GBA Logo horizontal Facebook LinkedIn Email Pinterest Twitter X Instagram YouTube Icon Navigation Search Icon Main Search Icon Video Play Icon Plus Icon Minus Icon Picture icon Hamburger Icon Close Icon Sorted

Community and Q&A

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.

GBA Prime

Join the leading community of building science experts

Become a GBA Prime member and get instant access to the latest developments in green building, research, and reports from the field.


  1. GBA Editor
    Martin Holladay | | #1

    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

  2. GBA Editor
    Martin Holladay | | #2

    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.

  3. Expert Member
    Dana Dorsett | | #3

    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.

  4. KeithH | | #4

    DIYer here: check out

    YMMV but I had a pro Manual J done and did a careful calc on 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.

Log in or create an account to post an answer.


Recent Questions and Replies

  • |
  • |
  • |
  • |