GBA Logo horizontal Facebook LinkedIn Email Pinterest Twitter Instagram YouTube Icon Navigation Search Icon Main Search Icon Video Play Icon Plus Icon Minus Icon Picture icon Hamburger Icon Close Icon Sorted

Community and Q&A

Using mini split systems on large open spaces

jtkilroy | Posted in Mechanicals on

I know there seems to be a millions suggestions on line for using mini splits to heat, cool, do your laundry etc. However, as much as I have googled, and my GoogleFoo is strong, I have found almost no discussion on how to heat and cool large open spaces with mini splits.

So my situation…

A large-ish room, 62×36, or roughly 2200 sq feet, 15ft ceilings (upstairs floor). (Shop / Work space ) The long side faces east west, short sides south and north. Solid brick walls, think late 1880’s construction, 18″ thick. No western exposure, blocked by another building.

Huge doors on north, east and south sides, big arched bastards, roughly 100 sq ft of surface area each. 3ft diameter round, single pane, ancient windows, 4 on the north side, 6 on the east side, no windows on the south side.

ZERO insulation, as in NONE, on any surface, unless you count paint or brick mortar as insulation. However, when this is done I would like to get foam sprayed under the 2nd floor to seal the space up a bit, mostly to allow an *attempt* at heating in the winter.

The 20+ year old 5 ton 0 SEER system, which managed to cool the space ok, except in the hell months (Jul, Aug, Sept) has finally give up the ghost.

I would like to remove the old central system, and the large, hanging, square duct system, and replace it with mini-split wall mounts. I don’t expect a wall mount can reach across 30ft but I can’t find a bit of information that addresses it. I could use ceiling mount I guess but I fear pumping condensate, of which this building and climate can produce a LOT of.

Is this a job best left to forced air? Should I just replace the old central unit with new and call it a day? Are large rooms a big no-no for mini splits?

GBA Prime

Join the leading community of building science experts

Become a GBA Prime member and get instant access to the latest developments in green building, research, and reports from the field.


  1. GBA Editor
    Martin Holladay | | #1

    A well-known net-zero energy building, the Putney School Field House (a school gym) is heated and cooled with ductless minisplits. Andy Shapiro designed the system.

    Here is a link to one article; with a little Googling, you'll find lots of articles about this building on the web: Putney School Field House.

    -- Martin Holladay

  2. Expert Member
    Dana Dorsett | | #2

    First, come up with the load numbers. Without them there's no way to really address it properly.

    There are many large commercial mini-split cassettes appropriate for larger spaces. Large retail spaces in Korea & Japan and elsewhere often use this technology.

    This building may do just fine with a pair of 2-3 ton wall coils on each of the 62' walls (4 total), but it's hard to make that call without a real analysis. With 15' ceilings you may have good ceiling-mount options too, but servicing the filters and drain issues may be more complicated with that approach.

    Even with fewer heads, in an open space convection can take care of a lot of temperature-leveling between zones. Thirty feet isn't much of problem if it's an 18" thick brick wall, 30' away but if it's a large wall of glass 30' away maybe not. How many supply registers are there? Where are they located relative to walls or windows? Take notes- there may have been design or evolution that led to the number and placement that could still be relevant to a ductless solution.

  3. jtkilroy | | #3

    I ran a pretty darn complete calc on load and the cooling load came in at roughly 54K btu's with one caveat, the software I used didn't have a choice for a totally un-insulated ceiling, with R-11 as the minimum choice. Where as I currently have R-0 ;-). The walls are solid brick all the way around, 18" thick.

  4. Expert Member
    Dana Dorsett | | #4

    What is the stack up, shape and color of the roof?

    Is this cooling-only, or is it heating + cooling?

  5. jtkilroy | | #5

    The second floor, with a 12ft cieling is a totally empty space, 15" solid brick all around, exposed on all sides, with a flat roof, hot tar and gravel. 48sq ft of window on the north and south sides, 144 sq ft. of window on the east nothing on the west. Heating plus cooling. My dear departed mother used to say, we have two seasons in Mississippi, TOO hot and TOO Cold! Heating has never been a huge problem, the passive heat from the massive brick walls does wonders.

    The only thing green about this building is the color of the money I spend on it!

  6. Anon3 | | #6

    A wall mount can blow 50 feet+

    Also, radiant barrier roof decking will do wonders to reduce your cooling load, for next to no cost if you DIY it.

  7. Expert Member
    Dana Dorsett | | #7

    Flat roofs don't convection cool and have pretty extreme peak temperaurs, but hopefully the ballast stone is a light color (that increases the solar reflectivity)?

    Assuming you're not replacing the roof decking, there are some silvery low-E paints that can make a difference if applied to the under side of the roof deck. Don't get sucked into the scammy nano-sphere bead type of paints or additives, so popular a handful of years ago- if it ain't silvery looking it's not the right stuff. If you combine a low-E painted roof deck with a perforated aluminized fabric type radiant barrier attached to the bottom of the rafters the net performance will exceed the R11 used in the load calculation. The cost of the rolls of RB for your 2250' of ceiling would be on the order of $300-350 (material cost only), the paint would run about $500-600 (two 5 gallon containers- it'll take on the order of 8 gallons), so as a DIY you'd be into it for about a grand (plus whatever it costs for the therapy to fix the stiff neck from spending that much time looking up. :-) )

    If you need higher performance than that you'd have to install real insulation and vent the roof, which would probably require some structural framing & materials to support the insulation, and designing & implementing the roof venting scheme.

    With the load calculation tool in front of you, how much of the 54K load was from windows, and how much window area do you have? Exterior shades or window films can cut that by well over half. If they are leaky multi-panes it's worth installing tight low-E exterior storm windows, which will cut the sensible cooling load some (not as much as some window-films), but would cut the latent load from air-leakage quite a bit, and lower the heating load.

    The R-value of 18" of brick is usually under R4, but the thermal mass and air tightness of masonry walls that thick help quite a bit. Insulating the walls wouldn't change the peak or average cooling loads by very much, but it would make a difference on heating energy use.

    The "right" way to insulate a flat roof in your climate would be 4-5" of rigid polyiso or 6" of EPS above the roof deck, with a high solar reflective index(SRI) roofing material (or mop-on coating) listed by the Cool Roof Rating Council (see: ) with a 3 year SRI north of 80. That's only going to be cost effective if/when it needs re-roofing, and can be made more cost effective (and greener) by using reclaimed roofing foam from commercial building demolition & re-roofing (typically less than 1/3 the cost of new material.)

    With a design cooling load of 54,000 BTU/hr you can probably get there with just two 2 or 3 ton heads, one mounted 1/4 the way from the end of one of the long walls, the other 1/4 of the way from the other end of the other long wall. In that configuration the air will circulate in the room air, and reach corners that the direct throw of the heads themselves might not quite reach. Four 1 or 1.25 ton heads would offer somewhat more even room temperature distribution though.

  8. Anon3 | | #8

    The bigger mini split pushes air a lot further and are quieter, look into the Midea Premier 3 ton units, mount it as high as you can (while still able to clean the filters periodically).

Log in or create an account to post an answer.


Recent Questions and Replies

  • |
  • |
  • |
  • |