PEX in a sub-slab sand box; where?
For you folks who put a sand box under your slab and heat it, where do you put the PEX? I’ve seen it at the bottom, but it makes more sense to me to have it near the center line.
My reasoning is thus (assuming you want the sand uniformly heated.): Say you have 2′ of sand under your slab, and foam below the sand. First, assume the heat in the sand is uniformly distributed, ie, it’s all at the same temp. To heat up that sand reasonably uniformly, you’d want the PEX at 1′ below the slab and 2′ OC. However, since the sand is likely going to be cooler at the bottom, you may want the PEX a tad below 1′. Too, the top of the sand will lose heat faster than the bottom, so you may want the PEX a tad above 1′.
Then you’ve got the problem of the sun (and your slab-heating method, if you have one) heating the top of the sand more than the bottom, so you may want your PEX below 1′. Not being able to move the PEX as conditions change, where are people generally locating the PEX, and what is the reasoning?
Second question: Since sand is not going to “take” heat as well as concrete, what size PEX, and what spacing? Siegenthaler says the pipe size does not make a big difference in delivered heat, despite the linear differences in circumference. BTW: I’m not interested in sub-slab water storage right now; I’ll have that super-slab w/ my wood gasification boiler.
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