Heat Loss Calcs for Superinsulated Hot Tub
Hi, I recently built a new super-insulated house and through that process learned a ton about building science, insulation, airtightness, etc. I am now working on an unconventional DIY hot tub project where the same principles are applying.
I am wondering if anyone can help me create a heat loss calculation and thus, an estimate of annual heating cost on a DIY concrete/stone hot tub?
Design temp (Northern Wisconsin) = -16°
Target hot tub temp = 102°
Dimensions = 10’6″ x 8’6″ (126″ x 102″) outside dimensions
Perimeter = 38′
Volume of entire envelop (including 10″ concrete walls) = 319 cubic feet
Wall is 10″ concrete poured wall with 4″ of XPS on outside
Walls are 43″ in height and are buried underground 50%.
No current insulation under slab, but I could add 1.5″ if it made a big difference.
Heating degree days at my location (degreedays.net) for base temperature of 102° = 21,560
I tried my hand using a calculator that I used for rough estimates on our home build. Treating the entire thing like a basement with an insulated lid on it. Those calcs show a heat loss of 6,392 btu/hr.
Can someone help me verify the basic accuracy of this estimate?
Then, how do I transfer this into estimated cost to heat? There will be a 5.5kw electric heater (Balboa BP7) and a pump to circulate the water. I am not sure what specs need to be known about the heater to estimate this part.
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