Rob Cox October 06 2015
Cyanuric acid is a chlorine stabilizer, and it's sometimes labeled and sold as 'Conditioner'. I prefer the term Stabilizer, because it refers to what it does - stabilizing the chlorine molecule by protecting it from the sun. It could also be called an 'extender', because it extends the product, making it last longer or go farther.
It does this by attaching itself to Free Chlorine molecules, and reducing their activity or 'vibrations'. You could also say that Cyanuric Acid 'suppresses' the chlorine molecule, like a heavy winter coat.
Sounds Great - what's not to like? Cyanuric Acid has come under fire over the years, first as a possible carcinogen with links to kidney damage. But for pool use, there is little danger, as long as you don't eat the stuff.
CYA is essentially non-toxic, according to the US Food & Drug Administration. However, when combined with another non-toxic ingredient melamine, it can form kidney stones, as seen in dogs and cats during the 2007 pet food contamination and in children during the 2008 Chinese milk problems.
And, Cyanuric Acid Reduces Kill Rates. In protecting the chlorine from the sun, like a heavy winter coat, it suppresses its potency or efficacy. When chlorine is stabilized, it behaves sluggishly, and kill rates are slower and fewer. Kind of like a Ninja wearing a Suit of Armor. Well-protected, but not very fast, plus it's hard to see out of the visor.
I detailed the supressive effect of CYA on Free Chlorine, in my article Fresh Thoughts on Cyanuric Acid. Experts disagree on the amount of killing power that is suppressed, ranging from 15% to 50% loss of potency, but all agree that using Stabilizer, or stabilized chlorine, (chlorinated isocyanurates) will reduce chlorine's kill rates and kill speed.
And furthermore, as you might expect - higher Cyanuric Acid levels (above 50 ppm), have a more pronounced effect. And when levels reach 100 ppm, it's no longer a heavy winter coat, but a straight-jacket on the chlorine molecule.
SO, What's the Problem? When the chlorine isn't as powerful as you think it is, your pool is not as sanitary, and possibly not disinfected. The validity of a standard minimum 1.0 ppm Free Chlorine has come into question.
1. Run your chlorine level higher. As compensation for the suppressive effect of CYA - use more chlorine tablets, until you test a consistent 2 or 3 ppm. See chart, adapted from Ben's Best Guess Chart at Pool Solutions, that has some recommendations for chlorine level, at different levels of CYA.
2. Monitor your Cyanuric Acid Level. Test your level at the start and end of every season. Don't add any cyanuric acid to the pool if you already have 20 ppm. Remember that chlorine tablets add a tiny bit of cyanuric acid, and the level will increase naturally. If your cyanuric acid level gets too high, dilute it by draining a portion of water and refilling. Test your level with a good test kit, like my favorite Taylor K2005, or use Test Strips that check all levels, like LaMotte 6-way test strips.
Wait - Do You Need Conditioner or Stabilizer? Yes, most sunny outdoor pools should have at least 20 ppm of cyanuric acid, or they can have trouble maintaining a good chlorine residual. A good range is 20-40 ppm. But, the higher thye CYA level, the higher your chlorine level should be, so it's best to keep it around 20 ppm. If your pool gets a lot of rain, and water drained during winterization, you may never have to worry about high cyanuric acid levels. But Test to be sure!
For a pool that has just been drained and refilled, add 2 lbs of Cyanuric Acid, per 10,000 gallons, to raise the level of Cyanuric Acid to 20 ppm.