Cyanuric acid management: the most ignored number in pool service

What CYA actually does

Cyanuric acid bonds reversibly with free chlorine, forming a chlorine reservoir that releases chlorine slowly as the free chlorine in the water is consumed or destroyed. Without CYA, free chlorine in an outdoor pool is destroyed by UV at a rate of 75 to 90% per hour in direct sunlight. With CYA at 30 to 50 ppm, the same chlorine lasts hours longer because the CYA-bound reservoir keeps releasing into the water.

The trade-off is that CYA-bound chlorine is less active than free chlorine. As CYA goes up, the effective sanitizing power of the same FC reading drops. At CYA 30, an FC of 2 ppm is plenty. At CYA 80, an FC of 2 ppm is borderline. At CYA 150, an FC of 2 ppm is almost no sanitation at all.

Why CYA accumulates over time

CYA is added to a pool in two ways. Direct: as stabilizer (granular cyanuric acid) when the operator dosed for low CYA. Indirect: as a byproduct of trichlor or dichlor tablet use. Every pound of trichlor dissolved adds approximately 6 ppm of CYA to a 20,000 gallon pool. A pool that runs 4 to 6 tablets per week through a season adds 30 to 60 ppm of CYA per year.

CYA only decreases through dilution (water replacement) or very slow biological breakdown (negligible in service settings). A pool that has been on trichlor for 3 years without a partial drain almost always has CYA above 100 ppm, and the operator wonders why the pool keeps fighting algae.

“Every pound of trichlor dissolved adds 6 ppm of CYA. After 3 years, you are at 150 ppm wondering why nothing works.”

CYA targets by pool type

• Outdoor chlorine pool: 30 to 50 ppm
• Outdoor saltwater pool: 70 to 80 ppm (the salt cell produces chlorine slowly, needs higher CYA buffer)
• Indoor chlorine pool: 0 to 30 ppm (no UV, CYA not needed)
• Spa or hot tub: 0 to 20 ppm (heat accelerates chlorine consumption regardless)
• Commercial pool (health code in most states): 30 to 80 ppm, often capped at 100 by code

How to test CYA accurately

The standard CYA test in pool service is the Taylor R-0013 reagent (a turbidity test using melamine). The test produces a precipitate that obscures a dot at the bottom of a graduated tube; the operator pours water and reagent into the tube until the dot disappears, and reads the CYA value off the graduation. The test has an accuracy of about ±10 ppm at typical pool levels.

To get a more accurate reading, test the same sample twice. If the results are within 10 ppm, average them. If they differ by more than 10 ppm, test a third time. Direct sunlight on the tube during the test throws off the reading; do it in shade.

Test CYA monthly during the season, at minimum quarterly off-season. Tracking CYA over time is more useful than any single reading because the trend tells you when a drain is coming.

The drain decision

When CYA crosses 90 to 100 ppm, the math on continuing to operate the pool with that chemistry tilts heavily toward draining. The FC target needed to sanitize jumps to 28 ppm or higher, which is impractical and expensive. The customer ends up with a green pool any time chlorine production lags for half a day.

Drain decisions are about how much to drain and when:

• Drain 50%: drops CYA by 50% (e.g., 100 ppm becomes 50 ppm). Right call when CYA is at 80 to 120 ppm
• Drain 70%: drops CYA by 70% (e.g., 150 ppm becomes 45 ppm). Right call when CYA is at 120 to 180 ppm
• Full drain: starts over. Required if CYA is above 200 ppm or the plaster needs acid washing anyway

Schedule drains in the shoulder season (October/November or March/April in sunbelt markets). The pool is not in heavy use, evaporation is lower, and refill chemistry start is less stressful for the customer.

Switching off trichlor

For customers whose CYA accumulates because of trichlor tablet use, the long-term fix is switching to liquid chlorine as the primary sanitizer. Liquid chlorine (sodium hypochlorite) adds zero CYA and gives the operator full control of the chemistry. The downside is more frequent dosing, which is easier for a service tech visiting weekly than for a DIY homeowner.

A reasonable hybrid: keep trichlor tablets for shoulder season and travel weeks (when the operator cannot visit as often), but use liquid chlorine as the primary in peak season. This caps CYA growth at a manageable rate and reduces the frequency of partial drains from every 2 years to every 4 to 5 years.

Common CYA mistakes

• Not testing CYA monthly. The number drifts up silently and the operator only notices when the pool turns green
• Adding stabilizer (granular CYA) when the pool is already at 60+ ppm. Overshoots and triggers an unnecessary drain
• Refusing to drain because "the water looks fine." It does until it does not, and the green pool event will cost more than the drain would have
• Ignoring CYA on saltwater pools. They need 70 to 80 ppm; running them at 30 to 50 burns the cell out faster
• Trusting test strips for CYA. They are wildly inaccurate. Use the Taylor R-0013 reagent kit