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

[bkp13]

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!

 

[Texas Splash]

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.

 

[Richard320]

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.

 

[chem geek]

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.

 

[bkp13]

Thanks everyone. We'll raise the CYA a bit and see what happens.

 

[Austintatious]

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.

 

[chem geek]

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.

 

[jmastron]

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.