Cyanuric Acid: Why Your Chlorine Quits On You In July

Cyanuric acid is the reason a pool that tested fine in March turns green in July with the same chlorine routine. CYA is sunscreen for chlorine. A little protects it. Too much locks it up.

What CYA actually does

UV light from the sun destroys free chlorine fast. Unstabilized chlorine in an uncovered pool in direct sun can burn off in a couple of hours. Cyanuric acid binds loosely to free chlorine and shields it from UV, releasing it back as it gets consumed. That's the whole job.

Trichlor tabs and dichlor shock both contain CYA. Every time you drop a tab in the floater or skimmer, you're adding stabilizer whether you wanted to or not. CYA does not evaporate, does not break down in sunlight, and is not removed by your filter. It only leaves the pool through dilution — splash-out, backwashing, partial drain, or rain overflow.

So it builds. And builds.

The lock-up problem

Here's the part that catches operators off guard in summer:

As CYA climbs, the percentage of your free chlorine that is actually available to sanitize and oxidize drops. The chlorine is in the water — your DPD test will read it — but it's bound up tight and reacting slow.

The working rule most operators use is the free chlorine to CYA ratio. Free chlorine should sit somewhere around 7.5% of the CYA level to stay active against algae. Run the numbers:

CYA 30 ppm → FC around 2–3 ppm is doing real work
CYA 50 ppm → you need FC closer to 4 ppm
CYA 80 ppm → 6+ ppm FC just to break even
CYA 100+ ppm → you're chasing a green pool with FC 3 and wondering why

This is why a pool that ran clean all spring at FC 2.0 suddenly goes hazy in July after a few months of tabs. CYA crept from 40 to 90 and the chlorine you're reading doesn't have the punch it used to.

How to test it

Standard 4-way and 5-way strips do not test CYA. You need a turbidity test — the little tube where you add reagent and look down at the black dot until it disappears. It's not as precise as FC or pH, but it gets you in the ballpark.

Test CYA at least at season start, mid-summer, and any time a pool starts misbehaving with chlorine that looks fine on paper.

Target ranges

Rough targets, not law:

Outdoor chlorine pool: 30–50 ppm
Saltwater pool: 60–80 ppm (SWGs run leaner FC, so the cell needs more UV protection)
Indoor pool: 0 — no sun, no need, and CYA just slows your chlorine down

Above 100 ppm and you're in lock-up territory on most pools.

How to fix high CYA

There is no chemical that removes cyanuric acid from a residential pool at any reasonable cost. The fix is dilution. Drain a portion of the water and refill. How much depends on the current CYA, the target, and the fill water — test the tap, some municipal water has CYA in it already from upstream pools.

A few notes on the equipment side:

DE and sand filters: backwash drops some water, but not enough to meaningfully move CYA on its own
Cartridge filters: you hose or soak the cartridge — you don't backwash it. Pull it, rinse it, soak it in cleaner if it's loaded, put it back
After any partial drain, retest everything. CYA, CH, TA, salt — all of it moves

On dosing chlorine and shock

Any dose recommendation depends on pool volume and a current test. A 10,000-gallon pool and a 25,000-gallon pool with the same FC reading need very different amounts of shock to get to the same place. Test first. Calculate against the actual gallons. Don't pour by habit.

If a pool is locked up with CYA at 90 and chlorine isn't holding, dumping more trichlor makes the problem worse, not better. Switch to liquid chlorine or cal-hypo for the season and let dilution from rain and splash-out slowly bring CYA down.

Where PoolPilot fits

PoolPilot is built around the solo operator and the truck — one person, a route, a phone, and customers who want a straight answer about their pool.

You can set per-pool target ranges for FC, CYA, pH, TA, CH, and salt. A pool with a SWG can run a CYA target of 70. The chlorine pool next door can sit at 40. Same route, different targets, no mental math at the gate.

Log readings, dosing, and notes per stop. The customer-facing message goes out after the visit — what you tested, what you added, when it's swim-ready. The same flat update you'd type yourself, just faster.

$30/month. 14-day free trial. No per-pool fees, no upsell tiers.

PoolPilot isn't trying to be Skimmer or Pool Brain or Jobber. Those are built for teams and offices. This is built for the operator running the route alone.

The short version

CYA protects chlorine from the sun. Too much CYA chokes the chlorine you're paying for. Test it, target it to the pool type, dilute when it climbs, and stop pouring tabs into a pool that's already stabilizer-loaded. The chemistry isn't complicated once you know what's actually holding your FC hostage in July.