Excuse the long post - I've tried to add all the relevant info
My new (to me) pool seems to like low FC for some reason. I bought the place Oct '09, so this is the first season I've been managing the pool.
33k gallons gunite
DIY acid dosing system
no overnight CL loss
Only thing that jumps out is low FC. It's been cruising at 1ppm since the pool opened in March. Water has been around 90F for a month. I'd expect to see issues by now if it was going to happen.
No algea, no scum, no smell, nothing. Water is gorgeous.
Once I shocked it up to 40ppm . It slowly stabilized right back to 1ppm.
Once I kept it at 10ppm for a week using bleach, according to the CYA/FC chart. The pool smelled like CL and stung our eyes. So I let it go back down.
Pump & cell run 6hrs a day, right at about 1 turnover. Cell setting is 100% output. Cell is the 24k model, a bit undersized. Cell seems to make lots of CL in the bucket test.
Once the pump lost prime and did not pump for 2 days (poor gasket seal). Water turned slightly cloudy, cleared back up in 6hrs of running. This tells me the pool is pretty dang stable , nothing going on.
Numbers are from the 'big' test kit. The FC readings from the various tests, strips, and pool store are consistant.
Such low FC numbers have been bothering me all season, but I'm tempted to shrug and accept it .