Hot tub cyanuric acid and when stabiliser becomes a problem

Hot tub cyanuric acid is the stabiliser that chlorine granules add to your water every time you dose. A small amount protects chlorine from sunlight, but in a covered hot tub it keeps building up. Once it climbs past about 50 ppm it slows your chlorine down, and the only reliable way to lower it is to replace some of the water.

This guide follows the ranges used by UK hot tub manufacturers and dealers, not the figures written for US public pools. You will see how the build-up happens, how to test for it at home, and how to work out how much water to change. Always check your own hot tub manual, because manufacturers do not all give the same numbers.

What is cyanuric acid and what does it do?

Cyanuric acid, often shortened to CYA, is a chemical that attaches loosely to chlorine in water. It is sold as stabiliser or conditioner, and it is also built into the most common hot tub chlorine, which is sodium dichloroisocyanurate granules (usually called dichlor).

When chlorine is attached to CYA, it is held in reserve. It is released gradually as the free chlorine in the water gets used up. This makes chlorine last longer, but it also means less of it is actively working at any one moment.

Cyanuric acid is not a sanitiser. It kills no bacteria, viruses or algae by itself. Its only job is to slow the loss of chlorine to sunlight, so it adds nothing to water safety beyond that.

Why chlorine needs protecting at all

Unprotected chlorine is broken down quickly by ultraviolet light from the sun. In strong direct sunlight, unstabilised chlorine can lose most of its strength, commonly quoted as around three quarters or more, within about two hours. Stabiliser was developed for open outdoor swimming pools to slow that loss.

A hot tub is a different situation. The water is warm, usually up to 40°C (104°F), the volume is small, and the cover keeps sunlight off for most of the day. Chlorine in a hot tub is mainly used up by sweat, skin oils, cosmetics and heat, not by the sun.

Does a covered hot tub need stabiliser?

Most covered hot tubs need little or no added stabiliser. If you use dichlor granules, you will get more than enough CYA from the granules alone, so there is rarely any reason to add a separate stabiliser product.

The main exception is a tub that sits uncovered in direct sun for long periods and is treated with an unstabilised chlorine. In that case a small amount of CYA can help. If you use bromine, do not add stabiliser at all, because CYA does not protect bromine.

How do chlorine granules add stabiliser without you noticing?

Dichlor granules are roughly 50-60% cyanuric acid by weight, depending on the grade, and trichlor tablets are in a similar range. In practice, about half of every scoop you add stays behind in the water as stabiliser.

Looked at another way, every 1 ppm of free chlorine you add with dichlor also adds about 0.9 ppm of CYA. Chlorine tablets made from trichlor add about 0.6 ppm of CYA for every 1 ppm of chlorine, because they contain more chlorine for each part of stabiliser.

As a dose-based guide, every 10 g of dichlor added to 1,000 litres (about 260 US gallons) of water leaves behind roughly 5-6 ppm of CYA. If you know how many grams you use each week, you can estimate how fast your level rises.

The chlorine gets used up, but the CYA does not. It does not evaporate, it is not removed by your filter, and it is not destroyed by shocking. Every dose leaves a little more behind, so the level only goes one way while the water stays in the tub.

Topping up after evaporation does not help. Evaporation removes only water and leaves the CYA behind, so the same amount of stabiliser ends up in less water. A tub that is topped up but never partly drained keeps concentrating it.

Here is a simple example. If you add 2 ppm of chlorine a day to keep your level in range, that is 14 ppm of chlorine a week. With dichlor, that also brings around 12-13 ppm of CYA each week, or roughly 50 ppm in a month.

In everyday use, owners who dose dichlor regularly and only top up the water can reach 100 ppm or more within about 3 to 6 months, often without realising it. A busy tub dosed as heavily as the example above can get there sooner, which is why a CYA test belongs in your regular routine.

Busy tubs use more chlorine, so they build CYA faster. This is one reason why regular water changes matter so much in a hot tub. Our guide on how often to change hot tub water explains how to plan a sensible schedule.

What level of hot tub cyanuric acid is too high?

For a domestic chlorine hot tub, most UK manufacturers and dealers suggest keeping CYA around 30-50 ppm. Many treat anything above 50 ppm as a sign to plan a partial water change, and anything around 100 ppm or more as a clear reason to drain.

UK commercial guidance from bodies such as PWTAG, the Pool Water Treatment Advisory Group, is cautious about stabilised chlorine in spa pools, because CYA builds up quickly in a small volume of hot water. US figures, such as those published for American public pools, use different ranges and different rules, so they are not a good guide for a UK garden hot tub.

Even so, US public health bodies support the same cautious view. The CDC and the New York State Department of Health advise against using stabilised (CYA-based) chlorine in hot tubs, because warm water needs disinfection that works quickly. Their reasoning matches the UK position, even where their exact figures differ.

