Having a problem with combined Chlorine in commercial hot tub!

[Huskermaniac]

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?

 

[JasonLion]

Welcome to TFP!

That suggests that there is a reservoir of organic material somewhere, perhaps the filter? I would try backwashing the filter and see if that changes things at all.

It is either that or there is something wrong with the test results (which seems unlikely).

 

[chem geek]

Also, if you weren't diligent about always maintaining a Free Chlorine level, you could have biofilms formed in piping or on spa surfaces (if it feels slimy, then that's likely to be biofilm). You may need to use a biofilm dislodging product such as Ahh-Some on your next water change.

 

[Patrick_B]

Great suggestions above and both sound very probable. I would sure want to see what was going on in the sand filter if it were mine. They do hold onto things over time.