Key takeaways
- CYA protects free chlorine from UV breakdown; without it, sanitizer disappears in hours under Austin sun.
- Trichlor tablets add CYA every time they dissolve; liquid chlorine and cal-hypo do not.
- CYA does not evaporate or break down; the only way to lower it significantly is a partial drain and refill.
- High CYA requires higher free chlorine targets to achieve the same sanitizing power.
- Over-stabilized pools (CYA above roughly 80 ppm) can stay persistently cloudy or green despite apparent chlorine levels.
- Salt system pools still need CYA managed in the water; the cell does not regulate stabilizer.
Austin context
Austin's long, bright summers make some CYA essential for any outdoor pool. From late March through October, unprotected chlorine can drop to near zero in just a few hours of direct sun. That is why many local pools run trichlor tablets or need stabilizer topped up after a dilution event. The risk is the flip side: a pool on tablets all year for multiple years can accumulate CYA levels that neutralize much of the chlorine's killing power.
Austin & West Austin pools also see heavy water top-up from evaporation and rain refill. Rain dilutes CYA slightly; topping up a pool with fresh fill does too. But the dilution is slow compared to tablet accumulation. If a pool has run mostly on trichlor for two or three years without a significant water change, CYA is often the hidden variable behind repeated algae fights and the need to run high salt output or add extra chlorine constantly.
How CYA works in the pool
CYA forms a loose bond with chlorine molecules that shields them from UV radiation. This bond is temporary: when the bound chlorine molecule finds a pathogen or organic contaminant to oxidize, it releases and does its job. The CYA molecule stays behind and is available to bond with the next chlorine molecule you add.
Think of CYA as a holding pattern for chlorine. It keeps more chlorine available longer on sunny days, but the chlorine inside that holding pattern is slightly slower to react than fully free chlorine without any stabilizer. That slowdown is acceptable in a normal range and catastrophic at very high CYA levels.
Where CYA comes from and why it builds
Trichlor (3-inch tabs and sticks) is the most common CYA source. Each tablet contains roughly 50 percent CYA by weight. A pool that relies on floaters or feeders loaded with trichlor will see CYA climb every week of the season unless dilution keeps pace.
Granular stabilizer or liquid CYA can be added directly to dial a pool in when starting fresh or after a drain. Liquid chlorine (sodium hypochlorite), cal-hypo, and salt cells do not add CYA. They raise free chlorine without changing the stabilizer level at all.
- Trichlor: adds CYA with every dose. Watch level monthly.
- Dichlor granular: also adds CYA. Good for salt-system boost but track the accumulation.
- Cal-hypo: no CYA added. Good shock choice when stabilizer is already high.
- Liquid chlorine: no CYA added. Common for maintaining residual on well-managed pools.
- Salt cell: no CYA added. You still need to maintain CYA in the water for UV protection.
Reading CYA levels and matching chlorine targets
The industry standard minimum for outdoor pools is usually around 30 to 40 ppm CYA. Above that, free chlorine targets need to scale upward to maintain the same effective sanitizing power. A common guideline is that free chlorine should be at least roughly 7.5 percent of the CYA level to have adequate protection.
At 50 ppm CYA, you want at least 3.75 ppm free chlorine. At 80 ppm CYA, you want at least 6 ppm free chlorine. At 100 ppm CYA, the target climbs to roughly 7.5 ppm, which is increasingly hard to maintain practically and expensive in product. Many techs and researchers now treat 60 to 80 ppm as a practical upper limit for most residential pools without special protocols.
- 30 to 50 ppm CYA: standard outdoor target range for tablet or liquid chlorine pools.
- 50 to 70 ppm CYA: acceptable but watch free chlorine targets carefully.
- Above 80 ppm CYA: investigate. Repeated algae with apparent chlorine is often CYA-driven.
- Salt system pools: some run a slightly lower CYA target. Discuss with your tech.
