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Thread: My readings....adding shock to eliminate combined chlorine..

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    My readings....adding shock to eliminate combined chlorine..

    Here are my numbers over a week after opening.

    FC 7.2
    TC over 10.0
    CC 2.8
    pH 7.7
    Hardness 310
    Alkalinity 160
    CYA 40


    I have been trying to restudy pool school and see if the pool lady's advice adds up. Her results indicated that I needed to add 3 more lbs of dichlor shock. She said that was because my combined chlorine is too high. I am having a hard time getting my mind wrapped around that idea, considering my Free Chlorine and Total Chlorine numbers are both hi. Can someone help explain?
    10,000 gallon
    Viking Freeform Fiberglass Inground Pool. FNS Plus DE Filter Pentair Challenger .75hp pump

    NC KANSAS44

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    dmanb2b's Avatar
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    Re: My readings....adding shock to eliminate combined chlorine..

    CC of 2.8 is too high...to get it down you need to raise your FC to shock level (higher) and hold it there until over time the CC comes down to 0.5ppm or less


    Looks like your getting good advice, but I would use liquid chlorine instead of the dichlor
    24'x52" AGP (13,500 Gallons), Intex SWG, (2)Solar Bear 4x20 panels, Hayward S220T Filter, 1/2hp Pentair Superflo

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    Mod Squad JohnT's Avatar
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    Re: My readings....adding shock to eliminate combined chlorine..

    She is sort of right. CC indicates that you have something using up your chlorine, but you don't have enough FC to get the job done. Adding chlorine gives you enough kick to get the job done and eliminate the CC.

    But, your CYA is 40, so be careful adding dichlor or you can end up with a high CYA level. Liquid chlorine in the form of bleach or liquid shock will do the job without the CYA.
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    Re: My readings....adding shock to eliminate combined chlorine..

    Quote Originally Posted by JohnT
    She is sort of right. CC indicates that you have something using up your chlorine, but you don't have enough FC to get the job done. Adding chlorine gives you enough kick to get the job done and eliminate the CC.

    But, your CYA is 40, so be careful adding dichlor or you can end up with a high CYA level. Liquid chlorine in the form of bleach or liquid shock will do the job without the CYA.
    HOw much chlorine bleach would i need to = 3 lbs of dichlor. Also i don't see where the pool calculator addressed combined chlorine? Or does it?
    10,000 gallon
    Viking Freeform Fiberglass Inground Pool. FNS Plus DE Filter Pentair Challenger .75hp pump

    NC KANSAS44

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    Re: My readings....adding shock to eliminate combined chlorine..

    Convential wisdom has it that you need to have tens times the FC level as CC level in order to break down the CC. So in your case you have a CC level of 2.8 ppm, you will need to raise you FC level at least to 28 and maintain it there. the other school of thought is that the shock level depends on the CYA level. Check out this link. According to that chart you need to raise you FC levels to at least 16 ppm.
    You can use the pool calculator to calculate how much bleach you need to change your FC. Be aware that you will need to add more then the calculated amount because some chlorine will be used up in reducing the CC and also in oxiding organic material in your pool. The idea is that you will have to maintain the "shock" FC level until all organic material is gone, and you do not lose more than 0.5 ppm of FC over night.
    7,500 gal, IG pool, L shape 22' x 15', 1.5 hp pump, cartridge filter, AquaPlus SWG/Controller, Pebble-Tec liner.

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    Mod Squad Bama Rambler's Avatar
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    Re: My readings....adding shock to eliminate combined chlorine..

    PC (pool calc) doesn't directly address CC's but it does give you the target shock level for your CYA level.

    Instead of asking how much bleach equals 3lbs of Dichlor, what you really want to know is how much bleach do I need to bring my pool to shock level. The Pool Calculator does tell you that. According to PC you need two 96oz jugs of bleach to get you to an FC of 15 or almost one 182oz jug. What it doesn't tell you is how many jugs you'll need in total to get your CC's down to .5 or less. Of course on-one can tell you that! I'd recommend you buy several jugs to begin with and then you can get more as needed. Remember, shock is a process not a product.

    dschlic1 and I were typing at the same time but I'm not wasting all this typing.
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    frustratedpoolmom's Avatar
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    Re: My readings....adding shock to eliminate combined chlorine..

    You should use the top section of the Pool calculator to calculate the jugs of bleach as Bama advised above - get the feel of using it correctly. Don't worry about dichlor for now.

    Then follow the steps in "How to shock your pool". If you follow those steps and go slow and ask for verification if you aren't sure of the process, you'll be fine.

    Just stick to using bleach - and each time you test, you put your current FC result in the "now" box and 15 in the "target" box, and hit calculate. Then when you move your mouse over the result box, it converts the number of ounces in to jugs for you.

    Just maintain shock level using bleach, by testing as often as you can (up to hourly) and adding bleach if necessary to maintain 15.

    Helpful links: Pool School; CYA/Chlorine Chart
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    Re: My readings....adding shock to eliminate combined chlorine..

    Thanks for the info. I was having trouble with the calculator but now have it figured out. I went to Target and stocked up on 2 gal jugs of bleach at lunch. I am starting to get the appeal of this BBB thing even though i have been reluctant.

    Thanks for helping me along so far. It's a learning process and for me has to be taken one small bite at a time.
    10,000 gallon
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    NC KANSAS44

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    JasonLion's Avatar
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    Re: My readings....adding shock to eliminate combined chlorine..

    Quote Originally Posted by dschlic1
    Convential wisdom has it that you need to have tens times the FC level as CC level in order to break down the CC.
    This is a myth. There is a relationship something like that involving ammonia, but it doesn't apply to CC (even though many people seem to think that it does). There is a somewhat technical discussion of what is really going on in this post.
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