By The Zagosa Team. Published .
Reviewed or updated .
Most of the shock failures we see in home UK hot tubs come back to cyanuric acid (CYA) creep. Once CYA climbs above 50 ppm, free chlorine gets steadily more bound and unavailable. Shocking at high CYA is like trying to light a fire with damp kindling: the heat is there but it cannot do its job.
How CYA blunts a shock
CYA binds loosely to free chlorine, releasing it back into solution slowly. At 30 ppm the binding is light and a shock works almost normally.
At 80 ppm, perhaps half your free chlorine is bound at any moment. A shock that should take you to 10 ppm of active chlorine actually leaves maybe 5 ppm working.
At 150 ppm the binding is so heavy that even a hefty shock has minimal sanitising effect. The water can fail health tests despite reading 5 ppm of free chlorine on a strip.
The test to run before the shock
Use a turbidity tube CYA test. Fill the tube with the test reagent and tub water, look down at the black dot, and stop pouring when it disappears. Read the level off the tube.
Strip-based CYA readings are unreliable. The colour patches match poorly above 50 ppm, and most strips jump straight from 30 to 100.
If CYA is above 50 ppm, do a partial drain (a third of the volume) and refill before shocking. The reset is far more effective than simply dosing more shock.
Stopping CYA from creeping
Switch from stabilised tablets (trichlor or dichlor) to unstabilised chlorine for routine top-ups. Calcium hypochlorite or sodium hypochlorite carry no CYA.
If you must use stabilised chlorine, test CYA monthly and refill at the first sign of creep above 30 ppm.
For bromine tubs, CYA is essentially never an issue, because bromine needs no stabilisation. The check is still worth doing once a year in case CYA has arrived from another source (a shock product, an algaecide, a top-up).
Common questions
Can I just add chlorine harder to beat high CYA?
Up to a point, yes, but you are spending more on chlorine for less return. Dilution by partial drain is cheaper.
Does refill water carry CYA?
Mains water in the UK has zero CYA. CYA only comes from chemicals you add to the tub.