- 1/2", 600' for $120. 
- Internal volume - 6 gal . 1 gal per 100 feet.
- Large bed of sand to store heat, or even pulverized, dry soil - is sufficient to store large amounts of heat. Even for seasonal storage.
- It is easier than having to maintain a large pool of water.
- Specific heat of water - 1 cal/gram/C vs rock at 0.2 cal/gram/C. Need 5x rock to water. But Void Ratio may be 50%, so one would need 10x the sand compared to water.
- Insulated basement with sand would be ideal.