SylvanLMP
Using solar energy to preheat the water is a great idea in certain parts of the country/world
Using a 2nd hand heater even for a storage tank is a BAD THING.
Most if not all residential tanks are not made to an ASME rating unless specified and they do cost more (lots more stringent testing and welding procedures)
A hot water tank is cheap and you can order one just for solar heating applications with all those cute bells and whistles like a circulator (Bronze or other non ferrous type of course)
Plus even the storage tank will require some type of pressure/temperature protection.
In this kind of application you would want to pipe in series as you want the preheated water to temper the incoming cold water when conditions are right YET you have the option of electric or gas back up.
The other piping arrangement was installed improperly BUT a home inspector is not really trained to know why or how to pipe in gas appliances.
There are several proven cost effective piping applications that solar power is designed for, just take your time and do your homework.
If you have any questions feel free to E mail me or contact a licensed Master plumber in your area NOT a handyman.
