My in ground, plaster/gunite pool really takes a beating over the winter here in Northern Virginia. It is bordered on two sides by tall trees, and all that stuff falls on it, of course. I always have it covered, but the cover is permeable, and it has less than a perfect fit, so when I open in the spring, I have lots of STUFF in the pool. Aside from grit, dirt, pine needles and leaves, it's a rare opening where there are not one or two dead critters. And of course algae! Nonetheless, for about a ten year period ending in 2015, I would use my net, my Polaris 380 pool sweep, and the Pentair FNS 60 pool filter to get things ship shape. All during that period, I would achieve good water quality, with all foreign material removed, in just 3 backwashes of the filter. It is large for my size pool (~14,000 gallons), but that is what I wanted.
My filter runs at 9-10 PSI when clean and freshly charged. I backwash when it hits 18 to 20.
Of course, along with all the filter ops, I followed the TFP regimen for water chemistry.
Last year, I noticed that to get the pool just right required 4 backwashes. One year later, this afternoon I'll probably complete my 6th backwash of this opening event. Something has changed, and I am not at all sure what it is. I completely disassemble the filter each fall and clean it; there is no evident damage to anything that I can note. Anyone have any ideas?