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Sizing a mini split/s in one large room (38×60)

4gvnsnr | Posted in Mechanicals on

Would anyone care to try and help take a guess at what type of mini split setup I may require in my current situation? I have read and read and read everything I can find but am still chasing my tail. Ran a few Manual Js with the free software available and came up with some numbers but I’d like to double check it if possible..Normally I wouldn’t even ask this type of question but I feel like this specific situation may lend itself to someone who knows the ins and outs being able to get pretty close with a ballpark guesstimate.

Indianapolis area 46254 zip code. I have a 38×88 building built with a 3ft stem wall and 2×6 construction. R19 batt insulation in walls (R10 rigid foam on the bottom stem wall), not sure on ceiling it’s blown cellulose but there is a lot of it. Concrete slab which I’m pretty sure is NOT insulated. ZERO windows and 2 steel man doors. 

The room in question is 38×60 that is being used as a recroom to watch football and play pool. It will only be used once or maybe twice per week. The rest of the time the temps will be set just to avoid freezing and drastic swings. There are 8 large TVs and will have between 10-12 people in it generally, occasionally up to maybe 20. 

Since it’s a part time space and cost is a factor I’d like to (and assume it’s wise to) use a cheaper brand/model single zone (or possibly 2 single zones) minis but I’m lost on approximate sizing. I will probably have a few additional space heaters available should the need arise but am hoping to not need them. 

Could anyone try and shoot me in the right direction please? Any help would be much appreciated. Thanks!

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  1. gusfhb | | #1

    That is a big room

    I am thinking that you will need more than one to get the air movement you need. I have used 2 ton units in 1000 square feet and they move air very well

    Obviously you should do a heat load calc,but if you do not need to maintain temp at 1 am on the coldest day of the year, you can choose to undersize. However if you undersize, then you lose the ability to bring the room up to temp quickly

  2. 4gvnsnr | | #2

    I was tentatively planning to use two on the exterior 38ft wall both facing in the same direction. I figured that will give me the best airflow and also 2 units will have an easier time keeping up than a single.

    I’ve done a heat load calculation but I’m not all that confident in my input numbers and so I’m hoping I can get a few replies to see if what I’ve go already matches up with their suggestions (somewhat closely at least).

  3. 4gvnsnr | | #3

    Anyone care to take a shot at a guess here?

  4. Expert Member
    Akos | | #4

    I would run your building through one of the online calculators ( or A rough stab at 38x80 with 10' celing r40 attic and 70F delta, probably around 50 000 btu.

    The best would be two ceiling mount 4 way cassettes to cover the larger space,

    If you want it to heat up fast, over sizing the units, say two 4 ton low temperature units like:!/product/29903

    Make sure whichever unit you go with that it comes with a pan heater, important in cold and snowy climate.

    1. Expert Member
      Dana Dorsett | | #15

      >" A rough stab at 38x80 with 10' celing r40 attic and 70F delta, probably around 50 000 btu."

      That may be about right if it has a large amount of window area comparable to a house or leaks tons of air, but it's probably under 40K, or even under 35K.

      The U-factor of a 2x6/R19 (really R18 after compressed to 5.5" in a stud bay) 16" o.c studwall is typically 0.065-ish. With 10' ceiling's you're probably looking at wall losses of 11-12KBTU/hr , ceiling losses probably less than 12KBTU/hr, plus air leakage, slab losses (but it's insulated at stem walls), plus windows & doors.

      Even at a 15% window/floor ratio with clear glass double panes it would only add another ~16K, but there are allegedly "...ZERO windows and 2 steel man doors...", probably U0.25-ish 21 square foot steel doors. I'm guessing with air leakage and slab losses it'll come in somewhere in the 30-35K range.

      >"The best would be two ceiling mount 4 way cassettes to cover the larger space"

      Two, yes, but probably no 4-way units, since they're a PITA to install as a retrofit. Most 4-way cassettes will require custom re-framing unless you can build out a box below the current joists or truss chords to accommodate the cassettes. Even 24" o.c. joist framing doesn't work for most.

      Sub-ceiling plane 1-way cassettes may be a better bet if you have the headroom.

