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Thread: Getting used to much lower CYA, stumped about what we're seeing

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    Join Date
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    Question Getting used to much lower CYA, stumped about what we're seeing

    OK, for the past couple of years -- in the midst of this California drought -- we've been making do with a pool that has an extremely high CYA because we didn't want to drain our pool. I forget the exact numbers (tested everything at the beginning of the season) but we were supposed to keep the chlorine levels high, at about 14-17, and we did so with no problems. No skin irritation, and all our other numbers except the sky-high CYA tested in a good range.

    According to my husband, he used on average about 6 cups of 8.25% bleach a day.

    Then we left town for the holiday weekend, and our pool was left "refilling" (read: overflowing)...for three days. When we came back, and saw what had happened, I tested all the numbers, and it was as if we had changed almost all the pool water. So, I got all our numbers back in range (added some borax and calcium) and saw in pool math that with our new CYA levels of 40, we want the chlorine to be between 3 and 7.

    When we tested it after coming home from our holiday weekend, it was down to about 2 but the water was constantly moving (refilling / overflowing). No sign of algae. My husband added about a half gallon of 8.25% bleach. the next day it was still 2. He added a gallon of 8.25% bleach. That morning it went up to 6, but by the afternoon, it was down to 2 again.

    To go from 6 cups a day to a gallon a day (and still not maintain the chlorine level) is very strange. We get pretty much sunlight, but we also have a good bit of shade before 9 am and after 3 pm. Would the low-ish CYA account for quick disappearance of the chlorine?

    We'd figured out the pool perfectly with the high CYA and now it feels we're starting form scratch.

    Thanks!
    ~13,000 gallon in-ground plaster pool, built in late 80s, refinished in fiberglass in 2013
    Pentair IntelliFlo 3HP Variable Speed Pump, 2014
    Old HP booster that runs a Polaris 280 sweep, ages unknown
    Old DE Filter, age unknown

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    Texas Splash's Avatar
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    Re: Getting used to much lower CYA, stumped about what we're seeing

    The fact that your CYA was reduced by so much can certainly impact your FC loss, especially in an area with a lot of sun. Even in my area, while the CYA range may ideally be 30-50, I end up going no lower than 50, sometimes 60-ish to save on chlorine. The sun just tears it up. All the Texas rains this season had me doing the same thing because it wiped-out my CYA level. I suspect you were just in a great routine with the relatively high CYA protecting your FC. If you are sure your CYA is only about 40, you always have the option to increase it a bit in increments of 10 to find that "happy balance" between CYA and FC. If at any time your water condition changes, or you believe something is just "off" in your water, you can always consider doing the overnight FC loss test, or post a full set of results for us to review. Hope this helps.
    Pat (a.k.a. Texas Splash) ~ My Pool: Viking Fiberglass; 17,888 Gal; Waterway Supreme 2-sp/2-hp pump; Hayward Ctg filter; TF-100 w/ Speed Stir
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    Richard320's Avatar
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    Re: Getting used to much lower CYA, stumped about what we're seeing

    You lose FC faster at higher concentrations as well as at lower CYA levels. So you're catching it from both sides.

    Anything more than half a jug of 8.25% Clorox in a day in a pool your size is excessive to me. It should be more like a quarter jug. Raise the CYA. Now's a good time to use up any leftover pucks. Four 8 oz pucks will add 10 CYA to your pool.
    16K freeform gunite with spa; Pentair 4000 DE filter; Century Whisperflow 1 HP; Pentair Minimax heater.
    Troublefree does not mean Maintenancefree. It's like brushing your teeth: You can spend a couple minutes a day and pennies a week or go to the dentist once a year and spend several thousand dollars.
    A pool is like a pet - you have to feed it every day, even the days you don't want to play with it!

