what's so bad about high CYA levels?

[beezar]

Seems like higher CYA levels (100 or more) would significantly reduce the amount of chlorine loss due to sunlight, hence making the frequency of adding chlorine conveniently less.

I understand that if you develop algae, you'll require much higher amounts of chlorine to shock the pool, but other than that, what's so bad about having higher CYA levels if you test your pool everyday and keep up with chlorine levels?

My pool CYA level is 140 which I've been trying to lower, but now I'm questioning if that is that important, as I've only needed to add chlorine maybe once every 5-7 days despite direct sunlight on the pool.

 

[duraleigh]

Briefly, the higher your CYA, the less effective your FC. At 140 CYA (How did you test that high, btw?) You should have an FC level up around 15-20ppm in the pool (24/7) to have an effective sanitation level.

Most people have difficulty maintaining that level. Shocking the pool would require FC in excess of 40ppm.

 

[chem geek]

The higher CYA level does protect the chlorine more from breakdown from the UV in sunlight, but as Dave points out it requires a higher FC level to prevent algae growth. Now you could be lucky in having the pool be poor in algae nutrients in which case a lower FC level won't have algae grow, but that's a risk which would require a heck of a lot of chlorine to fix due to the high shock level. Also note that chlorine demand is also a function of temperature so as your water warms up you'll probably find that the chlorine usage goes up as well.

When I was using Trichlor tabs 7 years ago in my own pool, my CYA climbed to 150 ppm after 1-1/2 seasons and I started to get an unusually high chlorine demand that eventually turned the water dull and then cloudy -- it was algae that was at first not visible. This was in spite of using an algaecide every other week and having an FC level of around 3 ppm. If I wasn't using an algaecide, I probably would have had algae when the FC dropped somwhere below 8 ppm or so. Had I known better, I could have raised the FC level as my CYA climbed or even better switched to using chlorinating liquid which is what I did and have not had such problems since (well, one time I did when I let the chlorine get to zero during a spring startup).

The minimum FC at 140 ppm would be 10.5 ppm for a manually dosed pool as that should prevent algae growth regardless of nutrient level.

The highest level most people use for CYA where chlorine alone is used to control algae is around 80 ppm, mostly for saltwater chlorine generator pools, though also for some manually dosed pools in very sunny areas. There is one pool service with 1000+ pools that uses 100 ppm, but they visit once a week raising the FC to 14 ppm where it generally drops to around 4 ppm by the time of the next visit. That's about as extreme as is typical. Managing a higher CYA can only be realistically be done using supplemental products such as phosphate removers and/or algaecides, but this costs extra.

 

[ThePoolNinja]

Drain the pool, start fresh.

 

[JasonLion]

A whole range of secondary problems develop when you are working with high CYA pools. For example, the typical PH test stops working when FC is above 15, yet FC often needs to be above 15 when using high CYA levels. Another issue is PH shift during shocking. Should you need to shock at high CYA levels, you would need to use soooo much chlorine that the associated PH shift would drive the PH out of range. Normally, at lower CYA levels, the PH shift is small and can be ignored. But at high CYA levels and thus astronomical FC levels the temporary PH shift from adding chlorine is so large that it takes you out of the safe PH range.

 

[beezar]

A whole range of secondary problems develop when you are working with high CYA pools. For example, the typical PH test stops working when FC is above 15, yet FC often needs to be above 15 when using high CYA levels. Another issue is PH shift during shocking. Should you need to shock at high CYA levels, you would need to use soooo much chlorine that the associated PH shift would drive the PH out of range. Normally, at lower CYA levels, the PH shift is small and can be ignored. But at high CYA levels and thus astronomical FC levels the temporary PH shift from adding chlorine is so large that it takes you out of the safe PH range.

Where would the pH go when you add lots of chlorine, up or down?

I've been maintaining my FC from 12-19, with a CYA of 140 and have been slowly bringing CYA down with backwashing/vacuuming to waste/etc. and just noticed my pool chlorine would drop very slowly while I had to add chlorine a lot more frequently to my spa (CYA 40). Both get lots of sunlight. What about maintaining a slightly lower CYA like 90? Perhaps that would be a good balance to avoid the pH shift/inability to measure pH accurately while still allowing less chlorine addition.

 

[JamesW]

Where would the pH go when you add lots of chlorine, up or down?

Adding a hypochlorite source of chlorine (calcium, sodium or lithium) will raise the pH and a cyanuric acid based source of chlorine (trichlor or dichlor) will lower the pH. Since you would (should) only be using hypochlorite sources, the pH would go up.

[edit]Hypochlorite sources raise the pH on addition and the pH will drop back down upon usage. Therefore, hypochlorite sources are mostly pH neutral. I was responding to Jason's comment about needing to add so much chlorine at once, which would temporarily move the pH out of range.[end edit]

 

[JasonLion]

When using bleach, liquid chlorine, or cal-hypo the PH goes up when you add chlorine, and then comes back down as the chlorine gets used up. With dichlor and trichlor, the PH goes down when you add chlorine, and then goes down further as the chlorine gets used up.