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Determining Minisplit Configuration

Max E | Posted in General Questions on

questions.
zone 1 is 815 sf of floor space and  has 512 sf of windows. beopt thinks that i need 28k btu and 1400 cfm to service this zone. can this be served by one 24k head or will I need three 9k heads attached to a multizone outside unit?

for zone 2 and 3 , should I do separate outside units, or 1 dual zone with a ducted unit for zone 2 and a wall mount for zone 3?

mini split zones

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Replies

  1. Expert Member
    Akos | | #1

    You have lots of rooms, wall mount in every room or there about is a very bad idea. A wall mount in your main floor will mostly work as it is an open space but the rest, the head is so oversized for the load that you'll get bad efficiency, humidity removal and comfort.

    Look at ducted units such as the Mrcool DIY universal with quick connects for the whole house or a slim ducted unit for upstairs for all the rooms and a wall mount for the main floor. A bit more work to install but well worth the effort.

    1. Max E | | #2

      Thanks akos I was thinking of a slim duct for zone 2,(four bedrooms) and a wall mount zone 3(master bedroom) and wall mounts for zone 1.

      But will one head be enough for zone 1? I think one head can't move enough cfm for the space

      The other problem with ducted is finding the transition plenum. They seem to be a custom item $$$

      1. Expert Member
        Akos | | #3

        I live with a wall mount in my bedroom and can tell you it is a very bad idea. I was very close to replacing them with a ducted unit and will do so if they every give me trouble. Even the smallest one moves too much air to be comfortable.

        I have used a length of standard ducting as budget plenums. Say 8x14 duct 3.5' long with the ends capped. Cut a slot in one side for the opening of your ducted unit (usually around 7"x32") and connect your takeoff on the remaining sides. There are also some pre-made plenums:
        https://www.airconplenums.com/mitsubishi-plenum/pead-rp35-140-ja/pead-rp100-125-ja/pead-rp100-125ja-return-air-plenum/

        If you are mostly using the main floor unit for cooling, than a single unit mounted high should work, set it up to blow along your windows as I'm guessing that is the big source of the heat gain. A 24k wall mount is a big one, place it in a way that it won't directly blow air at people sitting.

        1. Max E | | #6

          Thanks akos. Good suggestion if I can't find someone to make the plenum.

          So your bedroom mini split puts out too much cfm? Interesting. Is it worse than say what you get in a hotel with their ptac?

          Most of the heat gain is from the windows as you suspected. I told beopt there would be no windows coverings to approximate a worst case scenario. I would prefer to do a single head but I don't see any 24k units that come close to 1400 cfm.

          Plan b for living area would be another ducted unit if I can get into a setup affordably

        2. Max E | | #7

          I was also thinking, would a ceiling mounted cassette solve this problem?

          Ive read that ceiling mounted registers spread the air better. Could that be a possible simple solution?

      2. jrpritchard | | #4

        A hand drawn sketch with good measurements taken to any local HVAC shop should get you a relatively inexpensive plenum made. A bigger production shop with a plasma table would probably save a lot of labor cost but you may have more difficulty talking them into making it for you.

        1. Max E | | #5

          That's good to know. Great to know. It pushes me towards a ducted mini split for zone 2 instead of a standard unit. Thanks.

          Any thoughts on multi zone vs single ?

  2. jrpritchard | | #8

    People have a lot of opinions when it comes to mini splits and my opinions are based mostly on customer feedback and experience. We have installed fully engineered systems, customer specified systems and systems where we hire out the manual J SD and T and get to give our design input. I always lean towards as little equipment as possible which usually means ducted units placed in large zones. I prefer one to one units as opposed to multi zone units simply because they they generally allow more installation flexibility, shorter line sets, condensate lines etc. it also gives some redundancy in case of a failure and it is much easier to right size a system in a larger zone rather than simply putting a 9k head in each bedroom because that is the smallest unit made. Ducted unit also allow us to use whichever filter assembly we want and it’s much easier to integrate an ERV or
    Freshl air. Multi zone systems can be messy to install because each head throughout the house has several lines that all tie into the outdoor unit. Ceiling cassettes usually have built in condensate pumps that fail on a regular basis. My suggestion is always the same when it comes to mini splits - buy good equipment, do the job with as few pieces as possible, keep the installation as clean and simple as possible and keep up with the maintenance

    1. Max E | | #9

      Thanks so much for the advice jrpritchard!

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