CYA vs FC sunlight loss relationship

[Brad Owen]

Hi all, I've been doing a lot of searching and a lot of reading and found answers to most of my questions. But this one I can't figure out.

for at least 6 years+ I and the previous owner knew nothing more about pool care other than throw chlorine pucks in the pool, occasionally some acid and clean the filter. Surprisingly this worked. Never had a green pool. It did have the low PH high TA issue that was very slowly getting worse and I finally decided to work on that when I found this place. That problem is on it's way to being solved now.

But that is where the trouble begins. CYA level after these years was high. Very high. Best guess is 400+ based on the fact that I did a 2/3 drain and fill and now have a CYA of 190. FC back in the pre drain days was 20-30 but again it worked at least for me.

Right now overnight FC loss is less than 1ppm, but total daily loss is 2-3 ppm. Bather load is light to non existent. Only one using the pool right now is the dog. So to state my assumption, 1-2 ppm is being lost to sunlight.

First question is this assumption correct? I am in Arizona and there is very little shade around the pool. It gets sunlight from sunrise to 4pm but is not fully shaded till 6pm+. Figure 12+ hours of direct high Arizona UV sunlight. Pool is also shallow 3-5 feet. So it at least seems plausible to me.

Here is the next question. If I do another drain and fill to bring the CYA to 80-90 for a SWG or 40-50 to keep using bleach what is that going to do to my daily sunlight FC loss?

Is the relationship linear? Am I going to loose 2-4 ppm at a CYA of 90 or 4-8 ppm at a CYA of 50?

Long post I know but wanted to give as much info as possible. Thanks!

 

[JasonLion]

Welcome to TFP!

It is very difficult to give any kind of exact number because it depends on a great number of different factors: amount of sunlight, water temperature, bather load, organic debris load, and so on. That said, losing 2-3 ppm of chlorine a day with CYA around 200 doesn't seem right and suggests some kind of algae problem. With CYA that high and no one swimming, total daily chlorine usage should absolutely be below 2 ppm. But then it is difficult to say just how much chlorine usage comes from the dog swimming, especially if the dog is in the water a lot.

Chlorine loss due to sunlight is not at all linear with the CYA level. As CYA goes from 0 to 1 you get a huge savings in chlorine, while when CYA goes from 100 to 101 you would be hard pressed to measure any savings at all. Lower CYA levels always mean more chlorine lost to sunlight if everything else remains the same. So, assuming you don't have an algae problem, lowering CYA will increase your FC usage. Except that if you do have an algae problem, lowering CYA might allow you to get rid of the algae and thus lower your FC usage. So there are caveats and maybes to everything

 

[Brad Owen]

Welcome to TFP!
... That said, losing 2-3 ppm of chlorine a day with CYA around 200 doesn't seem right and suggests some kind of algae problem. ...

That is what I was thinking as well. But the overnight FC loss is near as I can tell 0 so easily less than 1ppm. Did I mention we get a lot of sunlight here? UV index is currently listed as 10+. Water is crystal clear, very sparkly, and there is nothing on the walls. Also constantly check the skimmer to see if any critters have fallen in. That will always play havoc with my FC numbers.

Welcome to TFP!
... But then it is difficult to say just how much chlorine usage comes from the dog swimming, especially if the dog is in the water a lot....

Ohh thinking more about this... We were gone for 4 days last week and the dog was boarded. For those 4 days we lost 10ppm of FC. So "not much" seems like a reasonable answer to dog vs. FC lost question.

I just can't think of any where else the FC could be going...

Chlorine loss due to sunlight is not at all linear with the CYA level. As CYA goes from 0 to 1 you get a huge savings in chlorine, while when CYA goes from 100 to 101 you would be hard pressed to measure any savings at all.

This is really what I was looking to hear. Might be closer to a log relationship then which is what I was hoping for.

Lower CYA levels always mean more chlorine lost to sunlight if everything else remains the same. So, assuming you don't have an algae problem, lowering CYA will increase your FC usage. Except that if you do have an algae problem, lowering CYA might allow you to get rid of the algae and thus lower your FC usage. So there are caveats and maybes to everything

So it sounds like there is no valid reason to run a CYA of 190 even here in sunny AZ and I should be ok to drop it down to at least 80 without doubling my FC loss (maybe only increase it a few 1/10ths of a ppm?) and see how it goes after that?

Any AZ locals with suggestions/insights for our somewhat unique climate?

Thanks!

 

[Brad Owen]

I don't like replying to myself but I really dislike coming across forum topics that ask the same question I have with no answer or update. So I'll go for the lesser of 2 evils and hopefully save some future searching person some frustration.

I dropped my CYA to 60 on 6/22. It has been hard to accurately compare chlorine use to pre 6/22 levels since then with vacations, 4th of July parties, and now giant dust storms. But there is no doubt that on average my daily FC loss is still somewhere between 2 and 3ppm. So if my daily loss went up after dropping CYA 130ppm, it was only a few 1/10ths of a ppm at most and lost in the noise. Overnight FC loss is still less than 1ppm.

So basically nothing to worry about.

 

[JasonLion]

Good. My guess is that you did have a very mild algae problem, which you have now gotten rid of, balancing out the increased chlorine usage due to the lower CYA level.