cwms27

I just installed a Hayward sand filter in place of my old cartridge filter. It has been running for a few weeks but I noticed the other day that after about 3 hours of running the pressure would jump up to 30-35psi from the normal run time of 15. I figured it was time for a backwash as it has been about a month since install. I backwashed it for about 1.5-2mins, then returned the pump to normal operation the next day I noticed the same thing. so I did the same, backwash and return on. This seemed to fix the pressure issue. until about 3 hours later I noticed it high again. I shut the system down, let it sit for about 15 mins before I got back to it and when I restarted it without backwashing or doing anything pressure was fine and again ran for about 3 hours when I checked the pressure was high again. Does anyone know what might be causing this? Thanks in advance for your assistance.