This is an IG pool with a sand filter, used by homeowners in a residential subdivision (I'm the new HOA president). 30' x 50' and approx. 39,000 gallons.
The filter and sand are new this year.
After we backwash, the pressure drops to 18 PSI and stays there for about 3 days. By the fourth day, it has spiked to 30+ PSI, flow slows and we backwash. I know this isn't right and the pool service tech seems as puzzled as I am.
Skimmers and strainer are cleaned regularly.