I know someone is heating there hot tub with thermal panels in the summer. I run a closed loop antifreeze system (Ohio) and I want to add it in-line some way. Maybe just a ball valve open in summer and closed in winter so I still heat my house water too. But how to control the temp. A flat plate exchanger will work but then how to control temp? Differential tied to some kind of electric valve? How are you doing it? Show me some pictures.