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    Re: Getting used to much lower CYA, stumped about what we're seeing

    You lose less absolute chlorine at higher CYA levels in spite of the proportionately higher FC levels. This is due to a non-linear CYA UV-shielding effect that we don't fully understand, but it's clearly real. Fortunately, a lot of the gain occurs as one approaches 80 ppm so going from 40 to 50 helps more than 50 to 60 and so on. The higher the CYA the less chlorine you'll need to add each day, but your risk is higher if you ever have to SLAM in that it will take a heck of a lot of chlorine to do such a SLAM. This is the primary reason we limit the high end to 80 ppm though like anything else on this forum you are welcome to do what you believe is best for your pool so long as you understand the consequences.
    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"

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    Re: Getting used to much lower CYA, stumped about what we're seeing

    Thanks everyone. We'll raise the CYA a bit and see what happens.
    ~13,000 gallon in-ground plaster pool, built in late 80s, refinished in fiberglass in 2013
    Pentair IntelliFlo 3HP Variable Speed Pump, 2014
    Old HP booster that runs a Polaris 280 sweep, ages unknown
    Old DE Filter, age unknown

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    Re: Getting used to much lower CYA, stumped about what we're seeing

    Quote Originally Posted by chem geek View Post
    You lose less absolute chlorine at higher CYA levels in spite of the proportionately higher FC levels. This is due to a non-linear CYA UV-shielding effect that we don't fully understand, but it's clearly real. Fortunately, a lot of the gain occurs as one approaches 80 ppm so going from 40 to 50 helps more than 50 to 60 and so on. The higher the CYA the less chlorine you'll need to add each day, but your risk is higher if you ever have to SLAM in that it will take a heck of a lot of chlorine to do such a SLAM. This is the primary reason we limit the high end to 80 ppm though like anything else on this forum you are welcome to do what you believe is best for your pool so long as you understand the consequences.
    Interesting...

    In my Taylor book it says that the benefit of being above 50 is negligible, with a 3-5 times longer lasting FC at 25ppm.
    7-10 year old In ground 20,000 gal, Aquabright ecofinish W/ Spa, waterfall and infinity jet, 60 sq foot DE filter, Single speed pump, Racer cleaner

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    Re: Getting used to much lower CYA, stumped about what we're seeing

    Taylor is absolutely positively wrong about this. Mark (mas985) did experiments described in this post and this post that confirmed that higher CYA levels had measurable benefits to protecting chlorine. We also saw this in numerous pools. See this post for a rough estimate based on SWCG pools seen that shows the benefit of higher CYA protecting chlorine loss even with proportionately higher FC levels.

    Taylor is wrong about several things, just as most of the pool industry has these myths or inaccuracies that get perpetuated over and over again, everyone repeating this information following others like lemmings falling over a cliff.

    What IS true is that a small amount of CYA has a LOT of savings compared to no CYA at all, but then saying there is negligible difference between 50 ppm and 25 ppm or between 50 ppm and 80 ppm would be incorrect.
    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"

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    Re: Getting used to much lower CYA, stumped about what we're seeing

    As another Central Valleyan, I can verify what you're seeing -- We're going through a lot more chlorine this summer at CYA 40 than last summer when it was 80 (inherited when we bought the place). Probably double. I think our summers here are just so sunny every day that high loss is a part of living here (contrast it to other places where it can be cloudy 1/3 to 1/2 the time). Covering the pool does help -- I think I lose 3-4+ppm/day uncovered, ~2 with the solar cover on.

    I'm planning to raise CYA to 50 to maybe 60 and see how that works; it's just a tradeoff of lower daily losses vs need for more chlorine if you need to SLAM (which should be rare to never if you diligently keep the levels above the minimum for your CYA). Just haven't decided whether to do it now with pure stabilizer or when we're away with the trichlor tabs left over from the previous owner.
    21000gal IG plaster, Sacramento CA area (late 1950s/early 60s)
    Filter: Cartridge, Pentair CCP420 (2014)
    Main pump: Pentair IntelliFlo VS (2015)
    Boost pump: 3/4hp (2011), Polaris 280 cleaner (unknown age)

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