My pool is 4 years old and this is the first time I am having this issue: Despite several backwashings, opening the filter, removing and cleaning the filter grids (several times, and the grids are not dirty)I am getting high filter pressure readings. Progressively and gradually over a period of days-running 7 hours/day, the pressure rises from ~11psi to 20psi or greater. This causes the flow to decrease to my heater and shuts the heater off(safety mechanism...but also tells me the gauge is correct and non-faulty). This past time I avoided adding DE to the filter to see if that made a difference and the pressure again gradually increased after 6- 8 hours of running. I had the filter apart to check for a physical obstruction-none, my returns are all open and unobstructed. I released the air pressure relief valve at the top of the filter and only water came out-no air, so I don't think its caused by a suction air leak. This is driving me crazy...any ideas? What am I missing?