Cyanuric acid protects chlorine from UV in pools but quickly turns into a problem in the small volumes of a hot tub. Here is the threshold to watch.
Cyanuric acid (CYA) is the unsung hero of outdoor pools. It binds to free chlorine loosely, protecting it from UV breakdown and extending its life by hours rather than minutes. In a hot tub, the same chemistry that helps in summer becomes a problem above about 30 ppm because the small water volume concentrates it fast.
In a 30,000-litre pool with a stabilised tablet feeder, CYA climbs slowly and rarely exceeds 40 ppm.
In a 1,200-litre hot tub, the same tablet contributes the same CYA into one twenty-fifth of the water. A few weeks of stabilised tablets and CYA is at 80 to 100 ppm.
At that level, free chlorine becomes sluggish. You can read 4 ppm on the strip and still get bacterial concerns because most of the chlorine is bound to CYA and not available to act.
Aim for CYA below 30 ppm in any hot tub. Above 50 ppm, free chlorine effectiveness drops noticeably.
Test with a turbidity tube (the one with a black dot at the bottom that you fill until you cannot see it). It is the only home test that gives you a usable number for CYA.
If you have a salt-water chlorinator, test even more often. Some salt cells slowly add small amounts of CYA-equivalent compounds.
Use unstabilised chlorine in hot tubs whenever possible. Dichlor adds CYA, calcium hypochlorite does not.
Test CYA at every refill and again after any prolonged use of stabilised tablets.
If CYA has crept above 50 ppm, dilute. There is no cost-effective chemical that removes CYA. Drain a third, refill, and re-balance. The refill is your reset button.
20 to 30 ppm. Just enough to protect free chlorine from UV without binding too much of it into the bound form.
No. Bromine is more UV-stable than chlorine and does not benefit from CYA. CYA in a bromine tub is a TDS contributor and nothing else.