Some health authorities and water specialists go further. They recommend keeping CYA as close to 0 ppm as possible in a covered spa, and treating about 30 ppm as a ceiling rather than a target. If you want the strictest approach, you can aim lower than the usual UK range, for example by using an unstabilised chlorine or bromine.

Your manual may give a slightly different range. If it does, follow the manufacturer, since their advice may also affect your warranty.

When a reading sits in the upper part of the normal range, around 30-50 ppm, act early. Cut back on dichlor where you can and test more often, for example weekly, so you catch the rise before it reaches the point where you need a water change.

Why does too much stabiliser make chlorine less effective?

The part of chlorine that kills germs quickly is called hypochlorous acid. The more CYA there is in the water, the more chlorine is held in reserve and the less hypochlorous acid is free to act. Your test may still show 3-5 ppm of free chlorine, but it works more slowly.

Some tubs and automatic controllers show an ORP reading. ORP stands for oxidation-reduction potential, and it measures how much sanitising power the water actually has. High CYA lowers ORP, so the reading can fall too low even when free chlorine tests at 3-4 ppm.

Speed matters in a hot tub. Warm water, several bathers and a small volume give bacteria good conditions to grow. The HSE explains that hot tubs and spa pools can spread Legionella if the water is not treated and maintained properly, which is why sanitiser that works quickly is important.

Pseudomonas aeruginosa, the germ behind 'hot tub rash' (folliculitis), copes well with warm water. When chlorine is slowed by high CYA, this germ has a particular opportunity in a hot tub compared with a cooler pool.

Some people call this effect chlorine lock. The name suggests the chlorine stops working completely, which is not quite right. It still works, but you need more of it and more time to get the same result. Our guide to hot tub chlorine levels and how to read them explains what your free and total chlorine readings are telling you.

Signs that stabiliser may be getting in the way

• Free chlorine reads in range, but the water still turns dull or cloudy.
• You need to add chlorine more often than you used to for the same use.
• The water smells stale or musty even after dosing.
• A strong, sharp 'chlorine' smell. This is actually chloramines, formed when chlorine reacts with bather waste without fully breaking it down.
• Skin or eye irritation after a soak, even when free chlorine tests in range.
• Surfaces or the waterline feel slippery soon after cleaning.
• Green algae in an outdoor tub despite normal chlorine readings.
• Shocking clears the water for only a day or two.
• Haze that lasts more than about 24 hours after a proper shock, when the water should already be clear.
• The water has been in the tub for several months on dichlor.

These signs can have other causes, such as low filtration time, a dirty filter or high total dissolved solids. Testing CYA is the only way to confirm it. If several readings drift at once, our article on hot tub TDS and when to change water can help you tell the problems apart.

How do you test for cyanuric acid at home?

Many multi-test strips include a CYA pad, which is useful as a quick check. For a more reliable number, use a dedicated CYA test. These usually work by mixing a reagent with a water sample, which turns the water cloudy in proportion to the CYA level.

Check what your strips actually measure. Many basic 4-in-1 and 5-in-1 test strips have no CYA pad at all, so you can test pH and chlorine every week and never see your stabiliser level. Make sure your strips or kit specifically list stabiliser or cyanuric acid on the pack.

Most home turbidity tests use a melamine reagent, and they are typically accurate to within about 5-10 ppm. If a strip suggests CYA above 50 ppm, confirm it with a dedicated liquid or tablet turbidity test before you commit to draining.

For the most stable reading, test before you add any chemicals, for example in the morning, rather than straight after a dose.

1. Take a sample from about elbow depth, away from the jets and any floating dispenser.
2. Let a hot sample cool for a few minutes, as many kits ask for water close to room temperature.
3. Add the reagent or tablet exactly as the kit instructions describe and mix fully.
4. Read the result in good daylight against the kit's scale, or by the black dot method if your kit uses one.
5. If the reading is above the top of the scale, mix one part tub water with one part tap water, test again, and double the result.
6. Write down the reading and the date, so you can see how fast it is rising.

Most home CYA tests are not precise below about 20-30 ppm, and strips can be hard to read between colour blocks. Treat any result as a guide, and look at the trend over several weeks rather than one single reading. Test CYA every two weeks if you use dichlor, and after every refill.

If you fill your tub from a private supply such as a borehole or well, test the fill water for CYA as well. Private supplies can occasionally contain traces from agricultural runoff, so your starting level after a refill may not be zero. Mains water in the UK is not normally a source of CYA.

How do you lower cyanuric acid in a hot tub?

