Heat Loss Calculations and Minsplit Sizing
I am about to build a new house and have been playing around with heat loss calculations to gain some perspective on the size of mini-split I need before I start asking vendors for their estimates. I was hoping people here could give a once over of my results and answer a few questions that I have.
The 26’x36′ 1.5 story house is located in midcoast Maine just west of Belfast. It will be constructed using 2×4 double stud walls, with 4″ separation between the walls. I plan to insulate using dense pack cellulose, I have estimated a whole wall R value of about 32 for the stackup. The windows will be triple pane with a whole window R value of ~5. The ~R50 vented roof will be built using 2×12 rafters, rockwool will be placed between the rafters and 1″ of polyiso will be attached inboard of the framing members. A shed dormer will occupy about two-thirds of the south side of the second floor and will have the same amount of insulation in the walls and roof. The house is being built on a full unfinished basement. It is assumed the basement walls are insulated to ~R20, a couple inches of foam (~R10) will be placed under the slab.
Excluding exfiltration losses, my spreadsheet calculations indicate a heat loss rate of ~18500 BTU/h. This loss is dominated by losses through the basement walls and slab, which account for close to half of the total. Assuming a mechanical ventilation rate of 60 cfm, exfiltration losses are estimated at about 4500 BTU/h.
A little more background then some questions. My main source of heat is going to be wood. I keep doors to upstairs bedrooms closed throughout the winter, warming those rooms with heat moving up from the first floor. My current plan is to install a ductless mini-split on the first floor which will serve as a back up. I am contemplating installing baseboard electric on the second floor to keep the insurance company happy and provide heat upstairs if it is needed.
First, I would love to get a sense from others if my results seem to make sense. And,, assuming they do, what they may translate into the size of the mini-split I should consider. Second, as stated earlier, losses in the basement dominate my non-exfiltration loss calculations. Should these losses carry the same weight as the losses from the first and second floors given the unfinished (and unheated) nature of that level? After all, heat does tend to rise. Any insight will be appreciated.
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