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Ducted Mitsubishi minsplits

nj_homeowner | Posted in General Questions on

* Updated *

We are selecting a cooling system for our house under construction in northern NJ.  It’s a colonial with about 2400sf across 3 finished floors above grade, plus a 900sf  finished basement.

Our plan is to install two ducted minisplit air handlers.  One in the attic will serve the second floor and attic, and one in the basement will do the first floor only.  Basement will have no AC for now.

I did Manual J load calculations, which show about 20k BTUH peak cooling load for the 2nd + 3rd floor system and about 15k BTUH for the 1st floor one.

Based on the feedback below, I have updated the plan!

Upstairs will get a Mitsubishi PEAD-A24AA7 ducted head unit, with a dedicated PUZ-A24NKA7 outdoor unit.

Downstairs will get a PEAD-A18 head unit, with a dedicated MUZ-FH18NA2 outdoor unit.

Giving each inside unit a dedicated outdoor unit is slightly more expensive for the equipment.  It will also take up more space.  But consensus seems to be that this will be much more efficient, and it does seem smart to have redundancy.

Any other thoughts on this system?

Thank you very much!

Brian

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Replies

  1. Expert Member
    Peter Engle | | #1

    Dana will probably weigh in here, as he knows everything about the various minisplit options. But I'll start: You are much better off with two separate minisplit systems that one multisplit. The multisplit turndown ratios are typically only 2.5:1, while some single-minisplit system have as much as a 10:1 turndown ratio. These systems don't operate at capacity most of the time, because the outdoor temperatures only approach design conditions 1% of the time. The rest of the time, they are operating at part load and minisplits with large turndowns are able to operate at very low capacities to match those low loads. This dramatically improves their efficiency and also improves indoor comfort. As a bonus, they also operate very quietly when they are at low load.

    1. nj_homeowner | | #3

      Thank you, Pete. I thought that might be the case. Giving each inside unit its own outside unit will cost $1800 more. It will also take up more space outside, and I imagine that two outdoor units running is louder than one. Do you have any idea what the overall seasonal operating cost efficiency advantage might be with dual outside units?

      1. Patrick OSullivan | | #10

        > It will also take up more space outside, and I imagine that two outdoor units running is louder than one.

        They are so quiet that I cannot imagine you actually noticing unless they are sitting on your outdoor patio while you are having a silent dinner.

        > Do you have any idea what the overall seasonal operating cost efficiency advantage might be with dual outside units?

        Also, don't discount the fact that having two outdoor units rather than one means you've removed a common point of failure. I'd rather be down 50% cooling/heating capacity than 100%.

      2. Austin G | | #16

        As far as your noise concerns, I have a 48k Mitsubishi condenser ON my patio, where we’ve often eaten lunch or dinner. It’s all but completely silent except in the winter... at which point we’re not out there eating anyway :)

  2. Expert Member
    Akos | | #2

    I would double check your man J. Unless you have silly sized west or south facing windows with no shading, I can't see a ~800sqft main floor having an 18k cooling load.

    With multi story houses with open stair cases lot of your cooling is done by the upstairs unit, you can get away with significantly undersizing the main floor unit.

    With a multi split, there is a bit more flexibility for sizing as there is very little chance that both units would run at max power at the same time. You can get away connecting more combined head capacity than the BTU rating of the outdoor unit. An 3C30NA2 with PEAD-A15 and PEAD-A24 head is probably better sized for you.

    With a bit of air sealing in the house and if you sharpen your Man J pencil a bit, you might be even be able to downsize to a 3C24NA2 with 9+18 or 12+18 heads.

    The dual outdoor units is always more efficient, not sure if it is worth it if you are using this for cooling only.

    P.S. You can get a surprising amount of ducting onto a SEZ-KD low static head if you are careful with layout. They are significantly cheaper.

    1. nj_homeowner | | #4

      Thank you. I just updated the calculations for some recent changes to our plan (including improved insulation and sealing), and indeed I am now getting only about 15k BTUH from the Manual J. The floor is about 1150 sf, with windows all around, including to the south and west.

