CYA protects chlorine from UV but reduces its activity. The minimum free chlorine to maintain effective sanitation must increase as CYA rises. Standard 1 ppm FC is only appropriate when CYA is below 30 ppm.
Minimum FC by CYA Level (PHTA / Taylor guidance)
| CYA Level (ppm) | Minimum FC (ppm) | Notes |
|---|---|---|
| 0 | 0.5 | Unstabilised — chlorine depletes in hours outdoors |
| 10 | 0.5 | Low stabilisation |
| 20 | 1.0 | Minimum outdoor pool recommendation |
| 30 | 1.5 | Standard minimum for stabilised pools |
| 40 | 2.0 | Common pool chemistry target |
| 50 | 2.5 | Adequate for normal bather loads |
| 60 | 3.0 | Salt pool / high UV exposure recommendation |
| 70 | 3.5 | Approaching upper management range |
| 80 | 4.0 | Upper limit; consider partial drain |
| 100 | 5.0+ | Problematic — plan partial drain |
| 120+ | 7.0+ | Chlorine lock zone — partial drain required |
Notes
- These minimums are for outdoor pools in direct sunlight.
- Indoor pools and covered spas do not require CYA.
- CYA above 100 ppm: plan a 40–50% partial drain and refill.
Sources:
- Pool & Hot Tub Alliance — Pool & Spa Operator Handbook, 2022
- Taylor Technologies — Pool/Spa Water Chemistry Reference