So I'd need 350 feet of 3/4 inch copper tubing to match the thermal
transfer of 1100 feet of Nick's True Value special. To get to 500K
BTU/hour for the pool HX, I'd need 700 feet of tubing, about $800.
That's 70% more $ than the True Value special.

Of course, this tubing would be NSF rated to 500 psi+, and would stop
any oxygen diffusion into the cistern. A similar chunk of 200 feet of
tubing would do the DHW heating, and would look a lot more like what
is normally used for potable water inside Californian houses.

(It looks like getting enough copper tubing to make a size X heat
exchanger, just sitting in a bath of hot water, is just as expensive
as buying a gas pool heater with the same BTU/hr capacity. Wow.)

My architect suggested that, since we'll be shooting several truckloads
of shotcrete to make the retaining walls, the incremental cost of a
shotcrete cistern will probably be small, as it won't take them very
long
to shoot it. So it sounds like that's what we're going to go with,
even
though I don't have any hard numbers on the option.

I called a spancrete manufacturer. The sales guy there said that
servicing a cistern with a spancrete top would require a big crane,
and that he sure wouldn't use spancrete. Why not a wood deck?
:-).