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Ductless minisplit: sizing advice for cooling

nefrusy | Posted in Green Products and Materials on

Hello, this is a followup to a question I asked almost two years ago about using a ductless minisplit to cool my upstairs: https://www.greenbuildingadvisor.com/question/retrofit-single-ductless-minisplit-to-cool-entire-upstairs

EDIT: found this separate thread I forgot about from 2017:
https://www.greenbuildingadvisor.com/question/cooling-load-calc-for-sizing-a-ductless-minisplit

So this is mostly answered by Dana (thanks again!)

But since I can’t delete this post, I guess I’ll at least shorten my question… according to Manual J, the load in the house is 7300 BTU main floor and 6000 BTU upstairs.  However, on a 93F day I observed that a main floor AC unit only needed to provide 2600 BTU, while an upstairs unit was maxed out at 5000 BTU (and struggling to keep cool).  It appears that air stratification is moving most of the heat to the second floor?

I’m about to get two quotes for an upstairs mini-split, was wondering what size unit to look at… based on Dana’s feedback from the 2017 post, I think I’m comfortable with trying a 9K unit (probably the Mitsubishi FH09 for lower modulation).  We’ll probably try surviving a summer with just the single unit upstairs handling the entire house, but if the main floor isn’t sufficiently cooled, then we might add a second 9K unit.  I’m hoping that air stratification causes most of the heat load to reach the second floor unit, while the cool air it provides also drifts down the stairs to keep the first floor cool.

If anyone wants to throw some suggestions out here, feel free… I’m getting a quote from a Mitsubishi dealer as well as a Fujitsu dealer in the next couple of days.

Thanks in advance, Nathan E

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Replies

  1. Expert Member
    Dana Dorsett | | #1

    Both the 3/4 ton Fujitsu RLS3 and Mitsubishi FH09NA "rated" for efficiency at 9000 BTU/hr cooling, but can deliver about 12,000 BTU/hr under AHRI conditions at maximum speed.

    Between those two, on cooling performance I'm partial to the FH09NA due to it's lower minimum modulation level, 1700 BTU/hr to the 9RLS3's 3100 BTU/hr, which will lead to much longer continuous modulating cycles. The calculated load for upstairs is less than 2x the minimum modulation level of the Fujitsu, but more than 3x the minimum modulation level of the Mitsubishi. (For heating capacity the 9RLS3 wins hands-down.)

    The "rated" capacity of the half-ton FH06NA is 6000 BTU/hr , which is nearly identical to the calculated upstairs load, but can deliver 9000 BTU/hr at maximum speed. But the upcharge for the 3/4 tonner is only a few hundred, and the minimum modulated levels are the same. If you're going to try cooling the whole house with one upstairs mini-split, the FH09 is your better bet.

  2. nefrusy | | #2

    Thanks again Dana!

    So I got a quote for Mitsubishi and Fujitsu, but the Fujitsu quote was for the 12K unit instead (I think the HVAC guy was worried about undersizing). Regardless, the Fujitsu quote was nearly $1000 less than the Mitsubishi, if installed in March due to a promotional rebate.

    I asked the Mitsubishi dealer about reducing the price and she said she'd look at the pricing for LG units. I have not looked up their specs at all until now... it appears they have a 9K "single zone high efficiency" unit, the LS090HSV5. It has a SEER of 23.5, but the capacity ranges from 1023-12625.

    That got me wondering... how much should I focus on the SEER of the units, vs. other considerations like minimum modulation?

    Also, I was quoted for the 12RLS3 instead of the 9RLS3, but the specs are very similar including minimum modulation. Would these two units perform almost identically under the same load, assuming it was somewhere around 5000 BTU load? Or is it likely that the 9K unit is tuned for lower loads and would perform better?

  3. Expert Member
    Dana Dorsett | | #3

    In heating mode the efficiency of the LGs at minimum modulation isn't very good compared to the Mitsubishi or Fujitsu units at minimum modulation. I'm not sure if the same is true in cooling mode. While more modulation range is always more comfortable, and not always more efficient, even though in most cases it is more efficient at minimum modulation than at max.

    A 3/4 ton Fujitsu is also good for 12,000 BTU/hr in cooling mode, the 1 ton is good for 13,600 BTU/hr. From your prior thread the calculated load was 10,600 BTU/hr, so it's not insane to go for the 1 ton Fujitsu, even though the 3/4 ton would cover it- the difference just isn't very much. The minimum modulation of either is still less than 1/3 of your design cooling load, so either one would run very long efficient modulating cycles in cooling mode.

    SEER is somewhat over-rated as an efficiency measure, especially in more humid climates. The slightly larger coils of the 12RLS3 might give it an efficiency advantage when operating in "DRY" mode on those days where it's too sticky to keep up with the humidity in the normal "COOL", mode. It it's a grand cheaper than an FH09, go for it!

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