David Meiland was intrigued by something he’d read in a Breaktime post at Fine Homebuilding magazine in which a Minnesota builder discussed plans for a heat-storing layer of sand 4 ft. thick below the house slab.
PEX tubing would dump heat gathered by solar collectors into the insulated layer of sand in summer, and extract it during the winter. Although sand isn’t an ideal material for this type of system, it’s cheap and easy to work with.
Anyone care to comment? Meiland asks in a recent Q&A.
Sand is not the best option
This idea isn’t new, writes J Chesnut. While Chestnut has heard architects discuss it many times, and despite the FHB poster’s experiences, he finds the sand-bed approach too complicated.
“There are too many variables that can’t be modeled and analyzed as a system (as far as I know),” Chesnut says, “daily weather patterns, heat storage capacity of the bed, efficiency of capturing solar heat, the rate of thermal transfer, the heating design load, heat distribution, etc.
“This strikes me as a ‘shooting from the hip’ approach.”
Given the vagaries of a sand-bed design, the house would probably need a back-up heat source capable of meeting the entire design heat load, a redundant second system that only adds to construction costs.
“Intriguing idea,” adds GBA advisor Martin Holladay, “but the data don’t back up the theory. In short, when winter comes around, the sand isn’t warm enough to provide useful heat.
“Building an insulated sand bed costs money — and the yield on the investment is nil or very low. Storing useful amounts of heat for more than three or four days is hard.”
“We did this about 25 years ago,” writes Tom of American Solartechnics.…