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Thread: Having a problem with combined Chlorine in commercial hot tub!

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    Join Date
    Mar 2014
    Nebraska City, Nebraska

    Having a problem with combined Chlorine in commercial hot tub!

    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?

  2. Back To Top    #2
    JasonLion's Avatar
    Join Date
    May 2007
    Silver Spring, MD

    Re: Having a problem with combined Chlorine in commercial hot tub!

    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).
    19K gal, vinyl, 1/2 HP WhisperFlo pump, 200 sqft cartridge filter, AutoPilot Digital SWG, Dolphin Dynamic cleaning robot
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  3. Back To Top    #3

    Join Date
    Mar 2007
    San Rafael, CA USA

    Re: Having a problem with combined Chlorine in commercial hot tub!

    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.
    16,000 gallon outdoor in-ground 16'x32' plaster pool; Pentair Intelliflo VF pump; Pentair IntelliTouch i9+3s control system; Jandy CL-340 square foot cartridge filter
    12 Fafco solar panels; Purex Triton PowerMax 250 natural gas heater (200,000 BTU/hr output); automatic electric pool safety cover; 4-wheel pressure-side "The Pool Cleaner"

  4. Back To Top    #4
    Mod Squad Patrick_B's Avatar
    Join Date
    Jun 2011

    Re: Having a problem with combined Chlorine in commercial hot tub!

    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.
    TFP Moderator
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