Guide
CYA and Free Chlorine — Why Stabilizer Changes Your FC Target
How cyanuric acid (CYA) protects chlorine from the sun and why higher CYA means you need higher free chlorine to stay sanitized.
Cyanuric acid (CYA) — “stabilizer” or “conditioner” — shields free chlorine from UV. That’s good in outdoor pools. It’s also why a flat “keep FC at 3 ppm” rule fails once CYA climbs.
What CYA does
Without CYA, sunlight can wipe out chlorine in hours. With CYA, chlorine lasts longer — but more of it is “held,” so you need a higher FC floor to keep the same sanitizing power.
The practical implication
- Low CYA → lower FC targets can work (watch the sun).
- High CYA → you must run higher FC, or you’re under-sanitized while the test strip still shows “some chlorine.”
- Very high CYA → draining/dilution becomes the real fix; you can’t dose your way out forever.
Trichlor tabs add CYA over the season. Liquid chlorine does not. That’s a product choice, not a moral one — just know the side effect.
Testing CYA
CYA tests are slower and fuzzier than FC. Retest after big fills, after months of tabs, and before you commit to a SLAM FC target.
How this shows up in Pooldex
Pooldex treats FC targets as CYA-aware, not a single magic number. Enter CYA when you have it; the verdict and doses make more sense. Log both so next month’s “chlorine won’t hold” mystery is already in the history.