical coastal NJ 99% design temp) either would be able to heat the place, but the Greenspeed would be cycling on/off at lower efficiency rather than modulating whenever it's over 35F, but the 18RLFCD would be modulating even into the 50sF, a function of both being better matched to the load and a MUCH bigger turn down ratio.

    A 2 ton Greenspeed delivers about 24,000 BTU/hr @ +15F at max speed ~9500BTU/hr at minimum, but can't drop below 10,000 BTU/hr when it it's in the mid-30s F outside. A house with a balance point of 60F and load of 20,000 BTU/hr @15F has a heat load of 10,000 BTU/hr when it's about 37F outside (which is roughly the mid-winter AVERAGE temp), and is still over 4000 BTU/hr @ +50F, still above the minimum modulation of the 18RLFCD.

    Bottom line: Sizing correctly is a critical factor for getting the efficiency out of modulating equipment, but big turn down ratios (=low minimum speed output) helps. Most multi-split Medusa solutions can't modulate below ~6000-7000 BTU/hr in heating mode, and if you have a pair of them (as the folks in Martha's Vineyard do) the minimum is 2x that. In those situations a mini-ducted solution with a lower minimum modulation will likely trump the bench tested HSPF on as-used efficiency.

    In new construction you don't need to install ducts in crawlspaces. Duct chases and soffits can be designed in on day-1 if you know that's where you're going with it.

  9. Cherylann Schieber | | #9

    Thanks for the lengthy, informative response, Dana. Am I right to assume that the higher efficiency of the ducted Fujitsu system will carry over to AC as well? Since these systems prefer short duct runs, do you think it would be ok to use a multiple head ductless system for the first floor and the ducted on the second and third? Fortunately, the builder uses an HVAC company that is very experienced with this technology. He told me they do more and more spit systems and that he primarily uses Mitsubishi and Samsung.

  10. User avatar
    Dana Dorsett | | #10

    Getting to the "right" solution starts with room-by-room Manual-J load calculation using aggressive rather than conservative assumptions on air tightness. I other words, for purposes of the load calculation pretend it's PassiveHouse-tight, and it must be very specific about insulation values and window U-factors & SGHC, not some code-min pro-forma boilerplate load calculation.

    For a three story house you'll probably need to set it up with a minimum of three zones (one per floor). Whether that's done with one compressor per floor or a multi-split depends a lot on the room by room load numbers and the floor plans.

    For most mini-duct cassettes the duct runs are of-necessity pretty stubby, but the Fujitsu RLFCD series can handle quite a bit more. That series can also be mounted vertically not just horizontally which can be a very useful option. The not-so-mini Mitsubishi air handlers have a lot more push than the SUZ/SEZ mini-ducted types. I'm less familiar with the Samsung line up- they're pretty rare in my neck of the woods (though the Korean vendor LG has a noticeable presence here.) Daikin has some decent mini-duct cassettes too.

    A Mitsubishi MXZ series multi-split using SEZ-KD series mini-duct cassettes to handle clusters of nearby small load rooms and standard wall coil heads or floor mounts for the more open zones can work just fine. Mounted in a soffit below a closet ceiling and serving 2-4 nearby rooms may require some adjustments in floor plan & door locations, but that can often work even in retrofits. An FH06 head per room is usually ridiculously oversized for the individual room loads, even when considering the fairly low minimum modulation range of FH06 heads. When married to a SUZ-KA09 compressor the smallest of the line SEZ-KD09 cassette has a minimum modulation of 4800 BTU/hr in heating mode, with a max of 14.1K, roughly a 3:1 turn down ratio, but the SUZ-KA15 has the same 4800 minimum, with a 21.1K max, which is a 4.4:1 turn-down. It's better to have just one serving the whole floor if you can. It sounds like you're going to be running ~1200' per floor, so it's not impossible.

    It's likely that with a code-min 3600' three story you'll be in the range of a 3 ton ton MXZ. It can handle up to 4 zones. With a better than code higher-R house that has been modeled and tweaked a bit it could be within range of a 2.5 tonner, which is good for 3 zones. The 2.5 ton MXZ-3C30 can deliver 28,600 BTU/hr @ +5F if it has the right heads/cassettes (~27K if all mini-ducted.) The 3-ton version is good for 45,000 BTU/hr @ 5K, ducted or not. A code min house that size that's had any thermal design adjustments/enhancements at all can come in under 35,000 BTU/hr, and as long as you don't have a huge amount of west facing windows the cooling loads should be no more than 3 tons. But a WAG like this doesn't trump a Manual-J.

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