DrDan, The floor doesn't care how you get the heat. You can burn your waste oil, lanolin from your sheep, peanut hulls, cotton linters, dried corn, or more practically your waste wood scraps. The only safe way I know to burn sawdust is after it is pressed into pellets. Long way to say you could have an alternative fuel source. The valving to accomodate it is simple and relatively inexpensive. In-floor hydronic does not lend itself to economical/convenient part time heating. The thermal lag of the recommended slab thickness (5 1/2 inches) for in-floor hydronic can be considerable.
Some sanity check items:
I would "run the numbers" on extruded unicellular insulation in thicknesses greater than 2 inches for under the slab. (Blue board etc.) You might get a payback in less than 5 yrs, maybe 3 or less. If this were a continuous occupancy structure (like a fulltime residence) then a heat bubble in the earth beneath the bld isn't quite so wastefull providing you stop the hemoraging of heat at the perimeter.
I sure wouldn't just throw in the towl on electric heat. Run the numbers on an in floor electric resistance heating grid. No, water, tubing, pumps, etc. Uses electricity 100% efficiently and delivers the heat where you want it without going through intermediate hocus pocus.
Heat pumps with a high SEER (seasonal energy efficiency ratio) can use electricity to heat the floor with an efficiency of maybe 300% or more (check some web sites). How do you get more than 100% efficient? Easy. With resistive heating (infloor grid, water heater, or the like) one Killerwatt (inside joke) of electricity yields one kilowatt of heat. A heat pump doesn't "burn" electricty to make heat, it uses electrical energy to move already existing heat from one place to another. One kilowatt of electricity in a high efficiency heat pump can yield 3-400 kilowatts of heat. That is until the heat source (outside air or whatever) is so low grade of "heat ore" that the process just stalls. With many heat pumps this is when the outside air goes into the teens or lower. But wait, there is a way 'round this.
Got a pond? Got enough land that will trench efficiently near the building to support a geothermal heat pump? Got a well that produces really good? Even in northern Minnesota or, dare I say it, Minot, ND the temp a few feet down in the ground is about 50 degrees F all the time, all winter and both days of summer. It is dead easy (with respect to the thermodynamics) to mine heat out of 50 degree dirt. So heating in the winter is easy. If you want to install some fan coil units to give you AC and dehumidification in the summer that is OK too as you already have the pumps. Do a web search on geothermal heatpumps (gex?) The pond? Toss a weighted coil of PEX in it and take heat out of the water. In summer heat it with your waste heat. All the heat need not be waste as you can use the rejected heat from the AC to heat your DWH (Domestic Hot Water, Jacuzzi, swimming pool, etc.)
No pond? Don't want a pond? OK, dig a trench and bury the PEX. The underground PEX or under pond water PEX replaces the air based heat exchanger with freon to a water filled heat exchanger with water or dirt instead of air supplying the heat (or accepting it in cooling season.
However you get the heat, insulate the perimeter of the slab to a level well below your frost line to avoid a continuous energy nose bleed.
I could go on for pages but I gotta go pick some pears as a gift for some friends. There is yet more you migt want to consider before throwing yourself on the mercy of someone with a different set of priorities than yours, As in the folkks who want to sell you something.
Patrick
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