There are many materials that will work with a drainback collector. If you are purchasing panels, the manufacturer will have specs and charts to help you do your design homework. However, if you DIY the collectors, that means more design homework up front for you. There are lots of case studies and plans at build it solar where others have tread the path you are taking.
If you install too many panels for your heat load or storage capacity, stagnated panels will exist. If any panels aren't strong enough, you will find out fast. If you don't have a relief mechanism, the system could explode from the high temp and pressure. Not a trial and error friendly rig. Extra homework up front and conservative overbuilding practice will eliminate bad days for you.
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