>The problem I'm facing though is in finding a suitable hot water storage tank/drainback tank combination that will fit in the existing built-in heater closet space.
My current thought is to replace the existing 50 Gal gas fired heater with a Rheem Solaraide 80 HE (80 gal with heat exchanger), but this will only leave 20" above for a conventional 10 gal drainback reservoir tank which is too tight. I would have plenty of space beside the main tank for something more rectangular (something the shape of a gasoline tank would be perfect), but so far, I haven't found anything like it. I did find something called a Copperstor drainback tank that is an array of spun metal cylinders, 2.5 gals each. This looks like it would work, but at $550 for an array of three, it seems I should be able to find something better.
Drainback tanks seem to be about the absolutely simplest things - especially tanks without heat exchangers.
One more thought, regarding sizing. Bob Ramlow's excellent book on solar water heating says drainback tanks should have a volume equal to the collectors plus pipes plus 4 gals. I measured the collectors and pipes in my set-up and get 2.5 gals total. I don't see what the extra 4 gals is for, unless to provide for eventual evaporative losses etc. Maybe he was thinking about drainback tanks with internal heat exchangers which would have to stay submerged. Any thoughts?
(p.s. if I were the film director, I would probably have bigger heater closets)
