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Thread: SWG & CYA level

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    SWG & CYA level

    The home we bought several years ago has a 20,000 gallon pool with an Intellichlor IC40 SWG that never seemed to work correctly. It was always reporting insufficient salt, while the chemical tests kept showing there was plenty (3500+) of salt in the pool. So our pool service used stabilized tablets to maintain our chlorine levels for the past several years.

    This year, we quit using the service and I'm trying to do it all myself. Looking around online, I found a reference that discussed steps for calibrating this SWG (it's a V1.6). [I'm disappointed that the pool service people never said anything about this. I'm also disappointed that, in several calls I made to Pentair Support about the IC40 "problem", none of the Pentair tech support people mentioned anything to me about calibration, either!]

    Anyway, I followed the calibration steps from that post and, now, that IC40 has green lights and appears to be generating chlorine just fine! So I'm pretty excited about that!

    My question, however, is about the CYA level in the pool. It tests at 110ppm (which I'm sure is due to the use of those stabilized tabs over the past several years), and the chart in the Taylor booklet says it should only be at 30-50ppm.

    From what I've read, the "only" way to reduce the CYA level is to partially drain and refill the pool to thereby dilute the CYA concentration. However, I've also read that the unstabilized chlorine the SWG creates will use up some of the CYA over time. So, is it possible that, if my SWG is running correctly, I can avoid the drain/refill because the unstabilized chlorine from the SWG will use up enough CYA to bring its level down to somewhere in the 30-50ppm range?

    If so, how long will it take? Or did I misread and the CYA actually doesn't get used up by the unstabilized chlorine, so I really will have to partially drain and refill the pool?

    Thanks.

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    Mod Squad jblizzle's Avatar
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    Re: SWG & CYA level

    Welcome to TFP!!!

    First how did you get the 110ppm reading?

    You are right the only practical way to lower the CYA is by draining and replacing water. It is true the the CYA will be broken down by any chlorine, but at a very slow rate, like < 5ppm a month so that is not a practical solution. It can also be lowered by a Reverse Osmosis service to the pool, but it is more expensive than replacing water and only available in very limited regions of the country.

    On the plus, side, we recommend a CYA between 70-80ppm for SWG pools, so you should not need to replace as much water as you think.
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    Re: SWG & CYA level

    Quote Originally Posted by jblizzle View Post
    Welcome to TFP!!!

    First how did you get the 110ppm reading?

    You are right the only practical way to lower the CYA is by draining and replacing water. It is true the the CYA will be broken down by any chlorine, but at a very slow rate, like < 5ppm a month so that is not a practical solution. It can also be lowered by a Reverse Osmosis service to the pool, but it is more expensive than replacing water and only available in very limited regions of the country.

    On the plus, side, we recommend a CYA between 70-80ppm for SWG pools, so you should not need to replace as much water as you think.
    The CYA level was based on the test in the Taylor K-2005 test kit (requires preparing a 14ml 50/50 mix of pool water & one chemical, then adding drops of another chemical until a "black dot" on the bottom of the container "disappears").

    Since the test is based on my subjective determination of when I can no longer see that "dot," I suppose my conclusion about the level could be off somewhat. Actually, I looked at my log book and found that I measured the CYA level at 95ppm on 6/13. So maybe my current level is closer to 100ppm than to 110 ppm.

    >> CYA will be broken down by any chlorine

    Hmmm. I had thought that the stabilized chlorine did NOT break down CYA, but that UNnstabilized chlorine does. Should I instead be thinking of it as: (1) all forms of chlorine break down CYA at a slow rate, but (2) because stabilized chlorine necessarily is adding more CYA to the system while that's happening, the "net effect" is that the CYA level goes up when using stabilizied chlorine?

    Come to think of it, I'm actually losing some water all the time (I guess it's evaporation, splashing, etc.). Anyway, it's enough that I have to add water every week. We use a water timer on the hose bib and it runs for ~1 hour each week. (I don't know how much water that is - I wish the device measure gallons instead of time).

    So if (1) as much as 80ppm CYA is recommended for a SWG pool like mine, (2) I'm already adding new water every week, and (3) if the chlorine will "eat" up to 5ppm/month, then why shouldn't I just let it go for 4-5 months and see if doesn't get down around the 80ppm range? Is that 100-110ppm CYA level dangerous or does it have some other undesirable effect on the pool chemistry?

