I have an (older-35+ yrs-Anthony Sylvan) in-ground sanitized by a Pentair FNS 48 DE filter and have struggled with this problem for some time. When set to recirculate, backwash or rinse, the flow of water is powerful end it appears perfect. When set to filter, everything starts off fine but after about 30 minutes The water level in the pump basket slowly begins to go down and eventually hovers just above the inlet port to the pump basket. At this point the flow of water also decreases significantly but the pressure in the filter jumps up about 10.
The increased air in the basket initially had me believing that there was an air leak. However, I am confused since the same conditions do not persist when on recirculate or backwash or rinse.
I use an aquabot to vacuum the pool so I doubt that is my problem. I’ve been here only 4 years. Replaced the pump motor last year after the existing one burnt out.
I am considering pulling out the fins and soaking them in muriatic acid to see if that helps.
The main drain has never functioned (not sure why) and when the pool company came to open the pool last week, (a new guy) tried getting it to work and was unsuccessful but I know he was opening the valve back and forth between the skimmer and main drain. By the way, there is only one skimmer.
Please help!!!