Cyanuric acid (CYA), also called stabiliser or conditioner, protects outdoor pool chlorine from UV degradation by the sun. Without it, chlorine in an outdoor pool depletes within a few hours. With too much, chlorine effectiveness is severely compromised.
Key Facts
- Target CYA for unstabilised pools: 30–50 ppm. Salt pools: 60–80 ppm.
- Above 80 ppm, CYA significantly reduces chlorine effectiveness — more chlorine is needed for the same sanitation.
- Trichlor tablets add CYA with every dose; this is the most common source of CYA accumulation.
- CYA can only be reduced by dilution — partial drain and refill with fresh water.
What Cyanuric Acid Does
Cyanuric acid forms a weak chemical bond with free chlorine in pool water. This bond protects chlorine molecules from being broken down by UV radiation from the sun. Without CYA in an outdoor pool, chlorine exposed to direct sunlight at midday will deplete by 90% within two to three hours. With 30–50 ppm CYA, chlorine is much more stable and persists throughout the day. CYA does not permanently consume chlorine — it releases it back when chlorine is needed for sanitation, then recaptures it. However, the bond also moderately reduces the sanitising speed of the released chlorine.
The CYA-Chlorine Relationship
The percentage of free chlorine that is in its most active form (HOCl) decreases as CYA increases. At CYA 30 ppm and pH 7.4, approximately 20% of FC is active. At CYA 100 ppm and the same pH, only 6–7% is active. This is why pools with very high CYA require much higher FC targets to maintain equivalent sanitation. Some industry guidelines recommend a minimum FC that is 7.5–15% of the CYA level. At CYA 80 ppm, minimum FC would be 6–12 ppm — far above the standard 1–3 ppm range. This situation is sometimes called "chlorine lock."
Managing CYA Buildup
CYA accumulates over time when stabilised chlorine products (trichlor tablets or dichlor shock) are used. Unlike pH, alkalinity, and hardness, CYA cannot be reduced with a chemical treatment — the only option is dilution by draining a portion of the pool and refilling with fresh water. Test CYA at the start of the season and monthly if using stabilised products. When CYA exceeds 80 ppm, plan a partial drain of 25–50% and refill. If CYA exceeds 100 ppm, a larger drain may be necessary. Switching to unstabilised liquid chlorine or cal-hypo will stop CYA accumulation and allow natural dilution from rain and splash-out to slowly reduce it.
Examples
A pool owner adds shock every week but algae keeps returning. FC tests fine at 3 ppm. A CYA test shows 120 ppm. At CYA 120 ppm, minimum recommended FC for adequate sanitation is approximately 9–18 ppm. The 3 ppm FC reading is inadequate, but standard dosing tables do not account for this extreme CYA level. The fix: drain 50% of the pool and refill with fresh water, which reduces CYA to 60 ppm. Switch to liquid chlorine. The pool stays clear with normal 2–3 ppm FC from that point forward.
Common Mistakes to Avoid
- Using trichlor shock repeatedly through a season without testing CYA — each shock dose adds CYA to an already elevated level.
- Not testing CYA at the start of the season, starting from an unknown and potentially very high CYA baseline.
- Trying to lower CYA with chemical products — no effective chemical treatment exists for reducing CYA; only dilution works.
- Pool & Hot Tub Alliance — Pool & Spa Operator Handbook, 2022
- Taylor Technologies — Pool/Spa Water Chemistry Reference