What Is CYA Lock?
Cyanuric acid (CYA), also called pool stabilizer or conditioner, is added to outdoor pools to protect chlorine from UV degradation. Without it, direct sunlight destroys chlorine in just a few hours. CYA is genuinely useful — at the right levels.
Here's the problem: CYA works by chemically bonding to chlorine molecules. It forms a reservoir of "stabilized" chlorine that sunlight can't destroy. But that bond doesn't just block sunlight — it also blocks the chlorine from sanitizing. At high CYA levels, the pool effectively locks up almost all available chlorine, leaving almost no active hypochlorous acid (HOCl) — the molecule that actually kills algae and bacteria.
Your test kit measures total free chlorine. It does not tell you how much is actively killing germs. At CYA above 80 ppm, a reading of 5 ppm FC might only represent the sanitizing power of 0.05 ppm HOCl — barely enough to keep a bucket of water clean, let alone a 20,000-gallon pool.
The FC/CYA Ratio — The Number Every Pool Owner Should Know
The relationship between free chlorine and cyanuric acid is the most overlooked concept in residential pool care. Pool chemistry research (primarily the Taylor study and subsequent work by the Pool & Hot Tub Alliance) establishes a clear minimum: free chlorine must be at least 7.5% of your CYA level to provide adequate sanitation under normal conditions.
| CYA Level (ppm) | Minimum FC Needed | SLAM Target FC | Status |
|---|---|---|---|
| 30 ppm | 2.3 ppm | 12 ppm | ✅ Ideal |
| 50 ppm | 3.75 ppm | 20 ppm | ✅ Good |
| 80 ppm | 6 ppm | 32 ppm | ⚠️ High limit |
| 100 ppm | 7.5 ppm | 40 ppm | ❌ CYA lock |
| 150 ppm | 11.25 ppm | 60 ppm | ❌ Drain required |
CYA Danger Zones
Signs You Have CYA Lock
CYA lock is easy to misdiagnose because FC tests appear normal. Watch for this combination of symptoms:
FC Tests Fine, Pool Goes Green
You measure 3–4 ppm FC but algae grows within days. The chlorine is there — it just isn't active enough to stop algae.
Shock Doesn't Last
You shock, pool clears, then turns green again within a week. Each shock temporarily overcomes the CYA lock but can't maintain it.
CYA Test Over 80 ppm
If you test CYA directly and it reads 80 ppm or higher, your chlorine's effectiveness is severely compromised.
How to Fix CYA Lock — Partial Drain and Refill
There is no magic chemical that removes CYA from pool water. The only reliable, proven fix is dilution: drain a portion of the pool and replace it with fresh water. Here's how to plan it:
- Drain 1/3 of pool: reduces CYA by approximately 33%
- Drain 1/2 of pool: reduces CYA by approximately 50%
- Example: CYA at 120 ppm → drain half → refill → CYA drops to ~60 ppm → manageable
- Multiple partial drains are safer than one full drain (avoids hydrostatic pressure issues with in-ground pools)
The SLAM Process After Draining
Once you've diluted CYA to 30–50 ppm, it's time to SLAM (Shock Level And Maintain) the pool to kill any remaining algae and establish clean water:
Test and Record Baseline
After refilling, test CYA, pH, FC, and TA. Record all numbers. Target pH 7.4–7.6 before proceeding.
Calculate SLAM Target FC
SLAM target = 40% of CYA. At CYA 40 ppm, target FC is 16 ppm. At CYA 50 ppm, target FC is 20 ppm.
Add Liquid Chlorine (Not Tablets)
Use 10–12.5% liquid chlorine or calcium hypochlorite shock. Do NOT use trichlor during SLAM — it adds more CYA and lowers pH aggressively.
Run Filter Continuously
Filter must run 24/7 during the SLAM process. Test FC every 2–4 hours and add chlorine to maintain SLAM level whenever it drops.
Pass All Three Criteria to End SLAM
(a) Water is visually clear. (b) CC (combined chlorine) is below 0.5 ppm. (c) FC holds at SLAM level overnight (loses less than 1 ppm in 8 hours).
Switch to Liquid Chlorine Maintenance
To prevent CYA from rising again, maintain with liquid chlorine or a saltwater system rather than trichlor tablets. Check CYA monthly.
Preventing CYA Lock in the Future
Once you've solved the problem, keeping CYA in range is straightforward:
- Test CYA monthly during swim season — it creeps up slowly and is easy to miss
- Reduce tablet use when CYA approaches 60 ppm — switch to liquid chlorine temporarily
- After heavy rain or topping off the pool, retest CYA — the dilution may actually help lower levels
- Consider a saltwater system — salt cells produce chlorine without adding CYA; you add CYA separately and it stays stable
- Annual partial drain — many professionals recommend draining 1/4 to 1/3 of pool water each spring to reset CYA and other dissolved solids