I'm getting ready to buy all the plumbing I need to hook up my solar panels to my pool plumbing. The builder is installing a 3 way valve and a check valve just before the 3 way. I'll be adding an additional check valve on the return line from the solar panels. I've seen some schematics that show a ball valve in the straight pipes going to and coming from the solar panels to isolate the panels. Why is this necessary? If I have a check valve and the 3 way valve, I can bypass the solar panels, so I don't see the purpose of the ball valves.