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Community and Q&A

Sizing Multisplits for Cooling

rhl_ | Posted in GBA Pro Help on

I’m in Climate Zone 4A. I’ve done my own Manual J, which is attached. I was hoping for some advice on how to layout the system, in terms of which sized indoor units to go with and how many outdoor units to go with? I was hoping to install an indoor unit in each bedroom, living room, and larger first floor rooms.

In particular, most bedrooms are << 3k BTU/hr load. The smallest indoor unit from mitsubishi is 6k BTU/hr load.

The living room (on this manual j divided into two rooms “living room” and upper living room), is about 14.5k BTU.

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  1. Expert Member
    Dana Dorsett | | #1

    Cooling only?

    The head per room approach almost always leads to grotesque oversizing and lower efficiency/comfort. The number of bedrooms and floor plan/layout isn't specified. A floor plan with the loads of each room specified would be more useful.

    Mini-duct cassettes can split the output between multiple low load rooms. The loads for the master bedroom seem a bit high for a 240' room, but possible, and it could use a 9K head on a multi-split or an FH06 single zone minisplit if those loads are real.

  2. rhl_ | | #2

    I want to size it for cooling. We already have hydronic heat. So the heating system in the minisplits could be supplemental. I did that manual J myself.

    The heating side of things does seem to roughly match my actual usage.

    I attached another version which is the same general idea, but assumes i insulate all the walls and attic etc. It also have the floorplan layout in it.

  3. twoodson | | #3

    Ryan, does this home currently have ductwork?

    1. rhl_ | | #5


  4. Deleted | | #4


  5. rhl_ | | #6

    Dana, i've attached a set of floorplans with the cooling loads from the manual J written directly on each room in the two floors i care about conditioning. I don't need to put into the basement. Perhaps this is not clear, but there is a 16' frustrum ceiling in the living room, and a large like 32-36 sqft window in the living room. (currently a very leaky, single pane thing).

    Also these are the numbers out of wrightsoft, it doesn't necessarily seem to add latent loads into each room, and the latent load calculations per floor seem surprisingly low for how humid it is in the summer months.

  6. monkeyman9 | | #7

    I'm in the same boat as you. My only option are oversized heads or mini ducted on attic floor with a foam board cover and spray foam on ducts and plenums...then blanket it all in maybe r60.

    My master suite area could have the mini duct in the closet with a lot of work. But the other 2 bedrooms are 10ft apart at the closest walls. So those ducts and mini duct unit would be in the attic no matter how I cut it.

    I really wish they'd make a 3k wall unit.

    1. monkeyman9 | | #8

      One thing you may consider that I thought about is having a head in the hallway with a through wall fan or jumper ducts to the small bedrooms. I'm not sure how this would work out and it complicates the system. Maybe someone else can comment there.

      1. rhl_ | | #9

        Or the other option: size the outdoor unit to the house load, and the indoor units to match as close as possible (but oversized relative to the outdoor unit), I wonder what happens?

        Air to water systems (such as space pal) have smaller indoor heads, however the outdoor units for space park are terrible. One question is if it’s possible to use a Mitsubishi outdoor unit, hooked up to a buffer tank with a heat exchanger. Could this be done while maintaining high efficiency.. (or just import the units from England or something)

  7. 1910duplex | | #10

    What did you end up doing? I am going to go with a ducted minisplit for cooling, and am in 4a, am interested in how happy you are with your set up.

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