CYA - Further Reading

What is Cyanuric Acid (CYA)?

Cyanuric acid, often called a stabilizer or conditioner, is an organic chemical compound added to pool water. CYA does not naturally occur in water and will only be present if added.

What does Cyanuric Acid do in Pool Water?

Cyanuric acid interacts with chlorine in two distinct and significant ways:

1. Cyanuric acid protects chlorine from degradation by UV light
2. Cyanuric acid buffers the harshness of chlorine.

Why Do You Need CYA in Pool Water?

CYA acts as a chlorine buffer.

Try splashing some pool water with zero CYA and an FC of 4ppm into your eyes … you’ll understand quickly why CYA is important in pools 😉

Actually, don’t …

CYA not only protects chlorine from UV loss, but it also acts as a chlorine buffer. The active chlorine species (hypochlorous acid and hypochlorite anion) are kept at much lower concentrations by holding the chlorine atom in reserve by reversibly bonding with it. Even though this buffering chemistry slows down oxidation and disinfection, it helps keep chlorine levels manageable and less harsh to the skin, eyes, hair, bathing suits, etc. It’s a benefit necessary for comfortable swimming and why many people hate public swimming pools. In many countries, public pools are not allowed to use CYA, so the water is very harsh and forms very irritating combined chlorine compounds, which is what most people would attribute to that “gross chlorine pool smell.”

CYA is a friend when used with the correct ratio to free chlorine.

Benefits to Using CYA in Pool Water

Benefit one is the UV protection.

Benefit two is the flattened pH dependency. With CYA 30, the HOCl concentration drops by 15% between pH 7.5 and 8.0.

Benefit three is the chlorine reservoir. All that "useless" chlorine bound to CYA stays in reserve. As soon as some HOCl gets used up, the equilibriums reshuffle (pretty much instantly), effectively releasing fresh HOCl.

Let's say you maintain FC 0.3 without CYA (that's halfway up to SLAM). A kid pees into the water, and FC drops to zero—a 100% loss.

If you maintained instead halfway up to SLAM with CYA 50ppm, you'd be at FC 10 PPM—the same as readily available HOCl. You lose 0.3 PPM due to a pee incident and are still at FC 9.7 PPM. This is a loss of just 3%, and you are still halfway up to SLAM—the party can keep going without the water turning cloudy or green.

CYA gives you a much more extensive FC working range without the water feeling too harsh.

The question now is, how much CYA is the optimum? The TFP recommendations are a good baseline, essentially based on experience.

Someone with an SWG in a desert climate will benefit from a higher CYA level of around 80 PPM in terms of UV protection and the CYA working range. You must account for up to 10 PPM CYA monthly degradation in these climates. Starting higher gives you more buffer until the pool begins behaving oddly.

In a less sunny climate, you probably get away with less CYA. Try out what works best for you.

Having the ability to maintain FC a bit higher with the higher CYA levels makes pool maintenance much easier once you get to know your pool. FC and pH testing may be done once or twice weekly to see that everything is still okay. There is lots of wiggle room. Occasional SWG adjustments with the seasons. Sometimes, a bit of acid.

How Does Cyanuric Acid Work in Pool Water?

When dissolved in chlorinated pool water, Cyanuric acid forms various compounds with chlorine called chlorinated cyanurates. These are chemical compounds very similar to the original cyanuric acid, where one of the hydrogen atoms has been replaced with chlorine.

In this way, the cyanurate compound buffers the chlorine in your water. When the chlorine atom attaches itself to the cyanuric acid molecule, it is effectively “held in reserve” and will not participate in disinfection or chemical oxidation reactions. This makes the pool water much less harsh for the swimmer's exposure to active chlorine compounds (hypochlorite and hypochlorous acid).

It is important to stress this point—the cyanuric acid concentration determines the amount of active chlorine (hypochlorous acid + hypochlorite ion) available in pool water. This is why the CYA level is so critical to know and why TFP teaches that there is no such thing as an absolute Free Chlorine level that is safe; the safe level of Free Chlorine is determined by how much CYA is in the water.

This is shown in the Chlorine/CYA Chart. Any Free Chlorine level up to the SLAM FC value for a CYA level is safe.

