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Thread: CYA and FC question

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    CYA and FC question

    I've lowered my CYA to around 70. I understand that at 70 I will have to keep a higher FC level as opposed to having a CYA level of 40-50.

    However, does having a CYA of 70 as opposed to 40-50 allow the FC to remain at that level for longer? IE: if I have a FC of 8 in a pool with a CYA of 40 and an FC of 8 in a pool with a CYA of 70 will both pools lose FC at the same rate?
    10,000 gallon, inground, pebblesheen, sand filter, TF-100 testing kit, Pentair Rebel

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    robdac's Avatar
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    Re: CYA and FC question

    All things being equal my understanding is that the higher the CYA the longer the FC will last because it prevents the sun for destroying it. So, yes.
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    Texas Splash's Avatar
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    Re: CYA and FC question

    Actually ... no, they will not reduce at the same rate. FC too high for it's associated CYA will not have adequate CYA/stabilizer to bond to and will be expended faster by the sun. That's why that chlorine/CYA Chart is so important. In this example, which I learned from personal experience, more is not always better.
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    Re: CYA and FC question

    Quote Originally Posted by Texas Splash View Post
    Actually ... no, they will not reduce at the same rate. FC too high for it's associated CYA will not have adequate CYA/stabilizer to bond to and will be expended faster by the sun. That's why that chlorine/CYA Chart is so important. In this example, which I learned from personal experience, more is not always better.
    Ok, that makes sense. Thanks for the answer.

    How about this example: will a pool with a CYA of 70 and FC of 8 lose FC at the same rate as a pool with a CYA of 40 and FC of 5?

    According to the chart those are the appropriate levels of CYA/FC.
    10,000 gallon, inground, pebblesheen, sand filter, TF-100 testing kit, Pentair Rebel

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    Mod Squad jblizzle's Avatar
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    Re: CYA and FC question

    I would not think of it as a rate. Think of it as a higher CYA will protect a higher % of chlorine each day. At a higher CYA, if you keep your FC at the recommend levels, then you should lose a lower # ppm of FC each day. But there are other drawbacks of having your CYA too high.
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    Re: CYA and FC question

    Quote Originally Posted by jblizzle View Post
    I would not think of it as a rate. Think of it as a higher CYA will protect a higher % of chlorine each day. At a higher CYA, if you keep your FC at the recommend levels, then you should lose a lower # ppm of FC each day. But there are other drawbacks of having your CYA too high.
    Ok, so at a higher CYA I'll have a higher % of my FC protected.

    So if I have a pool with a CYA of 70 and FC of 8 I'll lose a lower % of FC as opposed to a pool with a CYA of 40 and FC of 5?

    It sounds like it may be more cost effective to carry a higher CYA provided you keep your FC at the correct levels, correct?

    What are the drawbacks of carrying a high CYA? I understand that as the CYA gets too high FC becomes almost ineffectual.

    Thanks for all the responses!!
    10,000 gallon, inground, pebblesheen, sand filter, TF-100 testing kit, Pentair Rebel

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    Re: CYA and FC question

    Many people in your area run with a CYA similar to your because of the intense sun. The main reason I am posting is to discourage you from adding more CYA. I don't think you are going there, but just in case...

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    Re: CYA and FC question

    Quote Originally Posted by MarianParoo View Post
    Many people in your area run with a CYA similar to your because of the intense sun. The main reason I am posting is to discourage you from adding more CYA. I don't think you are going there, but just in case...
    No, I am not going above a CYA of 70. I am just asking out of pure curiosity. Thanks!
    10,000 gallon, inground, pebblesheen, sand filter, TF-100 testing kit, Pentair Rebel

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    Richard320's Avatar
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    Re: CYA and FC question

    Quote Originally Posted by skydude View Post
    Ok, so at a higher CYA I'll have a higher % of my FC protected.

    So if I have a pool with a CYA of 70 and FC of 8 I'll lose a lower % of FC as opposed to a pool with a CYA of 40 and FC of 5?

    It sounds like it may be more cost effective to carry a higher CYA provided you keep your FC at the correct levels, correct?

    What are the drawbacks of carrying a high CYA? I understand that as the CYA gets too high FC becomes almost ineffectual.

    Thanks for all the responses!!
    You will lose a smaller percentage to sun at a higher CYA. Will it be measurable? I don't know. You have a higher FC but a lower rate of loss. Maybe they'll cancel.

    The problem with running a higher CYA is that you have to run a higher FC. Under normal conditions, no big deal. But if you end up with algae, the FC levels needed to kill it are huge.

    If your CYA is 50 and your minimum FC is 4, you can squeak by for a day or two using just the colorblock FC test and add the usual amount of bleach. If something is going wrong, you'll catch it when you confirm things using the FAS-DPD. But if CYA is 70 and minimum FC is 5, your minimum is right at the top of the block. If CYA is 80, your minimum is off the scale. Which means you have to use the FAS-DPD test every single time.

    If CYA climbs to 100, your minimum is 8 and the target is 10 or 11. That's where you start getting interference with the pH test. It will read falsely high. So you never can be quite sure what your pH level is, and that can start causing eye irritation, dry skin, and problems with scale formation or plaster etching.
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    Re: CYA and FC question

    I have empirical evidence, also from a hot sunny summer climate -- Summer 2014 (we had just bought the house), CYA was 70-80 and I lost 1-2ppm of FC each day in the middle of the summer. Summer 2015 CYA was 30-40, and I lost 3-4ppm of FC each day.

    For this summer I'm raising the CYA to 60-70, which I think is a good level for sunny climates for those who are willing and able to maintain the minimum FC levels. And yes, it does mean testing with FAS-DPD more often, but I prefer that anyway and do so a few times a week (I have found some strips that go to FC 10, but I'm not as comfortable comparing colors anyway). And of course, the punishment for letting it drop too low and getting algae is more chlorine to SLAM
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