I just replaced my Hayward ProGrid 36 with the exact same filter. Starting pressure with No DE is 12 PSI. After about two hours the pressure goes to 20-30 PSI. Pool is clear so the filtration debris isn't causing the pressure rise. I run the backwash and it comes out clear. After running the backwash for just a few seconds the pressure will return to the starting pressure. I'm running no DE in it to rule out filtration debris for the pressure rise. Any idea what could be causing this rise in PSI over such a short time?