Cyanuric acid (CYA) protects chlorine from sunlight, but too much of it makes chlorine sluggish and ineffective. Once CYA climbs above 80 ppm, you’ll find yourself adding more and more chlorine for less and less effect. Unfortunately, CYA only comes out of pool water through dilution — there’s no chemical that removes it. Here’s how to lower CYA correctly.
When CYA is too high
- 30–50 ppm: ideal for traditional chlorine pools.
- 60–80 ppm: ideal for saltwater pools.
- 80–100 ppm: getting sluggish; chlorine effectiveness drops noticeably.
- Above 100 ppm: “chlorine lock” territory. Algae blooms despite high chlorine readings.
The only way to lower CYA: dilution
CYA is stable in water and doesn’t break down with chlorine, oxidation, or sunlight. The only methods are:
1. Partial drain and refill
The standard method. Drain a percentage of the pool and refill with fresh water. To cut CYA in half, drain 50% of the pool. Reasonably effective; uses a lot of water.
2. Reverse osmosis water treatment
Specialized service that filters pool water through an RO membrane and returns it to the pool. Removes CYA, calcium, and TDS without the water waste of a partial drain. Costs $0.20–$0.50 per gallon treated. Available in many U.S. metro areas.
3. Heavy rain
If you live somewhere with regular heavy rain, water overflow dilutes CYA naturally over weeks. Won’t hit target in time for the swim season, but contributes.
Step-by-step partial drain
Use a liquid drop test. Strip tests are too inaccurate at high CYA levels.
If current CYA is 120 ppm and you want 60 ppm, drain 50% (60 / 120 = 50%). If you want 30 ppm from 120 ppm, drain 75%.
Sand or DE filter: set the multiport valve to Waste position. Cartridge filter: drain via the pool drain plug or with a submersible pump.
Watch the water level carefully. Stop before water drops below the skimmer mouth (pump will lose prime).
Run a hose into the pool. Time varies — 8–24 hours for a typical residential pool.
After 24 hours of mixing, retest CYA and all other parameters. Adjust chemistry per our pool chemical levels guide.
Chemicals for after the drain
Taylor K-2006 Test Kit (with CYA)
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Durachlor Alkalinity Increaser
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Durachlor Chlorine Stabilizer
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- Stop using dichlor or trichlor (stabilized chlorine products) — they add CYA every dose.
- Use cal-hypo or liquid chlorine for routine sanitation; add cyanuric acid separately and only when needed.
- Test CYA monthly during swim season.
- Don’t shock with dichlor — use cal-hypo for shocks.
If you’re struggling with chlorine effectiveness despite high free chlorine readings, CYA is almost certainly the culprit. Send PST Pool Supplies your test results and we’ll walk through the right drain math for your pool size.