Cyanuric Acid in Pool – Understand CYA and Lower a High Level
Pool Guide · Chlorine & Water Care
Cyanuric acid, CYA, helps protect chlorine from the sun's UV light. At the same time, the level needs to suit the pool's water treatment. Start by measuring and checking which products you use.
What is cyanuric acid in pool water?
Cyanuric acid is used as a chlorine stabiliser. It can be added through stabilised chlorine, for example products based on dichlor or trichlor. With repeated use, CYA can build up in the water. A chlorine reading therefore needs to be assessed together with, among other things, pH, CYA and the treatment system's instructions.
Pahlén's guide to pH and chlorine describes both stabilisation and how cyanuric acid can accumulate. The product's ingredients list shows what type of chlorine you are actually using.
Which cyanuric acid level is right?
Follow the instructions for your chlorine product, salt chlorinator or automatic dosing system. A general guideline is not a prescription for all pools. Check the manufacturer's requirements carefully if you have a system that measures and controls dosing automatically.
Do not add stabiliser as a routine measure. First find out the current CYA level, which products already add stabiliser, and what your system is designed for.
Four things to check
- Measure with a CYA test. A regular chlorine or pH test does not automatically show cyanuric acid. Check the test's measuring range and follow the instructions.
- Confirm an unusual result. Repeat the test according to the instructions before planning a major action.
- Gather the product names. Note both ongoing disinfection, any shock treatment and separate stabiliser. Photograph the labels.
- Get the system manual. Write down the pool volume, current CYA level, pH and chlorine reading. This makes the advice more accurate.
How do you lower cyanuric acid in a pool?
Dilution with fresh water is an established method for lowering the level. The fresh water's CYA level and how much is replaced affect the result. Measure again after the change and once the water has mixed. Pahlén describes water replacement as a method for lowering cyanuric acid.
First check how much the water level in your pool may be lowered. The structure, ground and groundwater affect what is possible. Consult the pool supplier before carrying out a larger water change and follow local instructions for discharging pool water.
Calculation example: what does a partial water change mean?
If fresh water has no CYA and you first remove a certain proportion of well-mixed pool water and then refill the same amount, the CYA level decreases by roughly the same proportion. A 25 percent change therefore leaves about 75 percent of the original level.
Example with assumptions: 80 ppm × 0.75 = 60 ppm after a theoretical 25 percent change. In a pool with 40 m³ of water, the proportion corresponds to 10 m³. The figures explain the dilution; they are not recommended CYA levels or instructions to drain your pool specifically.
Several smaller changes do not give exactly the same result as one large change, because fresh water is also removed in the next round. Always use follow-up measurement.
Frequently asked questions
Is cyanuric acid the same as free chlorine?
No. CYA is a stabiliser and free chlorine is a different measurement. A test must explicitly support CYA to measure cyanuric acid.
Should I add cyanuric acid if the level is low?
Only if the instructions for your water treatment say it is needed. First check both the system requirements and whether the chlorine product already contains stabiliser.
What does Pooltime need to help me?
Send the pool volume, measured values, model of the filtration equipment and photos of the chemical labels. Also say if the water has recently been changed or topped up.
Get help with your water treatment
We can help you find the right basis for the pool's treatment and the products you use. Feel free to include a current set of measurements.
