This is very similar to a project that was on This Old House years ago (Bob Villa era). There was a group in Maryland that was building well insulated modular homes. To help with efficiency they buried a large water tank under the basement slab. They insulated the tank from the basement floor. They would run the water through a water to air heat exchanger. They could heat their house as well as cool the house utilizing this method. I'm not sure that sand is a real good source for storing heat. I believe that water is much better.
The specific heat of sand is .2 which is 1/5 that of water. If you can get a good recycled tank at reasonable cost the heat storage and mobility using a water system would be far more efficient and reactive to management since convective flow and conduction constantly brings heat to areas of loss.
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