The pool industry gets this concept wrong when they state that a 1-3ppm Free Chlorine is all you need. THAT. IS. WRONG! Your Free Chlorine level is determined by your CYA level. FULL STOP! The higher your CYA level, the more Free Chlorine you need to maintain a proper active chlorine level.

How Does Cyanuric Acid Protect Chlorine From the Sun's UV Rays?

Cyanuric acid and the related chlorinated cyanurates protect chlorine from degradation or loss due to UV light.

Chlorine degrades when exposed to UV light

When chlorine in water is exposed to UV light from the sun, the hypochlorite ion (OCl-) and hypochlorous acid (HOCl) both react with UV light and get degraded into oxygen gas and chloride ion (Cl-). Both types of active chlorine have different rates of extinction by UV light, but, on average, chlorine in water with no CYA present will have a half-life (i.e., the time it takes for the initial concentration to decrease by half) of approximately 35 minutes. So if you put chlorine in your water and wait 2 hours, less than 5% of the original dose will be left simply from loss to UV light.

Cyanuric acid protects chlorine from UV degradation

When cyanuric acid is added to water and chlorine can bind to it, the half-life of the chlorine increases to a maximum of 8.4 hours, depending on how much CYA is present. The greater your CYA level, the less chlorine is lost.

There are two reasons for this:

1. The presence of cyanuric acid causes chlorine to become bound to it and held in reserve. Because of this, the water has much less active chlorine (hypochlorous acid and hypochlorite). Since there is less active chlorine, there is less chlorine to react with UV and, ultimately, lose.
2. Chlorine bound to CYA is much more stable when exposed to UV light and will not readily degrade. Because of this, CYA is very good at protecting chlorine from UV loss. The CYA molecule, with no chlorine attached, also absorbs some UV and radiates it back at a different wavelength of light. So it, all by itself, will reduce the amount of UV hitting the water volume (as UV light passes into the water, it is just as likely to interact with a molecule of CYA as it is with a chlorine molecule). This inherent UV shielding effect is seen at higher levels of CYA.

What are the Correct CYA Levels for an Outdoor Pool?

The recommended levels of CYA depend on the way the pool is chlorinated.

When chlorinating with liquid chlorine, a CYA level of 30-50 is recommended. Pools in hot areas with intense sunlight can benefit from higher CYA levels to offset the degradation described below.

Saltwater chlorine Generation (SWG) pools require a higher level of CYA, about 70-80 ppm, to operate efficiently. The theory is that the CYA is slow to "store" the chlorine as it is being generated, so without enough CYA, there is a build-up of chlorine that degrades the performance of the salt cell.

An outdoor pool under a cage or cover does not need as much CYA as an outdoor pool in full sun.

What are the Correct CYA Levels for an Indoor Pool or Spa?

CYA is not required in an indoor pool or spa. It does two things: it buffers the harshness of chlorine and protects the Free Chlorine from the sun's UV rays. Bathers can benefit from CYA buffering the available chlorine, although an indoor pool does not need to protect its chlorine from UV rays.

CYA of 20-30 is all you need for an indoor pool. That would have you maintain a normal FC level of 4-6.

Does CYA affect TA?

See Is TA is affected by CYA levels?

How to Increase CYA Level in Pool Water

Solid stabilizer or Liquid Stabilzer?

You increase CYA by adding cyanuric acid, often sold as a stabilizer or conditioner. CYA is available as a solid granular or powder and as a liquid. The liquid costs a lot more (usually about 3X the price of granular solid CYA) and is generally not worth the extra expense.

Granular stabilizer is found in the United States. Powdered stabilizer is found in Australia.

If you use liquid stabilizer, you should shake the bottle well and plan on using the entire jug to rinse out the bottom to get all those solids out and into the pool. The liquid stabilizer may not fully dissolve and settle to the floor if the water is cold. Brushing is always a good idea when adding something to the pool.

Different Types of CYA have Different Strengths

There are three forms that CYA could be sold in aside from dichlor and trichlor:

• Anhydrous
• Dihydrate
• Liquid (sodium salt of CYA in aqueous solution).

The dihydrate may go into solution marginally faster than solid stabilizer since it's essentially dust. The liquid is already in solution and is the only instant version.

The active contents of the three do differ.

