MItsubishi or Fujitsu, dual or single
I am renovating an 800 sq/ft Shotgun style house. I have had icenyne spray foam insulation sprayed throughout; ext. walls, attic, and crawlspace are all sealed. I am trying to decide what to do with hvac and am looking into mini splits. I prefer the ceiling cassette unit for looks. I am in the Louisville, KY area. The floor plan is mostly open with the living room/kitchen being one zone and the bathroom, bedrooms and utility room being separated with two doors. I was looking into getting a dual zone system with a ceiling cassette in the living room/kitchen/bathroom area, and another ceiling cassette in the bedroom which would also feed the utility room. Do you think i will need -15F heating? it has been below zero here the last couple winters. I am a little overwhelmed at the options and capacities. Can you give me some suggestions as a direction to reasearch?
Thank you!
Erich
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Replies
Erich,
Q. "Do you think I will need -15°F heating?"
A. The 97.5% heating design temperature for Louisville, Kentucky is 10°F. (Source: Degree Day and Design Temperatures.) So no, you don't need a unit capable of operating at -15°F.
Q. "I am a little overwhelmed at the options and capacities. Can you give me some suggestions as a direction to research?"
A. Start with a heat load calculation and a cooling load calculation. For more information, see these articles:
How to Perform a Heat-Loss Calculation — Part 1
How to Perform a Heat-Loss Calculation — Part 2
Calculating Cooling Loads
Then you probably want to read this article: How To Buy a Ductless Minisplit.
Mini-splits without a specified capacity down to -4F or lower typically have both low capacity and lousy efficiency below +20F. Something like the Fujistu xxRLS2 series (specified to -4F) would still deliver very good performance though.
At 800 square feet a typical code-min house these days would come in with a heat load at about the output capacity of a single 3/4 ton mini-split @ +10F, not more than a 1-ton. That limits the number of options. If you have an insulated sealed crawlspace deeper than 12" or in the sealed-conditioned attic you can probably drop in a 1-ton Fujitsu mini-ducted version and run it as a single zone. The -12RLFCD and the 1.5 ton -18RLFCD are fully specified down to -5F.
A second opinion on design temps, shows a 99% outside design temps of 13-14F for Louisville are, and a 99.6% outside design temps of 7-8F:
https://www.captiveaire.com/CATALOGCONTENT/FANS/SUP_MPU/doc/Winter_Summer_Design_Temps_US.pdf
A third opinion puts the 99th percentile temparature bin at 10-12F:
https://www.extension.org/sites/default/files/7.%20Outdoor_Design_Conditions_508.pdf
So, you could correctly size equipment that specifies output only down to +5F and still do OK. But almost any 2- cassette solution is going to be sub-optimally oversized for one or both of the zones. It's either a unit sized for the whole-house load installed in the bigger open area, or a mini-duct unit.