I like your creativity. I've been doing solar thermal systems since the early 1980s. My only concern is in the event of stagnation your collector temperatures could easily exceed 200F. I first got into solar service following the installers mistakes. One example was using 50/50 solder at the collectors. 50/50 melts around 350F or so. I can not begin to tell you how many joints I repaired where the solder melted do to stagnation and the formation of steam .