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Manual J for Mitsubishi Hyper Heat Ductless Minisplits

kieran973 | Posted in GBA Pro Help on

I just finished doing a heat load Manual J on Cool Calc. This is for sizing a Mitsubishi hyper heat ductless system. Could anyone chime in if you think something is off with this heat load calc?

Zone 5A, southern Connecticut, 2 story house, built in 1925, unconditioned basement, fieldstone foundation. Total above grade sq ft: 1893. Total heated space: 1,533 sq ft. Total heated volume: 12,444 cu ft. Most of the house sits on the unfinished fieldstone basement, but there is a 250 ft, first-floor addition (one bedroom, one bathroom) that sits over an unconditioned crawl space.

Attic recently insulated to R-49, rim joists and crawl space walls recently insulated to R-15. The house is leaky. Blower door test done this week: 3618 CFM (or 201 ELA if I have that right). The guy who did the test also took infrared images of the first floor walls and said there’s some insulation in there, but not much. His best guess was R-5. All windows in the house, except for those in the first floor addition over the crawl space, are double pane, low E argon, .3 U value – but they’re not fancy, they’re cheap vinyl replacement windows from Lowes. The windows in the addition over the crawl space are ancient, failing single pane windows, but I’m about to replace those with essentially identical vinyl replacements to the ones in the rest of the house, so for the purposes of the Manual J, I inputted all the windows in the house as double pane, low E, .3 U value.

Winter outdoor 99% design temp: 13F
Indoor design temp: 68F

Total House Heating Load, according to Cool Calc: 42,377* btuh
1st floor (1,001 sq ft) Heating Load: 26,972 btuh (63.65%)
2nd floor (532 sq ft) Heating Load: 15,405 btuh (36.35%)

*For the wall insulation, even though the best guess is R-5, the only choices in Cool Calc were none or R-11, so I put none.
*I also have no idea what is in the shallow, almost flat roof over the addition (roof slope of about 10 degrees), so I again put no insulation. So gives these two conservative assumptions, the true heating load may in fact be something like 5,000 btuh less?

I also did a fuel-use heat load calc using Dana Dorsett’s method in “Out with the old, in with the new”, and I came up with 43,920 btuh whole house load. But importantly:
1.) this is AFTER multiplying by the 1.4 ASHRAE oversizing factor
2.) this is also AFTER then multiplying by .7 to account for the recent insulation added – my guess is that we cut 30% of our total heat load by insulating the attic from R-0 to R-49 and the basement rim joists and crawl space walls from R-0 to R-15.

I’ve also had two quotes from two Mitsubishi diamond dealers who have both more or less calculated the first floor heating load as 30,000 btuh. (One of them calculated the whole house heating load as 48,000 btuh. The other did not offer a whole house heating load calc.)

Does this seem like it’s in the ballpark?

If these numbers do look more or less OK I’m going to start sizing the specific indoor and outdoor Mitsubishi units.


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  1. kieran973 | | #1

    Did some more fine-tuning to reflect the R-5 wall insulation (assumed) and R-15 rim joist insulation (installed) and came up with a whole house heat load of ~35,500 BTUH.

    So I was thinking that one 18,000 btuh Mitsubishi hyper heat unit for the 1st floor and one 12,000 btuh Mitsubishi hyper heat unit for the 2nd floor would suffice. Because:

    99% design temp: 13F
    Downstairs heat load at 13F: 22,500 btuh
    Mitsubishi 18K wall mount unit max capacity at 13F: 25,666 btuh


    Upstairs heat load at 13F: 13,000 btuh
    Mitsubishi 12K floor mount capacity at 13F: 14,200 btuh

    But I'm also wondering about output vs. capacity during average outside temps. The average annual ambient temp in my area is 45F. This temp would obviously require much less output from the mini-splits. So what is the optimal ratio of a mini-split's output to its maximum capacity? What is the optimal ratio of a mini-split's output to its rated capacity? By optimal I mean both energy efficiency but also the long-term maintenance health of the unit and its parts. At 45F, the ratios of the 12K btuh and 18K btuh seem kind of low, but I have no idea whether that's a good or bad thing:

    18K wall mount output/max capacity ratio at 47F: 8927 btuh/30,000 btuh = 29.75%
    18K wall mount output/rated capacity ratio at 47F: 8927 btuh/19,000 btuh = 46.98%

    12K wall mount output/max capacity ratio at 47F: 5132 btuh/22,800 btuh = 22.5%
    12K wall mount output/rated capacity ratio at 47F: 5132 btuh/13,000 btuh = 39.48%

    In other words, what is the ideal (healthiest and most efficient) power level or gear for these units to be running at/in most of the time? Full-steam? Half-steam? 25% steam? Etc.

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