      On the pretty good non-Japanese vendors fronts, a pair of cold-climate Midea's Premier series DLFSFAH18XAK ceiling cassettes on separate DLCSRAH18AAK compressors can deliver about 36,000 BTU/hr @ +3F (the 99% outside design temp for Indianapolis) total for the pair of them.!/product/30297

      They make 2 ton and 3 ton versions too, but that's probably overkill unless the heat load really IS 50K:!/product/30368

  5. gusfhb | | #5

    I would not suggest oversizing, you 2 2 ton units sounds what I would 'assume'

    Worst case would be superbowl -10 night before kickoff, and unless you are out of town, you just bite the bullet and turn it up the day before,.

    i think what I would do for maintenance level heat is stagger the temps of the units such that one never turns on unless the temp drops below 'x'

  6. 4gvnsnr | | #6

    I came up with roughly 40k btu heating and 25k cooling but again it was my first time running one of those calculators and some of the inputs I wasn’t exactly sure on.

    Was tentatively thinking two 2 tons myself, maybe two 2.5 tons.

  7. 4gvnsnr | | #7

    And that sounds like a great idea on staggering the maintenance temps differently I didn’t think of that.

  8. 4gvnsnr | | #8

    Anyone else care to offer an opinion before I go ahead and purchase two 2 ton units?

  9. Jon_R | | #9

    I would look into adding a little foam to the floor and then measure and improve air sealing. Then re-do the Manual J and Manual S and go with what they come up with. Do not rely on guesses.

    1. johns3km | | #14

      The losses to the slab will be massive. What’s a slab, R1-2? At 2280sf, that’s at least 1,1000 BTU per degree hour. Assuming the slab stays at 50F, with a 15-20 delta or almost 15,000-22,000 BTUs. Get that insulated.

      1. Expert Member
        Dana Dorsett | | #16

        The slab edges are insulated at the stem walls, the local subsoil temps are in the mid-50s, and there's likely to be gravel under the slab, not a large copper ingot coupling it to the groundwater table. With a windowless semi-heated insulated building above it the soil under the slab isn't going to be 50F (even if the building is only heated to 50F.) Soil has an R-value too. In that location the losses to the slab are essentially zero (or even negative) when the building is just being heated to 50F.

        But raising the temperature of the thermal mass of the slab in a reasonable amount of time could add another ton of mini-split if it has to slew from 50F to 65F at slab level.

  10. johns3km | | #10

    12-20 people could account for 3000-5000 BTUs/hr.
    8 100W Big screen TVs could be another 1000 BTUs/hr
    Sounds system/amps, fridges, lighting all should be accounted for as well.

    I wouldn't be surprised if you could shave off another couple thousand BTUs/hr once you tally it all up. But if you size it tight for a 20 person Super Bowl party in February, it might not work as well if you went in there the next day with 4 people and 1 TV on. Just one thing to keep in mind.

    With a space that large and at those outdoor temps, I'd probably keep it set at 60 for the week and turn it up the morning before a game. Any pics of the space?

  11. Expert Member
    Michael Maines | | #11

    I ran it through BEopt, making some assumptions, and got 49.5 Btu/hr for heating and 18.7 Btu/hr for cooling.

  12. 4gvnsnr | | #12

    Im surprised the heating load is so much larger than the cooling, but that’s in line with what I came up with as well.

    Is it even wise to use mini splits with that much heating demand? I know they’re wonderful for cooling but Isn’t the heating more of a secondary feature or have they improved that much over the years?

    I wonder if it would be better to use two different sizes, say a 3 ton and a 2 ton and have the smaller one as a secondary supplement rather than both working all the time.

    1. Expert Member
      Michael Maines | | #13

      Funny, in the northeast we use them a lot for primary heating, especially in high performance homes, and their ability to cool is generally considered a bonus feature. They do have to be designed and specified properly, but units are available that will provide reasonably efficient heating down to -20°F.

      Whether they are the best choice for your situation or what configuration would be best is not something I can answer. I would be concerned about their ability to quickly heat that large a space if you keep it turned down low when you're not there.

  13. 4gvnsnr | | #17

    That is kind of funny. I guess the heating aspect has come I long way because I remember maybe 10-12 years ago reading about people needing to add supplemental heating to them to keep up. In any case, at least that assures me they should be good to go as long as I can size them reasonably well.

    I’m not concerned with how long it takes to get up to temp. I won’t set it back to 45 or anything and I’ll always have plenty of notice in advance to get it up to temp. If it takes 12 hours or more it’s not a problem.

