Small ducted mini-split question
My 93 year old mother lives in a small duplex in Chapel Hill, NC and her 25 year old Trane 2 ton central AC unit has failed. A gas hot air furnace heats the house through the same ducts in a single zone.
I think it is time for mini-splits. There is one main open living area of about 600 square feet that gets lots of sun and two small adjacent bedrooms of about 200 square feet each on the other side of the house that get no sun and are separated from the main living area by a winding hallway. So I think we need two separate zones and I am inclined toward two completely separate units.
A single unit will cover the main living area- size to be determined after measuring the space and guessing about the insulation in the walls but I suspect that a 9K or 12K unit will be sufficient.
My question is for the two small bedrooms. Do Fujitsu and/or Mitsubishi make small units that can be ducted through the wall separating the two rooms so that the doors can be shut without problem?
Thanks for any advice,
David
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Replies
David, it sounds like a "slim duct" or horizontally ducted system might work for you--a single unit with a wimpy but efficient fan that can supply a few closely-spaced rooms.
The individual room loads for a pair of 200' bedrooms is probably very low. You can probably do the whole house (main space include) with just one 1.5 ton -2 ton cool climate ducted minisplit, if zoning it as a single zone can work. The only way to know is to plod through the room by room load calculations.
Fujitsu makes a 7K slim duct cassette that may be more appropriate for splitting to the bedrooms:
https://www.fujitsu-general.com/us/resources/pdf/support/downloads/submittal-sheets/ARU7RLF.pdf
The 7K cassette is only compatible with their multi-zone compressors. A 2 zone 1.5 tonner still has a decent amount of capacity at NC type outside design temps, and you probably don't need 2 tons.
https://ashp.neep.org/#!/product/30267 (1.5)
https://ashp.neep.org/#!/product/30268 (2 ton)
Thanks Dana, Unfortunately I believe we need two zones because the big open room gets lots of sun while the bedrooms don't get any, so the heating and cooling needs are very different when the sun is shining than at night.
Am I correct in understanding that a single Fujitsu slim duct cassette can be ducted to the two bedrooms that are separated by a single wall? It is too bad that unit is not compatible with a single zone compressor. But I could put that unit between the two bedrooms and use a regular ductless unit for the big room, right?
I will try to get room by room calculations, but I suspect that the smaller figures will be sufficient, in part because the space above the ceilings is already conditioned by a separate system.
The Fujitsu units have a great deal of flexibility in mounting, and can even be mounted vertically. The picture at the top of this page is a 1.5 ton Fujitsu mounted vertically, with a short fat plenum branching immiately into 4 flex ducts, and one big common return below:
https://www.greenbuildingadvisor.com/article/getting-the-right-minisplit
https://s3.amazonaws.com/greenbuildingadvisor.s3.tauntoncloud.com/app/uploads/2019/08/02092349/Ducted-minispli-1-Dana-Dorsett-700x933.jpg
They can often be mounted just below the ceiling level in closets between rooms, with ducts directing air to both rooms. Figuring out the return ducting or path is as important as the supply path.
I too wish Fujitsu (or any vendor) had a decent half-ton single zone mini-duct cassette, given just how low single room loads are even in current code minimum construction.
If you want to build your own room by room heat load calculation spreadsheet using I=B =R methods, read this:
https://www.greenbuildingadvisor.com/article/how-to-perform-a-heat-loss-calculation-part-1
https://www.greenbuildingadvisor.com/article/how-to-perform-a-heat-loss-calculation-part-2