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How to size a mini split for a small addition?

onebassist | Posted in General Questions on

I’m planning a 2nd floor master bedroom addition and I’d like to heat and cool it with a mini split. Using a couple of online calculators ( and I’ve come up with about 5000 btu cooling load and 6000 btu heating load. Looking at the Misubishi H2i units, it looks like both the FH06 and the FH09 have the same minimum capacity range of 1600-1700 btu. The price difference between the two is about $100 with the 09 being somewhat more available.

My question is, will either unit be more efficient for my application and would I be problematically over sized with the FH09? I’m considering the larger unit because I expect we will often leave bedroom and bathroom doors open which will leave the addition somewhat open to the older parts of the house.

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  1. Expert Member
    Dana Dorsett | | #1

    Either can handle the load as long as your 99% outside design temp is above +5F. The FH09 won't be appreciably more or less efficient than the FH06, but would give you quite a bit of capacity margin on the cold end. Think Polar Vortex events- where the daily highs might not even reach the 99% outside design temperature for a couple of days.

    If your 99% outside design temp is +20F or higher it probably doesn't much matter which one you choose but if it's 10F or colder, go with the FH09.

    Opeing the doors and letting either unit heat/cool the adjacent spaces would give it more load to work with is a good strategy for efficiency during the shoulder seasons when the load of the MBR is below the minimum modulated output of the unit.

  2. onebassist | | #2

    Thanks for your reply. I think I will go with the FH09 and use it as you suggested. My town building department uses a design temp of 7F. An ASHRAE list I found has my 99% design temp at 12F and the 99.6% design temp at 7.7F.

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