    1. Expert Member
      Michael Maines | | #18

      Yes they have come a long way, both in terms of efficiency and cold-climate performance. Only a few models are rated for cold climates; you can find a list here:!/product_list/. I recommend clicking "download product list". Scroll to the right on the spreadsheet to see which models are rated for 5° or lower operation.

    2. Expert Member
      Dana Dorsett | | #19

      >" I remember maybe 10-12 years ago reading about people needing to add supplemental heating to them to keep up."

      Your memory is just fine. That was just about the time that vapor-injection scroll compressor technology that extended the low temp capacity and efficiency of heat pumps were starting to get commercialized.

      The Mitsubishi MUZ/MSZ FExxNA series mini-splits hit the US market around 2009 and were pretty much an instant success, since they had a specified capacity even at -13F. I'm not sure if they were the first to the market, but they were the first cold climate mini-splits that really took off. (I have a few family members who installed FE-series Mitsubishis in the early years.)

      Now several other vendors have cold climate mini-splits, some fully rated down to -22F.

      Midea's Premier series cold climate mini-splits are using Toshiba compressor technology. Midea is a large Chinese company selling everything from toasters & coffee makers to heavy equipment, and have a decades long partnership with Toshiba's refrigeration division (which built a large factory in China.) In recent years Carrier has started putting their name on Midea equipment in the US. World-wide Carrier and Midea have been in bed together for quite some time. The issue with installing internet-store sourced Midea equipment vs. the Carrier versions is the amount of technical & warranty type support, but in many cases it's the same stuff under the paint. Properly and professionally installed they should be equivalent.

      I've never personally been involved with installing or specifying Midea on a project, but with good installer support I wouldn't be opposed to it on a cost-sensitive project like yours.

      1. bfw577 | | #20

        Are you self installing these? I self installed a Midea premier floor console unit that I have been impressed with. It cost me 1100$ delivered and it has excellent cold weather performance rated at 100 capacity at 5 and 79% at -22. It delivers grear heat at cold temperatures.

        I also self installed a 12k Gree Sapphire ($1500)that blows away the Midea. Its at the very top of the AHRI directory for performance and beats out most of the big names. Its cold weather specs are insanely good and has an excellent modulation range. The heating capacity range is 3,071 to 18,766 with a HSPF of 14 and seer of 38. I attached the cold weather heating capacity chart from the service manual. It shows it puts out 100 percent heat at -17.

        Its too bad there is practically no professional companies installing them as they have been excellent units. I obviously have no warranty but I was willing to take the risk for the obvious substantial savings. I think there is a large increase of people self installing the cheaper chinese units for the same reason. Costco is even selling them now.

        1. Expert Member
          NICK KEENAN | | #24

          Just curious (and maybe this should be a separate thread) but when you self-installed, did you get a unity with pre-charged lines, or did you do all the installation and then hire a HVAC contractor to charge the lines?

  14. 4gvnsnr | | #21

    Thanks for the replies everyone.

    I was hoping I would get lucky enough for you to reply Dana as I have read countless posts of yours on minis and you obvious have a tremendous grasp on the technology. Do you think I’m in the ballpark with two 2 tons?

    I do plan to self install these, although i can have a tech finish off the lineset if need be. I’m not too worried about the warranty. With this being a part time space and with all the money I’ll save, I can replace one on my own and still be money ahead if need be.

  15. 4gvnsnr | | #22

    BFW, I assume you attribute the performance increase on the Gree vs Midea mainly due to the sapphire model upgrade and not so much the brand itself?

    Nevertheless there are some very affordable Gree models I was looking at but hadn’t heard much user experience with the brand. Nice to know they are strongly worth considering.

  16. nickdefabrizio | | #23

    I suggest you research your states rebate programs before you buy. In NJ they give you $1000 for high efficiency single head ductless units; $2000 for a multi head ductless and $2000 for a single head ducted unit. Given that the single head ducted is not much more expensive than the single ductless and much cheaper than a multi, the rebates make it much cheaper to install two ducted units, at least in NJ. So in NJ the best bet would be to get two Fujitsu ducted units and run short runs of ducting lengthways ( I like the look of the round spiral ducts to give an industrial look). Not sure about your state but it is worth a little research.

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