Solar heat storage in sand bed under house slab?
Can anyone comment on the following technique? This is excerpted from a Breaktime post at http://forums.finehomebuilding.com/breaktime/energy-heating-insulation/pex-layed-sand-between-concrete-insulation
What you are thinking of doing is what we are already doing in the solar side of heating. I am building a solar heated house in st paul Mn using FOUR FEET of sand below the slab. This sand has it’s own pex tubing for injecting the heat from the hot july and august sun, which will provide heat for the house through the new year!
In my house, the sand bed is fully insulated on all sides, top, and bottom, so it is a little different than what you are doing. Additionally, the slab has it’s own pex heating loop since it does not get heat directly from the sand.
What you are doing is very similar to the common design in central wisconsin advocated by Bob Ramlow. In his design, the sand below the slab is not insulated from the slab and it acts like a big ‘flywheel’ to hold heat energy for a few days and sometinmes months, if the sun doesn’t shine. This is a well proven design with many examples up and running for many years.
Sand is not an optimal material for moving or storing heat. We use it because it is cheap, easy to work with and it won’t leak all over the place causing your house to collapse. My gut feeling is that you want a bit more than two inches to make it through the day. That depends on the heat loss of your building, how hot you get the sand, and whetherit is a house or a utility building. The farther down the PEX is, the slower the response to a heat call will be. I think the Ramlow method uses pex in the sand and pex in the slab, but the slab PEXmay be optional. His followers tend to use two feet of sand, but thay are banking weeks worth of heat, not just the 8 or 10 hours you need.
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