      I would love to downsize any of the equipment for efficiency, and perhaps I can get away with a smaller multi-zone unit outside. But the PEAD-A24 is the smallest indoor unit with 0.6 iwc pressure. The next size down only has 0.2 iwc, which I am not sure is sufficient to distribute the air through the ducts.

      1. Expert Member
        Akos | | #7

        All PEAD will do 0.6", not sure which data sheet you are looking at.

        A one to one is about 2x as efficient as a multi at full power and about 1.5x at min modulation. Take a look at the cooling COP numbers bellow.

        https://ashp.neep.org/#!/product/31984
        vs
        https://ashp.neep.org/#!/product/29018

        EDIT:

        I think the NEEP data is messed up. The COP for the multi should be around 3. So the difference between a one to one and the multi split is large but not as bad as the NEEP data suggests.

        1. nj_homeowner | | #8

          Ha! You are setting me straight. The A15 looks like a much better fit then. I have read some of Dana's posts in which he recommends oversizing minisplit units... Do you think that does not apply here?

          1. Expert Member
            Akos | | #9

            There is some confusion here.

            You can oversize a one to one as you tend to get much higher efficinecy when it runs at partial load. You just have to be careful not to oversize too much as you can loose de-humidification performance at low load.

            You never oversize a multi split. They have much narrower modulation range, an over sized multi split will tend to cycle (I live with one and efficinecy/comfort sucks).

        2. nj_homeowner | | #12

          Thank you for the help. I updated my plan (and the original post above) to go with two units, an A24 with a PUZ for upstairs and an A18 with an MUZ for downstairs.

          I am actually very surprised that it would not be more efficient to run a multisplit setup for our house. It seems like running two units alone would reduce efficiency. With a single outdoor unit, the load is shared.

          But I guess they have not yet optimized for that.

          1. Expert Member
            Akos | | #15

            Not sure I follow your part numbers there, but what I would do, even in the milder NJ climate, is install a hyper heat unit for the main floor like:

            http://meus1.mylinkdrive.com/files/M_SUBMITTAL_PEAD-A15AA7_SUZ-KA15NAHZ-en.pdf

            This does close to 20kBTU in warmer weather but it won't derate much even during a polar vortex event.

            This combined with a non hyper heat ( http://meus1.mylinkdrive.com/files/M_PEAD-A24AA7_SUZ-KA24NA2_SUBMITTAL-en.pdf ) unit for the 2nd and 3rd floor can heat your entire place with electricity, if you ever choose to down the road, for minimal extra cost for the one hyper heat unit.

  3. Jeff Wasilko | | #5

    One thing to be aware of is the MXZ-8C48NA is not Energy Star rated. We didn't know that, and because of that we're ineligible for AECs in MA.

    The MXZ-5C42NA is Energy Star rated.

    1. nj_homeowner | | #6

      Good point! Thank you.

  4. Jay Thomas | | #11

    I am in North Jersey as well... make sure to size for heat as well. Since out 99% temp is 15f, you don't need the cold temperature units, and when it gets cold you will be happy with the comfort and convience.

    As others have said make sure to get two units instead of a multi split for efficiency. My overall energy bill is half what it used to be with gas heat in the shoulder months and about the same in the cold months.

    1. nj_homeowner | | #13

      Thanks! I am really surprised that 2x outdoor units is more efficient than one. Is your experience with improved efficiency only based on comparison with your previous non-minisplit system, or have you actually been able to compare dual minisplit with multisplit?

  5. Jay Thomas | | #14

    No I don't - real comparisons are exceedingly difficult. The issue is in NJ the climate is pretty moderate so much of the time, except the two months hot and two months cold, you will be in low gear on your mini split. Two units gives you twice the amount of gears (variable speed fractions) so you have a better chance of getting it right without a lot of short cycling. Plus you get redundancy as a bonus.

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