What Is Cyanuric Acid?
Cyanuric acid (CYA), also called stabilizer or conditioner, is a chemical that protects free chlorine from UV degradation. Without CYA, sunlight destroys up to 90% of free chlorine within 2 hours. With 30–50 ppm CYA, chlorine lasts 3–5x longer.
Think of CYA as sunscreen for your chlorine.
The FC/CYA Relationship
Here's the part most pool owners don't understand: CYA doesn't just protect chlorine — it also reduces chlorine's killing power. CYA bonds with chlorine molecules, releasing them slowly. This is good for longevity but bad for sanitization speed.
The critical ratio: FC should be at least 7.5% of CYA.
| CYA Level | Minimum FC | Effective Chlorine |
|---|---|---|
| 0 ppm | 1 ppm | 100% of FC is active |
| 30 ppm | 2.3 ppm | ~4% of FC is active |
| 50 ppm | 3.8 ppm | ~3% of FC is active |
| 80 ppm | 6 ppm | ~2% of FC is active |
| 100 ppm | 7.5 ppm | ~1.5% of FC is active |
| 150 ppm | 11.3 ppm | ~1% of FC is active |
At 100 ppm CYA, you need 7.5 ppm FC just to match the killing power of 1 ppm FC with no CYA. This is why high CYA is a problem.
Ideal CYA Levels
| Pool Type | Ideal CYA | Max CYA |
|---|---|---|
| Residential (chlorine tabs) | 30–50 ppm | 70 ppm |
| Residential (liquid chlorine) | 30–40 ppm | 50 ppm |
| Salt water pools | 30–50 ppm | 70 ppm |
| Commercial pools | 0–30 ppm | Varies by health code |
What Causes High CYA?
The #1 cause: trichlor tablets (chlorine tabs). Every trichlor tablet adds CYA to the pool. It cannot be removed by any chemical reaction — it just accumulates.
A 3-inch trichlor tab adds approximately 6 ppm CYA to a 10,000 gallon pool. Use one tab per week and you'll add ~24 ppm CYA per month.
Other sources:
- Dichlor shock — Also contains CYA (less common)
- Adding stabilizer when it wasn't needed
- Not draining periodically — CYA only goes up, never down (except dilution)
How to Lower CYA
There is only one way to lower CYA: drain and refill with fresh water.
No chemical, filter, or enzyme product removes CYA. Any product claiming to do so is misleading.
Partial drain calculation:
To reduce CYA by 50%, drain and refill approximately 50% of the pool volume. The math is proportional.
| Current CYA | Target CYA | Drain Percentage |
|---|---|---|
| 100 ppm | 50 ppm | ~50% |
| 80 ppm | 40 ppm | ~50% |
| 120 ppm | 40 ppm | ~67% |
Important: After draining, retest all chemistry. Fresh water changes everything — alkalinity, calcium, and pH will all need adjustment.
CYA Management Strategy for Pros
The best approach depends on your chlorination method:
If Using Trichlor Tabs (most common)
- Start the season with CYA at 30 ppm
- Test CYA monthly
- When CYA hits 70 ppm, switch to liquid chlorine until season end
- Drain 30–40% at winter closing to reset for next season
If Using Liquid Chlorine
- Add CYA at season start to 30–40 ppm
- CYA won't increase on its own (liquid chlorine has no CYA)
- Only retest if you suspect dilution (rain, leaks, splash-out)
Salt Water Pools
- Salt cells produce unstabilized chlorine (no CYA added)
- Add CYA separately at season start to 30–50 ppm
- Monitor throughout season — rain and splash-out reduce CYA
Track CYA Over the Season
PoolOps logs CYA at every deep test, so you can see exactly how fast it's climbing and schedule a partial drain before it becomes a problem.