Over-stabilization and what it looks like
A pool with CYA above 80 or 100 ppm can look fine on a test strip for free chlorine and still struggle to kill algae. The chlorine is present but so bound in the CYA holding pattern that its effective sanitizing rate is too slow for the demand. Techs call this chlorine lock or CYA lock, though the mechanism is more of a rate reduction than a true lock.
Signs include persistent cloudiness or a greenish tint despite what appears to be decent free chlorine, repeated algae that comes back after shocks, and a pool that seems to need far more product than it did previously. If you have been on trichlor for multiple years without a water change, CYA is worth testing specifically.
How to lower CYA
CYA does not break down meaningfully under UV, aeration, or chemical treatment at pool conditions. The practical solution is a partial drain and refill with fresh water that contains no CYA. A 25 to 40 percent water change can bring high CYA back into range.
During the drain, the pool needs to stay above the minimum operating level for the pump. We plan the sequence to protect equipment, maintain some sanitizer during the refill, and retest once the new water is circulated and balanced.
CYA strategy through the Austin year
Spring and early summer: confirm CYA is in range before swim season. If winter brought partial drain or heavy refill, it may have dropped; if the pool ran tablets all winter, it may have climbed.
Midsummer: UV demand is highest. Make sure CYA is present and free chlorine targets are matched to it. This is when over-stabilization shows up as a summer algae fight despite "enough" chlorine.
Fall and winter: chlorine demand drops but cedar season still demands sanitizer. CYA stays in the water, so a winter on tablets continues to raise it.
- Test CYA at least monthly, more often when troubleshooting.
- Log CYA alongside free chlorine and pH so the trend is visible.
- Avoid refilling a low CYA pool with trichlor only; use liquid or cal-hypo while stabilizer is too low.
- After any significant dilution (rain, partial drain), retest before adjusting dosing.
Salt system pools and CYA
Salt cells generate chlorine but do not add or remove CYA. A salt pool still needs a target CYA level for UV protection. Without any stabilizer, even high cell output will struggle to hold residual through a long sunny Austin afternoon.
Salt pools often run a slightly lower CYA target than heavy-tablet pools because the cell can regenerate chlorine continuously. Some operators prefer 30 to 50 ppm. The right number depends on your sun exposure, bather load, and how many hours the cell runs. We set a target for your specific setup.
Common mistakes
- Running a floater full of trichlor all year without testing CYA and watching it climb.
- Adding more chlorine when the pool keeps going green without testing CYA first.
- Assuming liquid chlorine is adding CYA the same way tablets do.
- Draining only a tiny amount of water and expecting a meaningful CYA reduction.
- Setting the salt cell to 100 percent output thinking CYA is irrelevant for salt pools.
- Skipping CYA tests in winter because demand is lower.
Safe for owners
- Use a CYA-specific test (liquid regents or a turbidity tube) rather than a basic strip for more accuracy.
- Note what chlorine source you are using and mention it on your service notes.
- Stop adding trichlor tablets when CYA is already high and ask about alternatives.
- Keep track of how often auto-fill adds water to gauge dilution rate.
Call a pro
- Accurately testing and logging CYA on a monthly or as-needed basis.
- Planning and executing a partial drain and refill to reduce over-stabilization.
- Adjusting free chlorine targets upward to match current CYA level.
- Switching sanitizer strategy (liquid vs tablets vs cell output) based on CYA trend.
- Diagnosing repeated algae tied to CYA lock vs other chemistry or circulation causes.
Why weekly care matters
CYA is slow-moving compared to pH or chlorine residual, but ignoring it between visits is how pools end up chronically over-stabilized. Weekly service logs the trend, catches a tablet-heavy winter that pushed CYA into the danger zone, and adjusts chlorine targets before the summer algae season exposes the problem. Year-round visits also give the data to decide when a partial drain makes sense before CYA becomes a bigger issue.
Common questions
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