Why Chlorine Disappears in Texas Heat (and How CYA Helps)

Why does chlorine disappear in Texas heat?

The sun destroys it. Ultraviolet (UV) light breaks chlorine apart. Swim University says, “it only takes 17 minutes of direct sun exposure to wipe out half your chlorine” in water with no stabilizer. Warm water and heavy swimmer use also drain chlorine. In North Texas, that adds up fast.

DFW averages 20.2 days a year at or above 100 F, according to the National Weather Service. As of its September 22, 2026 update, 2026 had 52 such days, tied for fifth most on record. A pool in that heat needs help.

What does cyanuric acid do?

Cyanuric acid (CYA) is a stabilizer. It binds loosely to chlorine and shields it from UV. Swim University says it makes chlorine last 3 to 5 times longer. Think of it as sunscreen for your chlorine.

Pool stores also call it stabilizer or conditioner. Outdoor pools need it. An indoor pool with no UV, or a bromine pool, can go without.

What CYA level should I keep?

Swim University says free chlorine should sit at about 7.5 percent of CYA. At 40 ppm CYA, aim for about 3 ppm free chlorine. In a salt pool at 60 to 80 ppm CYA, it says to keep free chlorine around 4 to 5 ppm.

Test your free chlorine and CYA with a good kit. Test strips can misread at high CYA. See our water chemistry chart for the full set of targets. Always follow the product label.

What are the signs of low CYA?

• You add chlorine in the morning and the test reads near zero by evening.
• You use liquid chlorine and CYA reads low. Swim University says that is likely why, because liquid chlorine adds none.
• The pool is a salt pool. Swim University notes that salt water does not naturally contain CYA even though it produces chlorine.

Low CYA is cheap to fix. Add stabilizer by label directions, and retest.

What if CYA is too high?

Too much is a problem too. Swim University calls it chlorine lock, or over-stabilization. Your test shows chlorine, but the water turns cloudy or green anyway. The chlorine is there. It reacts slowly because there is too much CYA compared to chlorine.

Swim University says that at 100 ppm CYA or higher, chlorine effectiveness “drops sharply,” and some test strips cannot give an accurate reading.

Where does extra CYA come from?

Stabilized chlorine tablets and powders carry CYA with them. Trichlor and dichlor raise CYA every time you use them. Swim University says, if you have chronically high CYA, temporarily stop using stabilized chlorine such as tablets and use unstabilized chlorine, such as liquid chlorine. CYA does not break down quickly or evaporate. The only reliable fix for high CYA is dilution.

Draining water has limits in a drought. Our post on high CYA during a drought covers your choices.

How do I keep chlorine steady all summer?

1. Test free chlorine, CYA, pH and alkalinity weekly.
2. Keep CYA in the range for your pool type.
3. Keep free chlorine at the right share of CYA.
4. Brush and clean the filter so chlorine is not wasted on debris.
5. Re-test after storms and heavy use. Rain and swimmers raise the demand for chlorine.
6. Keep notes so you can spot a trend.

If chlorine keeps dropping with good CYA, something is using it. Heavy chlorine demand can come from algae, phosphates or contaminants. A technician can sort that out.

Does weekly service help?

It does. Marina Pool Services’ weekly service covers water balance, including chlorine, pH, alkalinity, calcium hardness, conditioner level and salt if applicable. That check catches a CYA problem before the water turns. See the weekly pool service page, or the broader North Texas pool care guide.

If the pool has already turned, see our green pool cleanup guide and the algae treatment page.

What does a test routine look like?

Here is a simple routine for a chlorine pool, using Swim University’s ranges.

1. Test free chlorine and CYA with the same kit each time.
2. Compare free chlorine to CYA. At 40 ppm CYA, free chlorine should be near 3 ppm.
3. If CYA is below 30 ppm, add stabilizer following the product label, then retest.
4. If CYA is above 50 ppm in a chlorine pool, stop stabilized tablets and plan to dilute.
5. Write down each result with the date.

A salt pool has a higher CYA target of 60 to 80 ppm, so check your salt system manual for limits. Always follow chemical labels and store chemicals safely.

Next step

If your chlorine will not hold, see the weekly pool service page, then call Marina Pool Services at (817) 896-1120 or use the quote form at /contact. Marina gives free estimates for qualifying services.