Great forum! Thanks for the help!! I am hand dosing 10 % chlorine. Our pool is in the sun. I am confused as to why I want the CYA at 30 or above. If chlorine is more efficient with low CYA... it seems that keeping the CYA low at maybe 10 or 15 will use more chlorine due to sun but since it is more efficient at low CYA levels is it a wash since it the amount required for acceptable FC will be far less overall? I really don't want to end up with high CYA and go through draining again. The next bazillionaire is the one who comes up with lowering CYA without draining and replacing water. :-D
LOW CYA Chlorine Efficiency
- Thread starter jcmac
- Start date
You need some CYA or the chlorine will burn off and drop before effective levels too quickly. Unless you plan on testing several times a day and topping off the chlorine as it burns off...
svenpup
LifeTime Supporter
The appropriate CYA level is a balance between sun protection and chlorine effectiveness.
With CYA too low, the sun will burn off enough chlorine during the day that FC falls below minimum (or maybe even to zero).
With CYA too high, it becomes difficult to shock because of the high FC level needed.
The guideline CYA levels here are high enough to protect FC from the sun and ensure residual sanitizer in the water throughout the day, but not so high that shocking is impossible.
With CYA too low, the sun will burn off enough chlorine during the day that FC falls below minimum (or maybe even to zero).
With CYA too high, it becomes difficult to shock because of the high FC level needed.
The guideline CYA levels here are high enough to protect FC from the sun and ensure residual sanitizer in the water throughout the day, but not so high that shocking is impossible.
The way things work out, you actually use less total chlorine by raising the CYA level, even though you also need to raise the FC level to compensate for the lowered chlorine effectiveness. At higher CYA levels, you use higher FC levels, but the amount of chlorine actually lost to sunlight goes down, so the amount of chlorine you add each day goes down.
Thanks all! So below 30 ppm CYA there is no sun burn off protection? .. at 30 you have minimal burn off protection.. or is 30 where you get all you will get ...and at 40 ppm for example you would have the same burn off protection as at 30ppm? Thanks for bearing with me on this. I keep thinking as you move in the dreaded direction of 100 ppm CYA... you best be getting ready to pull the plug again and start all over. 
The higher the CYA level, the less total chlorine you use. This is essentially true at all CYA levels within reason. So there is some sun protection at a CYA level of 5, and there is more sun protection at a CYA level of 30, and even more at 60 and even more at 100.
I suspect you are trying to figure out why the recommended CYA level is between 30 and 50? That actually has to do with balancing out five or six different issues to come up with something that works well for lots of people. For example, there is no readily available CYA test that will give you meaningful results when the CYA level is below 20. If we recommended a CYA level of 15, no one would be able to know if they were at 15 or not. So that wouldn't make a good recommendation. Likewise, when CYA is around 100, it becomes so difficult to shock the pool that most people can't manage to kill an algae bloom. So recommending CYA levels around 100 is sure to create problems. Once you balance out all the different pros and cons, the best CYA level for the vast majority of people is between 30 and 50, even though higher CYA levels require less total chlorine.
Additional more technical discussion moved to The Deep End, here. JasonLion
I suspect you are trying to figure out why the recommended CYA level is between 30 and 50? That actually has to do with balancing out five or six different issues to come up with something that works well for lots of people. For example, there is no readily available CYA test that will give you meaningful results when the CYA level is below 20. If we recommended a CYA level of 15, no one would be able to know if they were at 15 or not. So that wouldn't make a good recommendation. Likewise, when CYA is around 100, it becomes so difficult to shock the pool that most people can't manage to kill an algae bloom. So recommending CYA levels around 100 is sure to create problems. Once you balance out all the different pros and cons, the best CYA level for the vast majority of people is between 30 and 50, even though higher CYA levels require less total chlorine.
Additional more technical discussion moved to The Deep End, here. JasonLion