Ductless minisplits – Zone 5 – with basement
Hello folks, this is my first post at GBA so bear (beer) with me. My son and I are building a new for my wife and I to live our days out in, two story bungalow style with a full basement under.
Exterior Walls – 2×6 16″ on center with 2′ XPS on exterior, wet blown cellulose in the bays. 3/4″ rain screen furring over foam, cedar lap siding. R19 in the bays + R10 = R29+
Ceiling – 2×10 sealed vault flash (core bond) + batt – R49
Basement Walls 8′ (6.5″ below grade/ 1.5″ above) with 1.5″ polyiso + R13 Batt in 2.4 stud wall over foam – R22
Basement Slab – 2″ XPS
Windows/Patio Door – Milarg Essence casement average U25
I have exhausted myself with spreadsheet upon spread sheet trying to come up with my own head load calculations, all from data on this site. Certainly my methodology is imperfect, but how much so I question. The below grade basement walls I used a Delta T of 20 degrees assuming ground temps of 50. Same for the basement slab.
Delta T used for all above ground surfaces was 82 degrees. Design Day = -15
I calculated in a 25% framing factor, the window U factor and ran my heat loads for each floor. I then ran heat loads by floor as I was hoping to use Mitsubishi Hyper Heat mini splits. I did it this way because of the extremely open layout of the main living floor, and the understanding that for the upstairs it appeared that more than a single head split would be overkill anyway despite the two separate rooms (bedrooms) with doors.
Unless I did something terribly wrong I shocked myself by seeing how much my weighted average R/U factor dropped as a result of the windows and doors. Here’s the data Assuming 8′ walls, and adding the floor joist bay to the basement above grade calculations, and adding the 1′ floor space between first and second floors;
Total First floor wall surface = 1,476; 906 @ R30 wall cavity, 308 @ R17 (framing factor) , and 268 @ R 4.348 (window r.o.)
I came up with a weighted average of R22.8 WOW!
Did the same for upstairs. R24.84 The upstairs has a 2′ knee wall before the vault starts. Added the 1′ of floor cavity to the upper level (had to choose one)
So, in trying to size the heating options for mini splits to within up to 50% greater BTU’s than the house calls for, I’m left with trying to calculate air leakage now, but having even more frustrations with that.
Additionally, as you can see by the attached floor plan’s, I plan to heat the house primarily with a wood stove in the living room opposite the stairwell wall. A Pacific Energy Alderlea T5. Of course for lazy days, vacations, and resale, I want to be sure that I have another “primary source” in the mini splits should for some unforeseen reason we sell the place.
To make it clear, I do plan on having an open stairwell both up to the 2nd floor bedrooms and down to the basement. I’d like to heat the basement, although I see only utilizing a portion of the basement marked laundry room/bathroom for now, and possibly the room marked family room down the road.
I came up with the following heat loads not including any air leaks being calculated yet. The house will be presumably tight, using Prosoco caulk at all window bucks, sheathing seams, top and bottom plates, rake wall plates, etc. We’ll do our very best.
Basement Wall Surface Area = 1,458 sq. ft., of which below grade is 1,053, above grade 339, and windows (egress) 66. And a slab of approx 1,330 sq ft. For the walls I come up with a weighted average R factor of just 20.06.
Again, the delta (if I’m correctly doing this) for below grade is 23 degrees, given ave soil temp of 45, same for the slab, though I know at the near ground surface the delta may be higher due to frost layer.
Below Grade = U.0476 x 1,053 sq. ft. x dt 23 = 1,152 btu
Above Grade (weighted for windows) = U.0567 x 405 sq. ft. x dt 83 = 1,905 btu
Slab = U.1 x 1,330 sq. ft. x dt 23 = 3,059 btu
*Total Basement BTU = **6,162
* Don’t plan on heating the entire basement unless useless to try not to.
**surprising how much the floor need here.
How many more BTU’s to add for leakage? 4 egress windows.
– First Floor Living Area – Wide open other than office with french doors.
Total Wall Surface Area x Weighted Average U x d.t. 83 = BTU + leakage
1,476 sq. ft. x U.0438 x d.t 83 = 5,365 BTU
– Second Floor Bedrooms Area – 2″ knee then vault begins, full sealed vaulted ceiling
Walls = 954 sq. ft. x U.0376 x d.t. 83 = 2,977 BTU + leakage
Ceiling = 1,644 sq. ft. x U.025 (24′ o.c. 20% framing factor on R50) x d.t. 83 = 3,411 BTU = Leakage
Total Upper BTU needs 6,388 + leakage
I’m exhausted !!! But there is more….
How to heat the basement. Baseboard electric? Small pezio electric started smallest gas decorative stove. I have a 12,000 BTU unit in my carriage home. The gas with no electric assist would act as a fall back for the whole house should we be gone and the electricity goes out deeming the mini splits inoperable.
How big and where to place mini split(s) for main and upper heat. And cooling, not that Bozeman requires much cooling at night when you’re sleeping.
Two splits, one upper and one lower, and some basement heat source is my guess
But the leakage question???
Looks like I can’t post attachments here.
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