CYA reduces UV chlorine loss
FC targets must scale with CYA
Lowering CYA = water replacement
CYA (cyanuric acid), also called stabiliser or conditioner, helps outdoor pools because it slows how fast free chlorine (FC) is destroyed by sunlight. At the same time, higher CYA means the same FC reading is less “active”. That’s why a pool can look clear, show “normal FC”, and still struggle with algae if CYA is high and FC targets are not adjusted.
Core idea: pick a sensible CYA level for your pool type, then keep FC proportional to that CYA. Most “chlorine disappears” complaints are either (1) CYA too low for sun exposure or (2) CYA crept up while FC targets stayed too low.
Why pools “lose chlorine” in the sun
- Low CYA: FC can drop sharply through the day on bright, warm weather.
- Heat + swimming: more organic load = higher chlorine demand.
- Right CYA band: FC holds longer, making daily control easier.
Choosing safe target CYA ranges
Outdoor pools (most backyards)
- Liquid/manual dosing: typically 30–50 ppm
- Salt chlorinator (SWG): often 60–80 ppm (only if your system can maintain the higher FC targets)
Covered / low-UV exposure
- Typically 20–40 ppm
If you’re unsure, start conservative (for example, 30–40 ppm), stabilise FC, then adjust. Raising CYA is easy; lowering it usually needs dilution.
How to test CYA correctly (so readings are reliable)
- Use bright indirect light; read at eye level (black-dot tests are visual).
- Follow the kit mix/shake timing precisely; repeat and use the middle reading if unsure.
- After adding stabiliser, allow circulation time before re-testing (commonly 24–48 hours).
- Big CYA changes usually mean water replacement: overflow, backwash, vacuum-to-waste, leaks, drain/refill.
FC/CYA quick reference (maintenance)
Use Min FC as the floor. Aim for Target FC so normal daily loss doesn’t drop you below the minimum. If algae appears while FC “looks normal”, confirm CYA and compare to these targets.
Practical rule: higher CYA protects FC from UV, but requires higher FC targets to keep sanitation strength consistent.
Maintenance targets — CYA 20 to 50
| CYA (ppm) | Min FC (ppm) | Target FC (ppm) |
|---|---|---|
| 20 | 2 | 3 |
| 30 | 2 | 4 |
| 40 | 3 | 5 |
| 50 | 4 | 6 |
Maintenance targets — CYA 60 to 100
| CYA (ppm) | Min FC (ppm) | Target FC (ppm) |
|---|---|---|
| 60 | 5 | 7 |
| 70 | 5 | 8 |
| 80 | 6 | 9 |
| 90 | 7 | 10 |
| 100 | 8 | 12 |
At very high CYA, daily FC targets can become difficult to maintain consistently.
If maintaining target FC becomes unreliable or algae pressure increases, plan dilution (partial water replacement).
High-demand cleanup level (reference)
When there is confirmed algae/organics demand, the effective approach is reaching a higher FC level that matches CYA and maintaining it until the pool is clean (not just a one-time spike). This table is a reference for “cleanup level” FC.
| CYA (ppm) | Cleanup FC (ppm) |
|---|---|
| 20 | 8 |
| 30 | 12 |
| 40 | 16 |
| 50 | 20 |
| 60 | 24 |
| 70 | 28 |
| 80 | 32 |
| 90 | 36 |
| 100 | 40 |
Common causes of CYA creep
- Tablets all the time: trichlor/dichlor add CYA as they add chlorine, so CYA slowly rises.
- Rare CYA testing: CYA is checked too late (only after problems appear).
- Fixed FC targets: FC is kept at “generic numbers” while CYA climbs into 70–100 ppm.
How to raise CYA (stabiliser): dosing formula
In metric pool math, 1 ppm = 1 gram per 1,000 litres. So the simplest dosing formula is:
Stabiliser grams ≈ (Pool litres × Δppm) ÷ 1,000
If your product is not 100% cyanuric acid, divide by its fraction (example: 96% product → divide by 0.96).
If your product is not 100% cyanuric acid, divide by its fraction (example: 96% product → divide by 0.96).
Add stabiliser gradually and allow time to dissolve and circulate before re-testing (commonly 24–48 hours).
Dosing cheat sheets
Raise CYA by 10 ppm
| Pool volume | Stabiliser needed |
|---|---|
| 30,000 L | 300 g |
| 40,000 L | 400 g |
| 50,000 L | 500 g |
| 60,000 L | 600 g |
| 70,000 L | 700 g |
| 80,000 L | 800 g |
Raise CYA by 20 ppm
| Pool volume | Stabiliser needed |
|---|---|
| 30,000 L | 600 g |
| 40,000 L | 800 g |
| 50,000 L | 1,000 g |
| 60,000 L | 1,200 g |
| 70,000 L | 1,400 g |
| 80,000 L | 1,600 g |
How to lower CYA: dilution math (no guesswork)
CYA does not evaporate. The dependable way to reduce it is water replacement. Plan the percentage first:
Fraction to replace ≈ 1 − (Target CYA ÷ Current CYA)
Examples
| Current → Target | Replace (approx.) |
|---|---|
| 100 → 50 ppm | 50% |
| 80 → 50 ppm | 37.5% |
| 70 → 40 ppm | 42.9% |
| 60 → 40 ppm | 33.3% |
After any meaningful water replacement, re-test CYA and set FC targets again from the FC/CYA chart.
Symptoms: what you see and what to test first
- FC crashes to near-zero late afternoon (sunny): test CYA + FC same day (often CYA too low for exposure).
- Algae despite “normal FC”: test CYA + FC and compare to chart (often FC target too low for current CYA).
- Dull water / irritation / persistent smell: test FC (and CC if available) + CYA; check filter condition.
- SWG runs longer but FC stays low: test CYA; verify output % and pump time; inspect cell condition.
- CYA “suddenly dropped”: look for dilution events (overflow/backwash/vac-to-waste/leak) and re-test.
FAQ
1) How often should I test CYA?
In summer for outdoor pools: about monthly. Re-test after overflow, backwashing cycles, vacuum-to-waste, leaks, or partial drain/refill.
2) What is considered “high CYA”?
Many residential pools become harder to manage once CYA is around 80–100 ppm unless FC targets are raised and chlorination can maintain them consistently.
3) Why do I get algae when FC looks “normal”?
Because FC targets depend on CYA. If CYA rises but FC targets don’t, sanitation strength drops and algae prevention becomes unreliable.
4) Can I lower CYA quickly without draining?
The most dependable method is water replacement (partial drain/refill or controlled exchange). Plan the percentage first so you don’t over-drain.
5) SWG pools: what’s different?
SWG pools often run higher CYA to reduce UV loss, but FC targets must rise with CYA and the cell plus pump schedule must keep up.
6) Do I need CYA in winter?
If the pool remains chlorinated and gets sun exposure, some stabiliser still helps. UV is lower, so FC usually holds longer.
7) What happens to CYA after heavy rain or overflow?
Dilution lowers CYA. After overflow/backwash, re-test CYA and adjust FC targets and SWG settings if applicable.
8) When should I call for professional help?
If algae keeps returning, CYA is very high and needs a careful dilution plan, pool volume is uncertain, there is a suspected leak, or test results are inconsistent.