Open loop solar pool heating systems are installed on the water side of  the pool pump. Closed loop systems are installed submersed into the  swimming pool. The two are different in that open loop has to be  manually drained periodically, while closed loop is usually left for an  entire season without attention. These solar heating systems are highly  efficient and economical with the installation costs, but require a  perfect condition for excess return. https://www.solartubs.com/solar-pool-heating.html