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Will a minisplit with an air handler be adequate for Zone 7A?

Terrance Kaase | Posted in Energy Efficiency and Durability on

I am building new construction with SIP walls in NW Wisconsin zone 7A. House will be 2000 sq.ft. single level with an insulated basement. I am interested in heating with a Mitsubishi minisplit using an air handler to distribute air throughout the house in a ducted ductless mode as seen on Ask This Old House feb19, 2015. The handler has backup elec. resistance heat for a heat boost when needed. Three basic questions.1 Will this be adequate to heat this house? 2. Can the air handler be mounted in the basement to distribute air and can we avoid having the units typically mounted on walls 3. Can this unit also heat and cool the basement? Any thoughts on this matter?

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Replies

  1. GBA Editor
    Martin Holladay | | #1

    Terrance,
    I haven't watched the TV show that you are talking about, and I don't know what you mean by a "ducted ductless" system.

    I found a link to the show, but I don't have time to watch it. I notice, however, that the online blurb says that in the episode, "Richard helps a homeowner install a whole-house ductless heating and cooling system." That sounds ductless to me -- not "ducted ductless."

    Here's what I know: you can design a heating system using ducted minisplits, ductless minisplits, or a combination of both types of minisplits. In all cases, the first step is to perform a heating load and cooling load calculation. As long as the equipment is properly sized to handle the heating and cooling load, and as long as you have chosen equipment that will operate at your expected minimum outdoor temperatures, and as long as the system is installed well, heating and cooling a house with minisplits will work.

  2. GBA Editor
    Martin Holladay | | #2

    Terrance,
    I just watched some of the TV show. It looks like the system discussed in the show has a ductless minisplit unit in the basement, and a ducted minisplit upstairs.

  3. Terrance Kaase | | #3

    To Martin
    What I would like to know , then, is if this system can be totally ducted? Do I need an air handler to do this and should it have the backup resistance heaters. Also will this be adequate for the basement? My house will not be passive quality but will be so-called good quality insulated and sealed. Also I will use a good quality large soapstone wood burner in the center of the living area.

  4. Dirk Denzin | | #4

    I saw this episode also, left me with a bunch of questions.
    to your questions,
    1) to know if this will be able to heat your house. You need to have a analysis done to see how much heat your house needs to maintain temp. They mentioned this on the show.
    2)&3) I can't answer this. I'd like to know this also. I noted that they mounted the air handler in the attic. I would think a basement mount would be better installation. I don't think you'll need much heat and cooling for a basement.

    I'm in about the same place as you. I'm designing a home for southeastern WI (zone 6) and am looking to do a ducted mini-split. I like the idea of doing away with a "traditional" forced air HVAC. Looking forward to some answers from someone with more knowledge.

  5. GBA Editor
    Martin Holladay | | #5

    Terrance,
    What you are calling an air handler is actually part of a Mitsubishi ducted minisplit unit. You can design a system with as many outdoor units and indoor units (ducted or ductless) as you want, as long as your budget can handle the cost.

    So the answer is: yes, you can do it. If you are designing the system, start with a heating and cooling load calculation. If you aren't designing the system, you need to talk to a competent mechanical engineer, energy consultant, or educated HVAC contractor.

  6. Ven Sonata | | #6

    Terrance, you can feel at ease because of that "large soapstone woodstove". It is especially efficient when temperatures push your heat pump COP down to 1 for a few weeks perhaps in January. That back up is great too if the power goes out...you are laughing then. For heat distribution leave the bedroom doors open at least during the day. The heat pump is nice in the shoulder seasons so you don't need much firewood or many fires through the year.

  7. Terrance Kaase | | #7

    To Dirk and Ven from Terrance. I also am trying to get away from traditional HVAC. I want to avoid using propane. The soapstone will actually be my primary heat source with another system as backup. We are not real familiar with minisplits in this area, thus my confusion, but I really want to go totally electric. I cannot afford photovolt. right now and just am trying to keep costs at a minimum with a good reliable system for heat and cooling.
    Another question to throw out is insulation of basement slab. If I don't put foam under the slab, but rather put it on top of the finished slab and then cover with plywood. Would I have a warmer floor using this technique? I read this technique in Finehomebuilding.

  8. Jerry Liebler | | #8

    Terrance,
    I faced a similar problem and rejected conventional air handlers as too expensive and consuming too much electrical power. I've posted my solution to ducting the output of a single minisplit here in post #8
    https://www.greenbuildingadvisor.com/community/forum/energy-efficiency-and-durability/33752/ductless-downside
    Basically I mount the minisplit in a closet along with 5 very energy efficient "smart fans" below the minisplit. The 5 fans each have their own duct to a distant part of the house and their combined flow rate is slightly greater than the minisplit's flow rate and the fans run whenever the minisplit is heating or cooling. Return air enters the closet above the minisplit either through grills or ducts in the ceiling.
    I too have a masonry heater and I've arranged my return ducts so the same fans can circulate air heated by (drawn from above) the masonry heater.
    When you choose your minisplit be sure to get one that works down to -13f or below. both Fujjitsu and Mitsubshi offer the low temperature ratings.

  9. GBA Editor
    Martin Holladay | | #9

    Terrance,
    Q. "If I don't put foam under the slab, but rather put it on top of the finished slab and then cover with plywood, would I have a warmer floor using this technique?"

    A. No. The best location for this rigid foam is under the slab.

  10. Nick DeFabrizio | | #10

    Jerry, I followed your original post and found it interesting. Did you actually implement your experiment and if so how does it work. If you have not implemented it I note that Mitsubishi now has a multi split h2 system that is rated down to -13F and compatible with ducted systems (see MXZ "C" line). These do pay some price in efficiency for ducted but less so in heating ( according to AHRI, the MXZ 4C36NAHZ has a HSPF of 10.1/ compare that to the Fujitsu 12RLS2H at 9.3). I understand Fujitsu will be coming out with a more efficient low temp multi 24 k system later this year also.

  11. D Dorsett | | #11

    Good heating solutions begin with a room-by-room heat load analysis of the heat loads at the 99th percentile outdoor temperature bin. With mini-splits in zone 7A you also have to look a the minimum operating temperature of the unit relative to your 99% temp, and it's output at that temp relative to your calculated heat loads. Without this fundamental information it's sort of like asking, "Will a car go fast enough for me to make it to work on time?" without knowing the distance or the condition or the road.

    No Miitsubishi unit has a rated output below -25C/-13F, though they still have a COP of about 1.8 at that temp. Most of them also automatically turn off at about -18F to -22F, to self-protect. There are locations in zone 7A with outside design temps of -13F or lower, and it's common to hit the -20s during cold snaps. .(Ashland WI's 99% bin is -16F, Eau Claire's is -13F.http://www.energystar.gov/ia/partners/bldrs_lenders_raters/downloads/Outdoor_Design_Conditions_508.pdf ) The COP never actually goes as low as 1, but it may hit 1.5 at the shut-off temp.The mini-duct cassettes often don't have a specified output below -20F/-4F, and the output falls off more rapidly with temp (in general) than the wall coils.

    With a wood stove to carry the load during the cold snaps below temps at which the heat pump will operate that may still be OK.

    But there's no way to make reasonable recommendations without knowing both the heat load and the outside design temp.

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