Solar should not get so hot that you can't use flex, BUT, there is no way you can tie that into your normal pump system, you would have to have a separate return to the pool (I think Poconos does it this way, he has a return running on the deck under his diving board just for solar - excuse me in advance if I'm wrong! Someone is doing it this way). Generally, an efficient solar system will return water that is no more than 5 degrees or so above the current pool temp.