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Thread: Adjusting CYA upward to slow chlorine loss due to sunlight?

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    Adjusting CYA upward to slow chlorine loss due to sunlight?

    Hello All,

    We are about to complete a nearly complete drain to reduce CYA. Yes, we were doing all the wrong things - using pucks!

    Anyways, I'm hoping to start off with a CYA in the very low end of the recommended 30-50 range.

    Once we get pool refilled, pH and FC adjusted for the CYA we have after completing refill, we will start a routine maintenance. Ideally, twice a week of adding bleach/liquid chlorine.

    My understanding is that CYA helps stabilize chlorine and keeps some "in reserve" that can be released when unbonded chlorine is needed to attack organisms (probably a very inaccurate, possibly erroneous summation - clarifications requested).

    We understand that we have to keep FC level above minimum level and the minimum level is determined by CYA level.

    My question is:

    If FC drops below safe FC level in between doses, I recognize I can increase chlorine dosage each time so that doesn't happen. But, would raising CYA produce a slower FC loss rate? (recognizing we will need a higher FC level to account for the higher CYA).


    Basically, with identical conditions (temperature, sunlight on pool, humidity, splash loss, etc.) will a pool with a 30 PPM CYA lose FC at a higher rate than a 50 PPM pool? Or is there something I just am missing here?

    Thanks!
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    Patrick_B's Avatar
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    Re: Adjusting CYA upward to slow chlorine loss due to sunlight?

    Welcome to TFP Glenn,

    No, you are correct, all things being equal, a pool with 50 Cya will see less FC loss to the Sun Versus one with 30PPM.
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    Re: Adjusting CYA upward to slow chlorine loss due to sunlight?

    Thanks Patrick_B!

    I failed to ask if there is any experience/chemistry knowledge that people can share regarding the different loss rates.

    Is it somewhat directly correlated? For example: would a doubling of CYA reduce loss rate by 1/2? Or is it more pronounced (a doubling of CYA would reduce loss rate by 1/8, 1/4, ....). (I don't plan on doubling CYA - but am looking for something to help guide my decisions).

    Worse case would be, even though a higher CYA reduces FC loss rate, the difference is negligible.

    Put differently: does anybody know (or have an opinion on) the X and Y in: An increase in CYA by X percent, reduces FC loss rate by Y percent.
    55K (not a typo!), in ground (is in ground redundant with 55K?).

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    Re: Adjusting CYA upward to slow chlorine loss due to sunlight?

    I guess I have a question based off that.
    The higher your FC, the quicker the sun burns through it, if all other things are equal. Because, from what I have read, the sun depletes a percentage of your FC, and the CYA level effects what percentage that is.

    Higher CYA requires a higher FC level, as well.

    So, without solid data, I have to wonder if there is a point where the higher required FC level outweighs the protection you get from the higher CYA. (I would assume it's somewhere outside of the ideal CYA range)


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    Re: Adjusting CYA upward to slow chlorine loss due to sunlight?

    Quote Originally Posted by phibroptic View Post
    I guess I have a question based off that.
    The higher your FC, the quicker the sun burns through it, if all other things are equal. Because, from what I have read, the sun depletes a percentage of your FC, and the CYA level effects what percentage that is.

    Higher CYA requires a higher FC level, as well.

    So, without solid data, I have to wonder if there is a point where the higher required FC level outweighs the protection you get from the higher CYA. (I would assume it's somewhere outside of the ideal CYA range)


    Sent from my phone-a-majig.
    Hi Phibroptic! That may be a valid assumption, but I hope it's wrong . Hopefully, the brains of TFP can help guide us!
    55K (not a typo!), in ground (is in ground redundant with 55K?).

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    Re: Adjusting CYA upward to slow chlorine loss due to sunlight?

    Hmm, answer may be in this article (forgot that Google is my friend!). Anyways, I'll read it in detail later (not sure I'll understand everything, but will read it!).

    http://www.troublefreepool.com/threa...ical-CYA-level
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    Re: Adjusting CYA upward to slow chlorine loss due to sunlight?

    I think I know what your getting at...

    you want to only add chlorine bleach 2 times a week, while that is "not recommended" you "could" raise your FC enough to allow a drop over 3 days to be over your minimum FC level..

    example:
    Min FC level 4, raise your FC level to "X" and test every day for 3 days, if you fall below FC 4 you would have to raise "X" till you stay above FC 4

    does that answer your question?
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    Re: Adjusting CYA upward to slow chlorine loss due to sunlight?

    Cowboycasey, that's pretty close.

    I can raise the dose levels, or raise the CYA level to slow the loss rate (with lower chlorine dose levels). Daily measurements and trends are important while we get this dialed in.

    Here is an excerpt from a chem geek article that was referenced by the link I posted above. It's not real encouraging:

    CYA ... % FC Loss ... FC Loss at 10% FC/CYA Start ... FC End
    100 .... 15 ................... 1.5 .................................... 8.5 (8.5%) OK
    80 ...... 20 ................... 1.6 .................................... 6.4 (8.0%) OK
    50 ...... 40 ................... 2.0 .................................... 3.0 (6.0%) on the edge
    30 ...... 60 ................... 1.8 .................................... 1.2 (4.0%) TOO LOW
    20 ...... 75 ................... 1.5 .................................... 0.5 (2.5%) TOO LOW
    0 ........ 99+ ................. N/A

    (link:http://www.troublefreepool.com/threa...l=1#post318157 )
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    JasonLion's Avatar
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    Re: Adjusting CYA upward to slow chlorine loss due to sunlight?

    At higher CYA levels you lose less and less total chlorine, even after taking the higher required FC levels into account. However, as CYA gets higher the FC level required for SLAMing also goes up, and with CYA above 80 several other problems start to develop, so there are limits to how high it is practical to raise CYA.

    Going three days between chlorine additions is not practical for routine use. Chlorine loss is a percentage of the starting FC level, so you lose way more chlorine on the first day, and somewhat more on the second day, vs daily chlorine additions. This is something that is practical for the occasional long weekend, but uses unreasonably large amounts of chlorine for routine use.
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    Re: Adjusting CYA upward to slow chlorine loss due to sunlight?

    Use of a pool cover that was mostly opaque to UV would let you dose twice a week. That's what I do.
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