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Mini-splits in a multi room house

DCContrarian | Posted in General Questions on

I’m in the design phase of a four-bedroom house. I just got preliminary drawings for the mechanicals from a ME. I had asked him to draw it up for mini-splits, which is a new technology for me but everyone raves about how great they are.

The first floor is an open plan with kitchen, dining room area, living room area and family room. The second floor is three bedrooms. Third floor is about a 200 SF office. Basement has the fourth bedroom, plus a workshop and mechanical room.

His proposal is to put a head in every room — four on the first floor, three on the second floor, one on the third floor and two in the basement. Ten heads total. The head specced is a Daikin FFQ09Q2VJU, which is a ceiling cassette rated at 9K BTU/hr.  He calls for four compressors outside.

I haven’t seen his Manual J, but I used an online calculator and got in the 30-40K range for both heating and cooling. I’m in zone 3. 

So my first question is, am I right in thinking that this system is way oversized, with 90K BTU when it needs about a third of that?

Second question is, OK, how should you configure a minisplit system when you’ve got lots of rooms? With the open plan on the first floor I could see having one or possibly two heads. But what about the bedrooms? What do you do when there’s a good possibility the rooms will have their doors closed a lot of the time?

He also calls for an ERV ventilation system with ducts in every room. Can that be counted on to distribute heating and cooling and possibly reduce the number of heads?

Thanks.

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Replies

  1. Patrick OSullivan | | #1

    > So my first question is, am I right in thinking that this system is way oversized, with 90K BTU when it needs about a third of that?

    Yes.

    > Second question is, OK, how should you configure a minisplit system when you’ve got lots of rooms?

    A ducted minisplit system is more suitable for the bedrooms.

    > He also calls for an ERV ventilation system with ducts in every room. Can that be counted on to distribute heating and cooling and possibly reduce the number of heads?

    Likely not.

  2. Trevor Lambert | | #2

    The proposal is ridiculous. Even 30k seems on the high side, though you don't mention insulation levels or total square footage. A 200sf third floor and three bedrooms on the second implies a modest size. An equal heating and cooling load leads me to guess you're in zone 4 or 5. Assuming "pretty good house" type construction, loads will probably come out to less than 30k, but definitely get that Manual J before proceeding. The online calculators are notorious for over estimating.

    With a lot of rooms, a ducted system is usually the best option. You might want one ductless for the big, open area and then duct the rest. I have one ductless on an open plan main floor that does a good job of heating the whole floor, and most of the second floor (second floor bedrooms supplemented with electric). It is entirely inadequate for cooling the second floor, so I added a mini split up there (eventually to be ducted to the bedrooms).

    ERV doesn't move enough air with a big enough temperature differential to meaningfully affect heat distribution.

  3. User avatar
    Minmax Design | | #3

    Agreed that 90 kBtu/hr is almost certainly way oversized and the ERV will not adequately distribute the conditioned air.

    I've done both a single story with a basement and a two-story, both with only two wall units. It works great for the open and quasi-open areas- kitchen, dining, living area in the single-story, and the first floor in the two-story. However, a single head is probably not the best option for the sleeping quarters unless the occupants have a very high tolerance for temp swings. When we close the doors to our bedrooms there is a noticeable temp difference.

    Definitely cannot rely on a lower floor unit to provide cooling for the upper floors and vice versa for heating.

    Individual heads in each room can be problematic because they end up being substantially oversized. Design day loads might come in at the lower end of their modulation range resulting in quite a bit of cycling for most of the year leading to a performance penalty. All of this is assuming there was increased attention toward minimizing heating/cooling loads in the design process and ultimately dependent on the room by room load calcs.

    As was said above, basically as the area becomes more and more closed off a ducted mini-split makes a lot of sense.

    1. User avatar
      Jon R | | #4

      > quite a bit of cycling for most of the year leading to a performance penalty

      Can you quantify this?

  4. User avatar
    Jon R | | #5

    I suspect the design is expensive enough that an air-water heat pump and fan coils would be less expensive. With a buffer tank, over-sizing is a non issue and it can retain the superior comfort of zoning.

    1. DCContrarian | | #6

      I had a similar thought. This is going to be a luxury-price system with sub-luxury performance

      Are there residential coils designed for both heating and cooling? I've seen them in commercial but in residential only ever for heating.

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