I'm constructing a homemade solar heater and I'm wondering if I need a check valve after the filter before the solar. I see that most all diagrams show the check valve after the filter but looking at the check valve from Lowes, it looks like it would really restrict the flow. I have an 18000 gallon gunite pool, Intelliflo VF pump, 420 sf cartridge filter, and have what I think is a pretty efficient piping/pump setup. I run the pump for 12 hours a day at 25 gpm and it takes about 175 watts for the pump to function at that flow. So I don't want to impact that efficiency very much when I'm not using solar. Since this is a cartridge filter, is there any harm if water does back flow thru the filter. If it's okay for that to occur, do I need a check valve after the filter?