I have had a 15,000 gallon swimming pool running with a Pentair IC40 Chlorinator for about 3 years now. The pool is solar heated in the southern california area, and is covered most of the time in a rural area. For the first 2.5 years or so I would keep the cell at about 40% and it would keep the chlorine level around 2PPM or so. For the past 6 months, I keep it at 100%, and I have to add chlorine.
I thought perhaps the cell was getting old so I replaced it, same thing. It can no longer keep up with the pool. When I test the chlorine levels out of the jets, its very high, like > 5ppm. The chlorinator cant seem to get the pool much over 1.5ppm or so regardless of how long I run it. It's clearly producing chlorine though because the chlorine readings directly from the jets are MUCH higher than those in the pool.
The water is clear, there are no stains or algae. I have tested the water myself and at several local stores, there seems to be nothing wrong with it. No nitrates or phosphates to speak of, high CYA level of 50-80, ph is normal, salt levels fine, no one can find anything wrong. It's fairly hard water but no different than it ever was. The pool is generally warm, like 90 degrees but its always been that way most of the year, even before this trouble.
All the indicator lights on the IC40 indicate everything is fine, and this is a new cell, it's not clogged or anything. Cleaning it had no effect (I did clean it even though it didnt appear to need it).
If I run the system even 12 hours a day I cant get the chlorine levels in the pool to raise much above 0.5 to 1.5ppm regardless. I have shocked the pool, no lasting effect. The chlorine just seems to go away somehow and I cant figure out why.
For now I just add a couple of cups of liquid chlorine every 3 days or so to keep the chlorine levels above 1. But I shouldnt have to do this, I never had to before.
What could have changed? The chemistry seems fine, no one can find anything wrong, and I cant think of anything thats changed.