We have a 2500 gal in ground hot tub that is 20 years old. It has a Pentair high rate commercial sand filter and pump. I am having an issue with the combined chlorine building on me. I will change out the water with new incomming water that has 0 combined chlorine in it. I can then let it circulate for a couple of hours in which it will turn over about 8 times. When I retest the water my combined is already at .5 with no bathers. If i let is run overnight my combined with elevate and be over 1.0 in 24 hours. I am feeding liquid chlorine through a Strantrol controler and keep the level at about 4ppm. My pH is at 7.3 and I use CO2 to regulate that. I have added a swim spa Ozonator that is sized for the pool and have it running into the feed lines after my heat exchanger. Since that did not correct the problem I have now added Potassium Monopersulfate. Even after shocking with the recommended dose I have a combined Chlorine level of 1.2 after 1 night. I am using the R-0867 Deox Reagent to give me the correctl readings in my DPD tests.
Anyone out there have any insight that will help me to control or fix this?