I've been struggling with startup on my pool since late April and not sure what to try next. I have a 22,000 gallon in-ground with 3/4HP Hayward Super Pump and S-244T sand filter. The pool is 13 years old, all original equipment. Live in NC, operate pool 5 months per year.
When I started up I had an algae problem and also an issue with lack of pressure at filter. Per conversations on this board back in April (this thread: lack-of-pressure-t20349.html), it was suggested that my problem had to do with a blockage or some sort or air leak centering on pump. I did the following: replaced o-ring on pump strainer basket, purchased and used pool lube, checked impeller (ok), checked weir (ok), checked for leaks (found none).
After all that, the pump did seem to be working better (no bubbles at top), so I began the shock process to get rid of algae. Maintained chlorine at 15 to 20 as best I could for two weeks. Algae obviously killed (color went from bright to dull green), but water was not clearing up. I was concerned that during this time the pressure at my filter remained at or near zero, despite the pump working fine and little or no air bubbles in pool. Moreover, during this time, the filter NEVER needed backwashing. I did backwash every few days during this period, but water was almost totally clean (unlike previous year's, where filter would need backwashing frequently during startup).
Based on that observation, I concluded that the culprit was the sand in my filter. (14 years old, never replaced). So I replaced the sand (vacuumed out old and replaced with new). The filter's backwashing mechanism seemed to work fine (I backwashed after replacing the sand), but after a few days with new sand, I noticed no change in the pressure level on the filter and a subsequent backwash didn't reveal any particles. I assumed my problem was particles suspended in the water, so tried a clarifier, but it made no difference. Two days ago, I resumed shocking -- no signs of new algae growth, but didn't know what else to try.
I'm contemplating replacing the filter totally, but not sure if that's the solution. I don't know anything about how water flows through a filter, but is there a way to test to see if there's some sort of mechanical problem there? No leaks around multiport valve, which seems to be working fine. I did replace the gasket when I replaced the sand. And replaced the pressure gauge.
Current numbers are CYA 45, TA 100, CA 20 (low, but don't think it matters), FC 16.5, CC .5
The numbers are as of this morning after overnight test, FC last night was 17.
Sorry for the lengthy post, but wanted to give as much information as possible in hopes that someone could suggest something else to try.