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    JasonLion's Avatar
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    Re: SWG & CYA level

    The CYA level could easily be far higher than you think it is. That test reports any CYA level over 100 as something around 100. You should do the CYA test again, but this time mix equal amounts of pool water and tap water together, do the CYA test on that, and multiply the result by 2.

    CYA levels over 90 can be exceedingly problematic. I strongly recommend replacing water to get the CYA level below 100.
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    Re: SWG & CYA level

    OK.

    I'll recheck the CYA level as you advise and will do the drain/replace. However, I've never drained a pool before. So can you tell me the steps to accomplish the required amount of draining after I determine that new CYA level? Specifically:

    1. How do I calculate what percentage of the pool needs to be drained based on knowing that CYA level and, once I do calculate that, how do I determine when I've drained that much water from this irregularly shaped pool (I think it's shape is called "Lagoon" - lots of curves, so it's hard to know how to calculate it. I've just been working off of the previous owner's statement that it's about 20,000 gal.)?

    2. Mechanically (after I turn off the pump) should the filter valve be set to "backwash" or to "waste" before I turn the pump back on to do the draining? And, before I start that draining, should I also close off the valves from the skimmers (which I think should cause the pump to pull water just from the drains at the bottom of the pool) or is it OK to leave those valves open? (I'm guessing that if don't close those valves, then when the water level gets below the skimmer level, the pump would be sucking air though those skimmer lines, which could keep it from sucking water from the drains at the bottom.)

    3. Anything else I need to know?

    Paul

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    Re: SWG & CYA level

    Close the skimmers, and set filter to waste.

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    Re: SWG & CYA level

    Rechecked CYA using 50/50 pool water & tap water. Multiplying result x2 still gives 110 CYA level.

    I understand I should close skimmer valves & set the filter valve to waste (though I may backwash some of it, as it's needed).

    But I'm not sure how to determine/measure the amount to drain in order to end up with 70-80 ppm CYA after refilling.

    It seems like, with a 20,000 gal. pool, I should drain just over 6,000 gallons, then refill (20000 x 75/110 = 13636; 20000-13636 = 6364).

    Assuming that math is correct, what I don't know how to figure out is identifying when just over 6000 gallons has been drained.

    I have a "lagoon" shaped pool (lots of curves, 4' - 8' depth), so how do I determine a point where I stop draining & start refilling?

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    JasonLion's Avatar
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    Re: SWG & CYA level

    You want to drain and replace just under 2' of water. That should be just under 1/3 of the total water (assuming an average depth of roughly 6'). It would be reasonable to actually replace about 18" of water and see where that gets you. That will save on water and very likely get you into the correct range. And if it isn't enough, you can always replace a little more later.
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    Re: SWG & CYA level

    Thanks! I'll be doing it tomorrow (if it doesn't rain) immediately after I replace the sand in my Tagleus TA100D filter.

    The instructions for replacing the sand in the filter say that, when I restart the system, I should start up on RINSE, then BACKWASH, then RINSE, then BACKWASH again, to prevent putting "sand dust" into the pool after the sand change. Therefore, it seems to me that - because both RINSE & BACKWASH end up discharging water to WASTE - these RINSE/BACKWASH cycles will at least get me started on draining that 18-24" of water from the pool. Does that make sense?

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    GreatCanadian's Avatar
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    Re: SWG & CYA level

    Not sure if this has been cleared up so I'll mention it anyway. You mentioned in an earlier thread that you are losing water to evaporation, and having to replace. It seemed to me in reading that you thought this would lower your CYA level. It doesn't. Replacing water that has been lost to leaks, and splash outs will lower your CYA, but not replacing water lost to evaporation. Thought I would mention it in case that was your impression.

    And yes. The rinse and backwash will help with your draining, thus lowering CYA when you replace that water.
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    Re: SWG & CYA level

    By now, you probably realize that you will also loose as much as 6-800 ppm salt during the drain, so depending on the required levels of your swg, you will need to add some back. Calcium levels will also be affected, based on your fill water, so that will require attention as well.
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