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Community and Q&A

Sizing a minisplit for supplemental heat

Jon_Harrod | Posted in Mechanicals on

I’ve been asked to quote a mini-split for a ~832 SF yoga studio that’s part of a larger physical therapy/gym complex in a recently renovated low-rise commercial building. The yoga studio has only one exterior (south-facing) wall with large windows; the other three walls adjoin spaces that are conditioned year-round.

Exterior surfaces [wall (185 sf net) , roof (853 sf) & slab floor (32lf exposed)] appear to be insulated; windows (85sf) and skylights (30 sf) are modern double pane. Maximum occupancy is reported at 15 people.

The studio is currently served by the central HVAC system, with three supplies and one return. The customers report that the room is cold in winter and undercooled in summer.

My question is how to account for the existing HVAC and air exchange with adjacent spaces in my sizing calculations.

Ultimately, I’m hoping to propose a Mitsubishi 1:1 high wall unit with Hyperheat. I’m used to aggressive sizing in residential settings but not as confident in my initial forays in light commercial.

Thanks in advance for your thoughts.

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  1. GBA Editor
    Martin Holladay | | #1

    Step one is a heat loss calculation, which you presumably have already done, or are about to do.

    Step two is to evaluate the existing HVAC system, to identify what's wrong. It's possible that the existing system simply needs larger ductwork.

  2. Expert Member
    Dana Dorsett | | #2

    Without the R-values or construction stackup AND the location there's no way to estimate the total heat load (there will usually be quite a difference in both the heating & cooling loads of 850' commercial building spaces in Mobile vs. Minneapolis.), and without the design output of the existing HVAC it's impossible to say how much the shortfall is.

    A group of 15 sweaty exercisers can add 3/4 ton or more to the cooling load, and there's probably another 2500-3000 BTU/hr of lights & plug load, so to be conservative you'd add at least a 1-ton, and a 1.5 tons isn't completely out of the question, but there's no way to specify the shortfall vs. the total zone load.

    For a single zone Hyper Heating Mitsubishi solution, an FH15N is probably going to more than cover the shortfall, and an FH18NA might even be able cover the whole load (even without the ducted HVAC), depending on the roof & wall-R. If you're going to just punt ( there probably isn't much choice, without going deep on the analysis of the existing HVAC), go with the FH15.

  3. Jon_R | | #3

    I second what Martin said. When the room is under heated/cooled, is existing hvac running at 100%? Also look at shading, air sealing, etc. Of course this doesn't help sell a mini-split....

  4. Jon_Harrod | | #4

    I'm in Tompkins County, central NY, southern end of climate zone 6. Part of what's driving this project is a desire for zoning, with the ability to have different set points for the yoga room compared to the adjoining spaces.

    If I design for the whole load, it points me toward FH18NA. It's reassuring to me that no one is saying bigger.

  5. Expert Member
    Dana Dorsett | | #5

    The FH18 may cover the whole load, but also may NOT cover the whole load. There is no substitute for running the load calculations here.

    But the FH18 is the largest of the single zone Hyper Heating units- if iit turns out you need something bigger you'll have to go to a different class of compressors, and multiple mini-split heads.

  6. Jon_Harrod | | #6

    Yes, of course, sizing calculations are absolutely being done! Right-J is giving me a design heating load of 13,229 Btuh and a design cooling load of 17,951 (13,998 sensible, 3953 latent). If I size the system for the whole load, this points me toward the FH15 or FH18 (they both have the same minimum heating/cooling capacity; leaning toward the 18 for quicker recovery from setbacks).

    I think my question is this, and maybe I'm thinking too hard about it: Is there any scenario in which the central HVAC would increase the maximum heating or cooling load beyond what I calculate for standalone design loads?

  7. GBA Editor
    Martin Holladay | | #7

    Q. "Is there any scenario in which the central HVAC would increase the maximum heating or cooling load beyond what I calculate for standalone design loads?"

    A. Not really, unless the existing forced-air system is operating as an air conditioner during the winter (when you are trying to heat the space) or operating as a heating system during the summer (when you are trying to cool the space). That would be unusual but possible.

    As long as you are doing a heat loss calculation, you should check out the rating of the existing furnace (its BTU/h rating) and the rating of the existing air-conditioner (in tons). The existing equipment may be adequate -- there may simply be a duct design issue here.

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