Montana Shop Hydronic Heating
Thanks for taking the time to read this. I’m currently in the process of building a 42 x 80 x 18 shop with SIPs. I used LoopCAD to design 5 ~450′ loops of 3/4″ pex for hydronic heating in a 6″ thickened edge monolithic slab with monoslab ez form arctic forms and 2″ XPS under the entire floor. LoopCAD calculated my Manual J8 heating load at 56,500 Btu/hr for 60F design temp. Electricity is $0.05 kWh after a base customer fee through my electric co-op, Propane has averaged around $1.30 gal over the last couple years. So it seems based on the calculations from those numbers, I should probably install an electric boiler, like the ones from Electro Industries. As I understand it, my peak heating need would be 56,500 to maintain 60F, so I should size my electric boiler around 55-60kBtu. Using a base temp of 50F, I estimate that it should cost around $1300/yr to heat. Does this all sound probable with an electric boiler being the best solution? I may install a couple of heat pump mini-splits in the future that would obviously have better efficiency to supplement a warm floor, but I really wanted the comfort of a warm floor for working on. The slab is finished, and the building is just starting to go up. I appreciate any suggestions and am happy to provide any details that I should have added.
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At least look into a water to air heat pump (eg, Spacepak or Chiltrix) to handle the majority of the load at a much lower $/btu. Or possibly even a water-to-water heat pump (you would need a source).
With that cheap of electricity, the resistance boiler (or a pair of 5kW water tanks modified to run both elements) is the way to go for the floor heat. A single oversized hyper heat mini split would also help in reducing your running costs with probably less than 5 year payback. It would also help in getting the place up to temperature quickly.
I would also run your place through some of the on-line load calculators (ie loadcalc.net), my rough hand calc puts the building heat load at about 3/4 of that assuming R30 walls/roof and -10F outdoor design temperature. Leaky garage doors can definitely add a lot of load, so maybe not oversized after all.