Symptoms of High Cyanuric Acid in Your Pool

High cyanuric acid does not announce itself. There is no smell, no color, no cloudiness that belongs to it specifically. What you get instead is a pool that misbehaves while every test you run says it should be fine.

The signature is chlorine that reads normal and water that goes green anyway. If you have been shocking a pool every couple of weeks and it keeps coming back, test CYA before you buy anything else.

The symptoms, roughly in the order people notice them

Free chlorine tests in range and algae appears anyway. This is the one that sends people to search engines. The test is not lying and the algae is not imaginary. Cyanuric acid protects free chlorine from the sun, and the cost of that protection is that a greater share of your chlorine is bound up and unavailable at any moment. The more CYA you have, the higher the free chlorine number has to be to do the same work.

You shock, it clears, and two weeks later it is back. The shock worked. It just never got above the threshold for the CYA you are carrying, so the die-off was partial and what survived came back.

The cell is running harder for the same result. Output crept from 40 to 60 to 80 percent. Pump hours went from 8 to 12. Chlorine still sits at the bottom of the range.

Chlorine demand feels bottomless. You never hold a level, you just top up.

The water looks dull rather than dirty. Flat and slightly grey, with a tile line that films over faster than it used to.

Your CYA test tops out. Most home kits stop around 100, so a reading of “100 or more” is a ceiling, not a measurement. None of our sources publish a method for reading past it, so get a sample tested on better equipment before planning a fix around a number you do not have.

None of these is proof alone. Together, especially the first three, they justify a test.

Is high CYA dangerous?

The risk is not toxicity. None of the sources we rely on describe cyanuric acid as a health hazard at the levels found in pools. What they describe is a chlorine problem. Trouble Free Pool puts it simply: the free chlorine you have to hold depends on your CYA level and rises as CYA rises, and if free chlorine drops too low for the CYA, you get algae.

The danger, if you want the word, is a sanitation failure you cannot see on a test strip. You are running a pool that reports chlorine and does not disinfect like one.

The arithmetic that makes it concrete

Trouble Free Pool establishes the relationship but publishes no numbers. The numbers come from Pool Chem Tracker’s FC/CYA chart, which reproduces the Trouble Free Pool methodology as a table. Salt pools get their own column, and the minimum is lower than for a manually chlorinated pool because the generator continuously replaces what gets consumed.

The rules behind it are short. Minimum free chlorine for a salt pool is about CYA multiplied by 5%. Shock level is about CYA multiplied by 40%.

Now run your own pool through it. At CYA 100 the minimum free chlorine is 5.0 ppm. Not the target, the floor. Very few owners have ever held 5 ppm on purpose.

So a pool at CYA 100 with free chlorine at 2 ppm is below half its minimum. The test says there is chlorine in the water, and there is. It is not doing the job, and functionally that pool is unchlorinated. That is the pool described at the top of this page: normal test, green water, baffled owner.

It also explains the shock that keeps failing. At CYA 100 the shock level is 40 ppm, so a dose that took you to 12 ppm did not shock the pool, it fed it. In The Swim points the same way: above 20 ppm CYA add 50% more shock, above 40 ppm double it. The boost button will not get you there either, since most salt systems in shock mode raise free chlorine by only 5 to 7 ppm, slowly, while placing heavy demand on the cell.

The contradiction nobody names

Read Hayward’s own water balance chart and you get two instructions on the same page. Free chlorine 1.0 to 3.0 ppm. Stabilizer 60 to 80 ppm, with 80 as the target.

Put 80 into the chart above and the salt generator minimum is 4.0 ppm. Hayward’s chlorine ceiling is 3.0.

Follow Hayward exactly, hit their stabilizer target, hold the top of their chlorine range, and you are still under the minimum free chlorine for the CYA level Hayward told you to run. That is not a rounding difference. It is a real disagreement between two authorities, and we are not going to resolve it by picking a favorite.

What we will say is how to settle it in your own backyard. Running Hayward’s numbers with clean water all season means the disagreement is academic for your pool. Running Hayward’s numbers and fighting algae means you have your answer, and the chart above tells you which number to move. Algae is the referee.

Manufacturer ranges are also where warranties live, which is why we are not telling you to ignore your manual. The fuller comparison is on cyanuric acid for pool, and the table gets its own page at the CYA and chlorine chart.

How it got high

Cyanuric acid does not evaporate and is not consumed. It leaves only with water, so the level moves one direction until you remove some.

The usual culprit is stabilized chlorine. Orenda’s number is the one to remember: “trichlor is only 45.8% chlorine (by weight). The majority of it is cyanuric acid.” Every tablet added stabilizer that stayed behind. Salt pool owners get there by running tabs as a backup while the cell is down, or by dosing granular stabilizer at every opening out of habit rather than in response to a test.

One footnote. A large stabilizer dose is mildly acidic, so it pulls pH and alkalinity down with it, which sends people to muriatic acid for saltwater pool chasing a balance problem they created here.

What to do about it

Two answers, and the order matters.

Short term, raise free chlorine to match the CYA you have. Find your row and hold the target column, not the minimum. This is not the fix, it is how you keep the pool clear while you arrange the fix.

Then remove water. Dilution is the only practical method, the arithmetic is fraction to replace = 1 - (target / current), and the procedure, the cost and why the reducer products disappoint are on reduce cyanuric acid swimming pool. Budget for salt too, since it leaves with the water at the same rate, and the pool salt calculator will size the replacement dose.

Overshoot on the way down and going back up is the cheap direction: one bag, one afternoon, on how to raise cyanuric acid level.

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