As I said, building a heater is potentially life threatening. You seem familiar with the safety devices typically used on water heating systems, so I presume you can experiment safely. But I didn't know that when I posted. There have been 3 or 4 prior posters this year with some other DIY pool heating system ideas and not one of them had a clue about safety.

When I post as I did above, I'm not only writing for you -- since you hadn't mentioned T&P valves, etc -- but also for all the lurkers who will Google "DIY pool heater", and end up here.