On my solar heater flow lines I have a two position valve which I believe is used to control the amount of water to be heated. The valve is positioned between the input line to the solar panels and the heated water return to line to the pool. In my picture the input flows to the right side of the valve and the return line flows to the left. I have tinkered with the valve setting for a couple of months but cannot see any difference in the water temp in my pool. Can someone explain how I can use the valve properly to control my water temp?