Sometimes the opening to the pool returns from the chlorinator doesn't close all the way, even with the chlorinator at zero. My inline chlorinator is a Rainbow 320 (I think made by Hayward), and when I use it, it still will introduce some chlorine into the pool with it set at zero. When I'm trying to up my CYA (which is frequently), I leave mine set at just below "1", and it keeps my Cl level at about 4 in an 18 x 36 pool.

I'm really surprised that the CYA is only 40 if she's using trichlor--but if those numbers are true, you can't ask for anything better!!

Janet