Oddly enough, I do too :-) I've been thinking about our very cheap solar pond
water heater concept. This might provide all the hot water a house needs with
no backup fuel over a long period of time if the top part of the pond is 140 F.
If it's only 90 F, the conventional water heater might have to add heat every
day, even though the solar collection might be more efficient. This might work
with a "variable flow rate" if the $0 10 W fountain pump only starts when
the shallow collector pond water reaches 140 F and the fountain pump shuts
off when the pond bottom reaches 110, to avoid overheating the pump.