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Thread: Minimum Recommended CYA Level Question

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    Join Date
    Jun 2011
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    Minimum Recommended CYA Level Question

    Quick CYA level question. Is the 30-50 for a BBB pool just to help preserve the FC during the day or does it serve another purpose. I am currently at 25 and will add more when I have to next top off the pool so that it doesn't go too low, but I like the lower FC and shock levels (should I have to) that poolcalculator says that I need with the lower CYA level. I do not lose FC very quickly and was wondering if I should still be at a 30-50ppm of CYA.
    20' x 40' / 33,000 gallon / IG / Vinyl
    200lb sand filter / 1hp pump
    4hrs full sun with about 45 min part sun before and after

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    linen's Avatar
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    Re: Minimum Recommended CYA Level Question

    I think as long as your pool stays above the minimum FC level (or maybe a little higher than minimum), 2 ppm FC for 25 ppm CYA (I would use the 25 cc method with the 0.2 ppm resolution to determine these low levels) in your case and you do not have visible water problems and/or CC you may be alright. I think most people struggle keeping any chlorine in their pool during the day at low CYA levels, but it depends on temperature, cover, shade, how diligent you are everyday at adding/testing FC, etc. Also, you do not have much of a safety margin at those levels. If you are an experimenter (like me) you could record your FC loss every hour (or a least morning to evening) for a few days to judge if you could run at lower CYA levels.
    TFP Expert who uses Pool School and my TF100 test kit along with PoolMath for my: Round 11K gallon AGP with deep end, 20" sand filter, Matrix 1hp 2spd, 6 2ftX20ft solar panels (and solar cover!), Intex SWCG (copper bars disconnected) and a Rubadub hot tub (chlorine). The SLAM process is not finished until: 1. CC < 0.5 ppm, 2. An OCLT < 1.0 ppm and, 3. The water is crystal clear.

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    JasonLion's Avatar
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    Re: Minimum Recommended CYA Level Question

    There are a couple of issues. One is that at lower CYA levels you lose more chlorine to sunlight each day, so end up using more total chlorine. Second, the CYA level tends to go down slowly through the course of the season, so it is best to not have it right at the low end before it gets later in the season. Third, at lower CYA levels, you have less chlorine buffered and held in reserve, so it is easier for the FC level to get to zero when there is excess chlorine demand. And finally, the CYA test gets less precise below 30, so it becomes more difficult to know what your CYA level really is.

    None of those prevent CYA around 25 from working. If you don't mind using more chlorine than you really need to be using and running a slightly increased risk of chlorine going to zero, it can work well enough.
    19K gal, vinyl, 1/2 HP WhisperFlo pump, 200 sqft cartridge filter, AutoPilot Digital SWG, Dolphin Dynamic cleaning robot
    Creator of PoolMath and Pool Calculator. Other handy links: Support this site, TF Test Kits, Pool School

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    linen's Avatar
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    Re: Minimum Recommended CYA Level Question

    Quote Originally Posted by JasonLion
    There are a couple of issues. One is that at lower CYA levels you lose more chlorine to sunlight each day, so end up using more total chlorine. Second, the CYA level tends to do down slowly through the course of the season, so it is best to not have it right at the low end before it gets later in the season. Third, at lower CYA levels, you have less chlorine buffered and held in reserve, so it is easier for the FC level to get to zero when there is excess chlorine demand. And finally, the CYA test gets less precise below 30, so it becomes more difficult to know what your CYA level really is.

    None of those prevent CYA around 25 from working. If you don't mind using more chlorine than you really need to be using and running a slightly increased risk of chlorine going to zero, it can work well enough.
    Those are good points JasonLion! It seems somewhat counter intuitive that have lower concentration levels of FC required due to lower CYA concentration, still might require more Chlorine than at higher CYA/FC concentration levels. But I think you are right (not that that matters what I think). At low levels of FC, seems like testing/dosing errors would become magnified?
    TFP Expert who uses Pool School and my TF100 test kit along with PoolMath for my: Round 11K gallon AGP with deep end, 20" sand filter, Matrix 1hp 2spd, 6 2ftX20ft solar panels (and solar cover!), Intex SWCG (copper bars disconnected) and a Rubadub hot tub (chlorine). The SLAM process is not finished until: 1. CC < 0.5 ppm, 2. An OCLT < 1.0 ppm and, 3. The water is crystal clear.

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    Join Date
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    Re: Minimum Recommended CYA Level Question

    Pool Calc recommends 2-6 as a target FC. I shoot for an FC of 6-8 to account for any daytime loss and I would love to see some stains continue to fade. I do plan on adding CYA at my next fill up, I just don't have any in stock. Didn't know if there were problems similar to low pH or low FC.

    I do not lose much FC in a day, so that wasn't a concern. I am most convinced by Jason's point that below 30, accuracy, and not the dot at the bottom of the cya test tube, begins to fade. I will aim for 40.

    Just trying to keep learning.
    20' x 40' / 33,000 gallon / IG / Vinyl
    200lb sand filter / 1hp pump
    4hrs full sun with about 45 min part sun before and after

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