• Anhydrous, assuming 100% purity, would be 7.75 moles per kg.
• Dihydrate again assuming 100% purity would be 6.06 moles per kg
• Liquid typically 35% as the salt with maybe 1.5% CYA dissolved would give you 1.95 moles/kg, or 2.43 moles/L.

So the dihydrate only gives you 78% of the active content of the anhydrous material. The liquid only provides 25% of the active content of the anhydrous material on a weight-for-weight basis.

How Should Solid Granular Stabilizer be Added to Pool Water?

A solid granular stabilizer is best added by placing it in a sock or nylon and hanging it in front of a fast-moving return(s). Shaking and squeezing the sock in the pool water will help the stabilizer dissolve faster.

Some members have found that the thickness of the sock affects the stabilizer's ability to dissolve. Using nylon or a laundry bag allows more water flow and faster dissolving time.

Alternatively, the sock can be placed in the skimmer basket, but it should only be left there if the pump is continuously running and not turned on and off by a timer. CYA is acidic when it dissolves into water, so leaving the sock in the skimmer without the pump running is not a good idea.

It is not recommended to pour granular CYA directly into the pool or the skimmer, as recommended on stabilizer bottles. It takes a long time to dissolve, and the granules can sit against the pool surface, creating locally low pH levels.

The pump should be run for 24 hours after adding solid stabilizer, and if stabilizer was added to the skimmer, you should avoid backwashing/cleaning the filter for a few days.

How Should Solid Powdered Stabilizer be Added to Pool Water?

When using the sock method with powdered stabilizer, the stabilizer can instantly cake up to a brick-like consistency once wet. Left like that, it would probably take weeks to dissolve. If you give the "brick in a sock" a good knead with your hands, it will turn the brick back into individual grains that would then pretty quickly dissolve through the sock and get dispersed in the pool water.

Alternatively, powdered stabilizer can be added to a pool using the "swirling water in bucket" method. When water is added to the powdered stabilizer in a ratio of approximately 1:4 (CYA:water), you will get the ‘clumping,’ but if it is allowed to sit for about an hour, it becomes a slurry. When the slurry is mixed with a larger volume of water, some clumping can re-occur, but the clumps are smaller and much easier to break up.

When to Test for CYA after Adding Stabilizer?

No matter the method of addition, solid stabilizers can take up to 24 hours to dissolve into the pool water, so it is essential to wait for them to dissolve and for the pump to homogenize (mix) the pool water well before testing. If you wait approximately 24-48 hours after the last bit of CYA has dissolved to test, those results will appear on a test.

Be aware that the current test for CYA (melamine turbidity test) can only detect a CYA difference of 10 PPM at best. So, if you add small amounts of CYA to your pool, the test might not show it.

How to Lower CYA Levels in Pool Water

In nearly all cases, replacing water is the best way to lower CYA. Pool water can be replaced by draining or by a water exchange process that we describe in Draining.

If replacement water is extremely expensive or your local water utility has restrictions, you might consider a Reverse Osmosis water treatment.

How is CYA Lost and Degrade in Pool Water?

In general, once you add CYA to pool water, it will remain in the water unless you drain the water that contains it. Some CYA are lost as water splashes out from pools.

Water leaking from pools will take CYA with it. Unexplained drops in CYA levels, especially if you have an auto-fill that automatically adds fresh water to the pool, can indicate a water leak.

Evaporation of water from the pool's surface will not cause a loss of CYA. The water will evaporate, and the CYA will remain in the remaining water.

High pool water temperatures will cause the chlorine to oxidize Cyanuric Acid. This tends to show in water temperatures of 90+ degrees. Every 10F increase in temperature results in roughly doubling the rate of degradation.

Chlorine breakdown in sunlight causes CYA degradation by hydroxyl radicals. Depending on the amount of sun the pool is exposed to, this loss can range from 2 PPM to 10 PPM per month.

In an area with 90+ pool water temperatures and extreme sunlight exposure, degradation can cause the loss of 10+ PPM of CYA a month.

Some bacteria feed on Cyanuric Acid and create ammonia in pool water. This can only occur if the Free Chlorine falls to 0, allowing the bacteria to live. This generally happens in pools closed for the winter and discovered at pool opening.

CYA will degrade in the winter with a closed pool.