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Mini Split comparisons

PatriciaCranberry | Posted in Mechanicals on

I have a recommendation from my designer. And I’ll follow up with them on Monday…but anyone out there have any insight into various Mistusbishi models?

Recommended MUZ-FH18NA2/MSZ-FH18NA2. Unit is sized to primarily heat a workshop to approx 60Deg. Per Man J I need 20.9K for heating the space to 60degF. The unit referenced above will provide 20.2K. So thats max capacity.

I found a unit online that is maybe an older model on markdown/clearance: MUZHM24NA2-U1/MSZHM24NA. Its bigger capacity…but what are the differences in the model /what will I loose compared to what I will gain.

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  1. Jon_Harrod | | #1


    What is your outdoor design temperature? The HM unit is rated at 26,000 Btuh at 47F but only 18,500 at 17F:

    The FH unit is rated at 30,000 Btuh at 47F and 20,300 at 17F:

    They have similar minimum outputs (~5150 vs 5400 Btuh) but the FH has higher efficiency (COP) across the operating range.

    If you've checked the inputs on your Manual J, I wouldn't worry about the ~500 Btuh shortfall. Manual J is a pretty conservative calculation. Chances are high that you will never need the full calculated load. And if you find that in extreme cold, the temperature is slipping a bit, you can add 500 Btuh by turning on a few lights.

  2. Expert Member
    Dana Dorsett | | #2

    The outdside design temperature used in the Manual-J is still important, but so is your 60F indoor design temperature, which is different from the 70F design temperature a which they are tested for capacity & efficiency. The difference in indoor & outdoor temperature bears directly on capacity.

    With an indoor design temp of 60F the FH18 would still be able to deliver ~ 20.3 K even at +7F.

    The HM24 would still be good for ~18.5K @+7F.

    At warmer outdoor temperatures they will be able to deliver more.

  3. PatriciaCranberry | | #3

    Design Temp was 18.--which we probably have 8 or 9 days out of the year. Yes.. The 60F is not the normal. I'm wondering if the Hyper-heat unit (improved performance at low temps) is really necessary in my climate zone 3, central Oklahoma, especially given this workshop will be unoccupied every evening/overnight, especially on those cold nights. I'm leaning toward the MSZ 24GL or MSZ 24HM. My designer says 26K would put me at 70F...more normal...and might afford me SOME cooling in the hottest months. I'm seeing online equipment costs for the larger 24K units lower than the 18K unit.

  4. Expert Member
    Dana Dorsett | | #4

    With an outside design temp of +18F and an indoor design temp of 60F you can use the submittal-sheet numbers for sizing the thing.

    The HM24's 18,500 BTU/hr @ +17F outdoors with a 70F indoor design temp will more than cover your 21K load with a 60F indoor design temp.

    The GL24 has a higher tested HSPF, and 24.6K of capacity @ +17F(70F indoors) but also a higher minimum modulation of 7500 BTU/hr vs. 5400 BTUh/r @ +47F. (or +37F with a 60F indoor design temp- it's minimum output will be higher at +47F)

    If your 20.9K @ 18F outdoors, 60F indoors is correct your heat load rises at about 500 BTU/hr per degree below 60F. Your January mean temp is about 40F, so you're looking at a mean load in January at a delta-T of 20F or about 10,000 BTU/hr. It'll be even less the rest of the heating season. See:

    Ideally you'd want something that can still be modulating the ~5000 BTU/hr load you'll have at 50F to keep it from cycling itself into lower efficiency.

    The GL24 can't modulate much below 7500 BTU/hr @ 37F at your 60F indoor temperature even less at higher outdoor temps and would be cycling on/off before outdoor temps reach 42F. That's well below the typical mid-January daily high of ~50F.

    The HM24 will probably still be modulating at ~44-45F outdoors with a 60 F indoor temp, but not at 50F.

    The advantages of the FH18 is both higher tested efficiency, and a lower minimum modulation, and still wouldn't be a bad choice. It's maximum cooling is only 1500 BTU/hr shy of the HM24's max, and if priced comparably to the HM24 it would be an better choice for your situation, since it can dial back to 5150 BTU/hr for at least a hint more modulation.

    A Fujitsu 15RLS3 is a better fit, with a few degrees more modulation range and even higher as-tested efficiency. Even though it's only rated for 18,000 BTU/hr @ +17F outdoors/ 70F-indoors it should still deliver over 20,000 BTU/hr @ +17F out/+60F-in, and it can throttle all the way back to ~3100 BTU/hr @ +37F-out/60F-in, which means it'll probably still be modulating at ultra high efficiency at +50F, so it'll be modulating most of January at least, if cycling a bit at the beginning & end of winter, and cycling pretty much all the time during the shoulder seasons.

    With the load information presented and the weather data inferred, in the preference would be:

    1 15RLS3

    2 FH18NA

    3 HM24NA

    4 GL24NA

    You'd really only want to look at the 2-tonners if cooling was a priority.

  5. PatriciaCranberry | | #5

    Reply to Dana: What about a larger Fujitsu with desirable modulation range? . I "say" I want the workshop to stay at least 60F in the winter but I'm not opposed to 65F...and maybe some cooling capacity in the hottest month of summer?
    Thanks for schooling me on cycling vs modulating. ..

  6. Expert Member
    Dana Dorsett | | #6

    The 15RLS3 is the biggest single-zone wall coil system Fujitsu makes. To go bigger with a wall coil would require a multi-split compressor (all of which have minimum modulation rates in the 5 KBTU/hr or higher range.)

    Fujitsu does make a 1.5 ton mini-ducted system (18RLFCD) that still modulates down to 3.1K @ 47F out/70F in, and delivers 21.6K @ 17F out/70F in.

    One nice feature of the _ _ RLFCD series is that (unlike the competition) they can be mounted vertically. For a heating dominated climate like yours mounting it a few feet from the floor with the return plenum pulling from near the floor improves the as-used efficiency compared to a wall-coil mounted at head height or higher. That also makes maintenance & filter cleaning easier. If you go this route use only hard wall duct and keep the runs ultra-short, with only radiused ells (no squared off sharp throated ells).

    Take a peek at John Semmelhack's pictures of a vertical RLFCD installation with super-short ducting in a tiny framed out utility closet in post #14 of this thread:

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