my problem with water is; at the heat the evacuated tube heaters run, the tanks would have to be vented to be safe. There will be a lot of evaporation in a vented tank with 185° water. I think this will fog up the glass in the greenhouse. fogged glass will greatly reduce the efficiency of the whole system.
as far as what goes under the slab. At the forty forth parallel we have a frost line of six to seven feet. That is why everybody has basements.
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