Chlorine not working? Check your stabiliser first

Chlorine not working? Check your stabiliser first

Cyanuric acid protects chlorine from the sun, but too much of it switches your chlorine off. What the right level is, how pools drift over it, and the only real fix.

This is the single most common reason we find for “I keep adding chlorine and nothing happens” on a Florida pool.

What stabiliser does

Cyanuric acid — CYA, stabiliser, conditioner — binds to chlorine and shields it from ultraviolet light. Without it, an outdoor pool in Vero Beach can lose most of its free chlorine in a single sunny day. With it, chlorine survives long enough to do its job.

Why more is not better

The same bond that protects chlorine also holds it back. The more stabiliser in the water, the slower the chlorine works, and past a certain point a normal chlorine reading is effectively inactive. The pool tests “fine” and still goes green.

The commonly used guidance is 30–50 ppm of cyanuric acid for a residential pool, and free chlorine held at roughly 7.5% of the CYA level. At 40 ppm CYA that means about 3 ppm of free chlorine. At 100 ppm CYA it means 7.5 ppm — which almost nobody is actually maintaining, which is why over-stabilised pools bloom.

How pools drift high

• Stabilised chlorine products. Trichlor tablets and dichlor shock both add cyanuric acid every time you use them. A pool run on tablets year-round climbs steadily and never comes back down on its own.
• Top-ups instead of dilution. Cyanuric acid does not evaporate and it is not consumed. Only splash-out, backwashing and draining remove it.
• Salt pools. A chlorine generator makes chlorine without adding stabiliser, but owners often still drop tablets in the feeder “to help”, and the level creeps up anyway.

The only real fix

There is no chemical that reliably removes cyanuric acid from pool water at a sensible cost. You lower it by replacing water — a partial drain and refill, sized to the reading. On a pool that is far gone, that is a drain-and-refill job; on a pool that is only somewhat high, a partial exchange plus switching off stabilised tablets will bring it back into range over a season.

What we do about it

We test stabiliser on a schedule rather than only when something goes wrong, and we choose sanitiser products with the reading in mind — liquid chlorine when the level is already high, tablets only when there is room for them. It is a boring, invisible piece of the job, and it is the difference between a pool that behaves and a pool that fights you every August.

Further reading: Orenda Technologies on cyanuric acid limits.