I have a 22,500 gal IG pool with a Pentair IC40 SWG. This is my 3rd season on the cell. I also have a Pentair Intelliflo VS pump. My first year no issues with chlorine production but last year and again this year I have noticed that I have to supplement with 6-8% bleach (about a gallon a week) to maintain my chlorine level at 3-4ppm. My FC slowly drifts down. I maintain my stabilizer at about 80ppm. I use the Trouble Free method. My water looks great but I wonder why the chlorine output is not enough to maintain my sanitizer levels. I have had to gradually increase the cell out put from 75% year one, 85% year two and this year I am at 100%. The cell is clean and maintained as directed. It has no scale. Salt level is 34-3500. I have my system cycle 4 3 hour runs for a total of 12 hours per day. I run my pump at 1150 rpms for a total flow across the cell of 30 gal/min.
It occurred to me today that perhaps if I increase the water flow across the cell to 1500 rpm/40 gal per min that I would potentially be exposing the cell to at least a 25% increase of NaCl molecules and consequently could expect an increase in chlorine production. Does this sound reasonable? Perhaps I have been running just enough water across the cell to have it function but not enough for max output. I know there is a law of diminishing returns in regards to water flow but I have been running my pump slow to save energy and get enough turns.
So I either have to increase my pump runs or increase flow right? I am also aware that the cell output drops with age and I probably am experiencing some of that. The only other thing I can think of is phosphate levels in the water. I have not tested for phosphates but am considering it. The pool is located in a rural area and is subject to lots of dust and some agricultural chemicals especially in the spring with planting. I don't use any pool chemicals that would introduce phosphates. It is filled with well water and so it is possible that it contains some natural phosphates. I do know it is naturally high in alkalinity at around 230 out of the tap. I have got the total alkalinity down to about 130 and my CSI runs about 0.10-0.14 at all times so I am able to maintain good balance. This is a vinyl liner pool.
Do any of you have any other ideas? I have had a pool for 30+ years and so I have had lots of experience but learn something new all the time,