tanks (pre-cast models) can also be a very good choice because they are designed to contain water. They have one significant disadvantage, though: they must be buried. Septic tanks are not designed to be free-standing and will crack and fall apart unless they're installed underground. If that suits your needs, the tank must be waterproofed on the inside with a plaster made of cement and latex.

The best option for most standard residential solar space heating installations is almost certainly fiberglass. The material is rated for high temperatures (check that, of course) and is reasonably priced. Most manufacturers produce tank kits that ship in pieces so that they are easier to get through doorways for retrofits. The kit is constructed in situ and either bolted together (with rubber gaskets to seal everything watertight) or cold-welded using epoxy. If you have a choice, opt for the epoxy seals – they're better.