By The Zagosa Team. Published .
Reviewed or updated .
Cyanuric acid (CYA) is the quiet workhorse of outdoor pools. It clings loosely to free chlorine, guarding it from UV breakdown and stretching its life from minutes into hours. In a hot tub, that same helpful chemistry turns into a problem once you get past about 30 ppm, because the small volume concentrates it so quickly.
Helpful in pools, harmful in tubs
In a 30,000-litre pool running a stabilised tablet feeder, CYA rises slowly and rarely tops 40 ppm.
In a 1,200-litre hot tub, the same tablet drops the same CYA into a twenty-fifth of the water. A few weeks of stabilised tablets and CYA is sitting at 80 to 100 ppm.
At that level free chlorine turns sluggish. You can read 4 ppm on the strip and still meet bacterial trouble, because most of the chlorine is bound to CYA and not free to act.
The threshold, and testing it
Aim for CYA below 30 ppm in any hot tub. Above 50 ppm, free chlorine effectiveness drops off noticeably.
Test with a turbidity tube (the one with the black dot at the base that you fill until it vanishes). It is the only home test that gives a usable CYA number.
Running a salt-water chlorinator? Test even more often. Some salt cells slowly add small amounts of CYA-equivalent compounds.
Keeping it under control
Use unstabilised chlorine in hot tubs wherever you can. Dichlor adds CYA; calcium hypochlorite does not.
Test CYA at every refill and again after any long stretch of stabilised tablets.
If CYA has crept above 50 ppm, dilute it. No affordable chemical removes CYA. Drain a third, refill, and re-balance. The refill is your reset button.
Common questions
What CYA suits an outdoor tub in summer?
20 to 30 ppm. Just enough to shield free chlorine from UV without locking too much of it into the bound form.
Does bromine need a stabiliser?
No. Bromine is more UV-stable than chlorine and gets no benefit from CYA. In a bromine tub, CYA is just a TDS contributor and nothing more.