Sizing a minisplit for a 900-square-foot Chicago carriage house
halcali
| Posted in Mechanicals on
I have a two-level carriage house (900 sq. ft. total). Brick structure, framed in with R-19 insulation, two levels, open loft style.
I’m wondering if the 12000 BTUH Mitsubishi MZ-FE12NA would be enough to heat the space, or should I go with the 18000 BTUH MZ-FE18NA?
Also, is there any reason why I should consider the Fujitsu minisplit too? I’ve had good experience with the Fujitsus in a warmer climate (than Chicago), but the specs on the Mitsubishi Hyper Heats seem hard to match in a Fujitsu.
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Replies
Timothy,
Sometimes I sound like a broken record, but I'll say it again: the first step to sizing a heating system is to perform a heat-loss calculation.
For more information on this topic, see:
Saving Energy With Manual J and Manual D
How to Perform a Heat-Loss Calculation — Part 1
How to Perform a Heat-Loss Calculation — Part 2
Timothy,
One more thing (and I realize that this information is probably coming too late to be helpful, but other GBA readers may benefit from my advice): If the type of insulation you used to insulate your old brick building in Chicago is fiberglass batts, you may have made a mistake.
For more information on this topic, see Insulating Old Brick Buildings.
He might be OK with the R19 fiberglass if the house has bungalow-deep roof overhangs (at least 1 foot per story), little direct rain wetting of the walls, and little to no splash-back wetting at the drip lines.
Without running the actual calculations, unless the place leaks air ike a sieve, and/or has only single-pane windows (and lots of them) it's unlikely it would have a heat load at 0F (Chicago's approximate outside design temp) of more than 13,000 BTU/hr, which would be the approximate output of the FE12NA @ 0F. A single MSZ-FH15 NA would have plenty of margin for even polar-vortex days, but it might be overkill.
You might swing it with an -FH12 or FE12, but do the math, eh? The -FH12 units are more efficient than the FE12s, rating an HPSP of 12.5, to the FE's 10.6. The rated output capacities are the same, but getting 21% more heat per kwh is enough of an efficiency difference to be worth paying a bit extra for.
DIYer here: check out loadcalc.net
YMMV but I had a pro Manual J done and did a careful calc on loadcalc.net. The numbers were within 10% of each other (load calc was higher). I tossed some ball park numbers in (for gods sake, do a sizing yourself - this is just as an example) for a wild guess at your place. I got 31k btus. That number is worth what you paid for it. The point is that you can do a loadcalc quickly and a decent careful one reasonably quickly.
It also implies that 1.5 tons of heating might be on the small side unless you have tightened the house considerably.
Remember that below 5 F, you lose capacity on the mitsubishi hyper heats. 82% at 4 F and 62% at -13 F. If you are thinking that's ok, it will just be cold if it gets to -10 F, double check your thinking. You might find that the heat increase possible at 62% of rating won't keep you much over freezing. Depending on what plumbing there is and where it is location, you might have issues.