Quote:
Originally Posted by abogart
I have been wondering if there would be any advantage to...
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Interesting idea.
Some things to keep in mind, when your furnace fires, the fuel can reach very high temperatures (400F to 600F and higher), so the area of the furnace's HX does not need to be very large to supply the amount of heat that is required to the house.
With fossil fueled hot water heating, temperatures of the water can run to about 160F to 180F, so the area of the HX would need to be larger to transfer heat at the rate you want.
If you are going for lower temperature heat, such as what solar or maybe even a heat pump can provide (90F to 140F), your HX area needs to be larger still.
One of the ways to get a really large area HX that will work for lower temperature heat sources is to use the whole floor as a heat source!
Most of the radiant floor design work that has happened to date in the US assumes fossil fuel heated water, and the water pipe that carries heat to the floor is assumed to carry water in the range of 140F to 180F, so spacings of 12 to 18 inches work out pretty well.
If you want to take advantage of lower temperature water (90F to 140F), you would be disappointed by the performance of a floor that was designed for higher temperature water.
So you would want to increase the 'heat density' of the floor's hydronic system. In other words, the spacing of the tubes that carry the water would need to be closer together.
European hydronic designers seem to be more aware of this than designers in the US. One reason is that energy in Europe costs about twice what it does in the US.
So, long story short is that your idea will work, but you will need a large energy-dense radiating surface area to make it work the way you envision.
-AC_Hacker