A little while back, we visited a friend in Edgewood who’d installed passive solar water heating. He had hoses on a panel on the roof being heated by the sun. The water from the hoses was pumped into his water heater, making it so that it often doesn’t even need to turn on. He made the external water tank himself with wood and pond lining, keeping the cost really low. He opened the water tank to show me and the water was steaming just from the sun. Amazing! I think our water heater is used to heat our house, so such a setup could be an interesting idea. If I were to try to build it, I’d probably blow up our roof or something.