Thanks for your help! It is a neighborhood public spa. Does the dichlor come from the chlorinating tablets in the chlorinator, the shock, or both? Also, our health regulations mandate a minimum CH of 200 so I'm not sure if I can go that route.
I work at an outdoor swimming pool and have been having issues with controlling the combined chlorine in the spa. The combined chlorine usually rises at least 1 ppm per day throughout the day. The spa sits in sunlight most of the day. Here are the stats (as we speak):
FC: 4.6
CC: 1.6
CYA: 110
Alkalinity: 90
PH: 7.2
Calcium: 270
300 gallons
Combined chlorine is the only chemical I've had trouble balancing all summer. I've tried shocking at 10x the FC level (something I read) while exposing the tub to sunlight all day and this gradually works, but as soon as it gets low enough I set FC down between 3-5 and combined rises once again. I've tried thio-trine Cl neutralizer and this works to get combined chlorine to zero, but again once I get FC back to the desired range whaddaya know the CC rises sharply once more. It's a busy spa which gets many users over the course of a day so super-chlorination is a frustrating procedure as it requires us to close the spa. Our health regulations mandate a CC of no greater than 0.5