I just finished a batch hot water heater from 2 tanks that i scrounged, a salvaged sliding double glazed glass door, OSB, and a piece of foil covered fiberglass. The glass from the door is 2ft x 6 ft so I made a 2ft x2ft x 6ft box with the OSB and the glass from the doors on top and 1 side. Ilined the box with the fiberglass painted the tanks black put everything together sealed the joints. after being in the sun for 5 hr the water temp was 140 deg I don't know the size of the tanks but enough for a hot bath, large tub. cost so far about $30 mostly for plumbing parts and beer for the friends helped get it on the roof.
