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Getting the Right Size Minsiplit

RustyShackleford | Posted in General Questions on

I’m looking to install a minisplit for the main living space of a mountain cabin (that currently relies on electric-resistance heat).   I’ve run the Manual J calculation (using https://www.loadcalc.net) as well as the alternative sizing tool at https://hvac.betterbuiltnw.com .   They agree remarkably well, that an 18Kbtu/hr unit would suffice.  Almost – and I’d hate to get it wrong.

I like an 18K Fujitsu unit, specs here: https://ashp.neep.org/#!/product/51830/7/25000///0 , particularly its remarkable minimum output of 2800 btu/hr at 47-degrees.   That is less than a hair dryer !

But the 24K version of that model has a minimum 47-degree output that is barely higher, at 2900 btu/hr: https://ashp.neep.org/#!/product/51831/7/25000///0 , while providing a bit more margin for error at 5 degrees, and costing maybe $200 more.

Why not go with the 24K unit ?

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Replies

  1. Expert Member
    NICK KEENAN | | #1

    I have to wonder if those are the same unit or built on a similar frame.

    If you look at the min column, both units have identical numbers for BTU/hr and COP. But the 24k has an EER of 9.2 and the 18K has an EER of 12.0. At rated output the 24K has a COP of 2.71 and the 18K has a OP of 3.52. That's what you expect to see if you just took the same unit and derated it.

    Usually the issue with oversizing is when you're cooling, how low you can turn it down. If the cooling is oversized for the space you get short cycle times and poor dehumidification. But both units seem to have identical low cooling performance.

  2. RustyShackleford | | #2

    Interesting catch. Maybe so.

  3. RustyShackleford | | #3

    Although the larger one specs 20amp breaker, instead of 15amps.

    1. Expert Member
      NICK KEENAN | | #5

      Also what I would expect if they're the same equipment and one is just the derated version.

  4. joshdurston | | #4

    You should confirm the min fan speeds as well. The algorithm these things run off isn't purely matching output to load. They seem to have some sort of discharge air temperature control as well as fan speed control. So depending on the entering air temperature and your lowest fan speed, the real-time min output may be substantially more than the rated min.

    If the min fan speeds are similar and the min outputs then in may be worth considering stepping up. It's often the case that several units in a size range may share indoor/outdoor components, and this can lead to some odd steps, where going up one size leads to a doubling of the min output, or in your case the min staying the same despite a higher capacity.

    1. RustyShackleford | | #6

      According to the product pages at ecomfort-DOT-com, the minimum airflow (for the indoor unit) are 241 and 306 cfm, respectively. Quite a bit larger difference than the minimum heat output that I quoted.

      1. RustyShackleford | | #7

        I also notice that although the maximum cooling performance is quite different, the heating performance (at 5 degrees) isn't very different, 18.6 and 19.5 kbtu/hr, respectively. Although, I suppose if I'm sizing the unit for an indoor-outdoor temperature differential of 60 degrees or so, that 900 btu/hr difference is worth about 3 degrees in higher inside temperature.

        I don't care about the cooling very much. This house sits at 4200ft and is always cool. Most of the nearby houses don't even have A/C (and heat with electric resistance).

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