Dilution is the only method that reliably lowers CYA. Because a hot tub holds a small volume, often around 1,000-1,500 litres (about 260-400 US gallons), replacing water is usually quick and cheap compared with any other option.

A water change is usually cheaper than it seems. If you keep dosing dichlor into water that already holds 100 ppm or more of CYA, you are paying for chlorine that can barely sanitise, and every dose pushes the level higher still. Fresh water lets each gram of chlorine do its job again.

Draining and refilling is the only dependable way to bring cyanuric acid down. Chemicals sold to lower it give mixed results, and no common household product removes it.

Working out a partial water change

The maths is simple. If you replace half the water with fresh tap water that contains no CYA, the level halves. If you replace a quarter, it drops by a quarter. For example, 80 ppm with half the water replaced becomes about 40 ppm.

To work out how much water to replace, divide your target reading by your current reading, then subtract the result from one. For example, to go from 60 ppm to 40 ppm, 40 divided by 60 is about 0.67, and one minus 0.67 is about 0.33. That means replacing about a third of the water, or about 500 litres in a 1,500-litre tub.

If you would rather not drain a large share at once, replace about a third of the water, refill, retest and repeat. Each round lowers the reading by about a third, so a high reading may need two or three rounds to get below about 30 ppm. For example, 90 ppm becomes about 60 ppm, then about 40 ppm, then about 27 ppm.

If your reading is over 100 ppm, or the water is also old or high in dissolved solids, a full drain is usually the better choice. It also gives you the chance to clean the shell and flush the pipework.

Draining usually takes 1-2 hours through the drain valve, and much less with a submersible pump. A full drain, clean and refill typically takes around 2-3 hours, plus time for the water to heat up again.

1. Test CYA and decide how much water you need to replace to reach about 30-40 ppm.
2. Stop adding chlorine for a day or two beforehand, or use a chlorine neutraliser such as sodium thiosulfate, so free chlorine is near zero before the water goes anywhere.
3. If you used a neutraliser, let the water circulate, then test to confirm free chlorine reads 0 ppm before you start draining. Add a little more neutraliser and retest if it does not.
4. Switch the hot tub off at the isolator before you start draining.
5. Drain the planned amount through the drain valve or with a submersible pump, following local rules on where treated water can go.
6. If you are doing a full drain, use a pipe cleaner product before draining and clean the filter while the tub is empty.
7. Soak the filter in filter cleaner for at least an hour, or replace it if it is due. Filters that have sat in high-CYA water often carry biofilm that would recontaminate fresh water.
8. Refill with fresh water, ideally through a hose pre-filter, putting the hose into the filter housing to reduce airlocks.
9. Switch the power back on, let the water circulate, then balance total alkalinity first (about 80-120 ppm), then pH (about 7.2-7.6), then calcium hardness (about 100-250 ppm, with many manufacturers aiming for 150-250 ppm), before adding sanitiser.
10. Test CYA again the next day to check your new starting point.

Never drain treated hot tub water into a surface-water drain, a stream or ditch, or near plants. In the UK, surface-water drains often lead straight to rivers. If you are not sure where you can discharge it, ask your local water company.

In the UK, the usual acceptable place for dechlorinated hot tub water is the foul sewer. That might be an outside gully or a utility sink that connects to the sewer. Check where a gully leads before you use it, because some outside gullies take only rainwater and are part of the surface-water system.

After a refill, the order in which you adjust your chemicals matters. Our guide to hot tub water balance walks through pH, alkalinity and sanitiser in the right order.

Do cyanuric acid reducer products work?

Some products claim to break down CYA using enzymes or bacteria. Reports of their results are mixed, and they are generally designed for swimming pools rather than hot water. Many need specific conditions, such as low chlorine for a period, which is not ideal in a hot tub.

If you choose to try one, follow the label carefully and test before and after. Keep the water at the lower end of its temperature range during treatment, add no dichlor while the product works, and retest after 3 days and again after 7 days.

The reason for keeping the water cooler is that high heat reduces the activity of the enzymes or bacteria these products rely on. At normal soaking temperatures they may work slowly or hardly at all, so lowering the set temperature for the treatment period gives the product a fairer chance.

Choose a CYA reducer made for pools and spas rather than a general enzyme cleaner. Household enzyme products are not designed to break down cyanuric acid, so they will not lower your reading.

Reducers are at most a stopgap for borderline readings. They are not a replacement for draining when CYA is well above 50 ppm. For most domestic tubs, a partial drain is faster, more predictable and usually cheaper than a reducer product.

Things that do not lower cyanuric acid

• Baking soda (sodium bicarbonate) raises total alkalinity, not CYA.
• Tennis balls can absorb some surface oils, but they do not remove CYA.
• Hydrochloric acid, called muriatic acid in the US, lowers pH and alkalinity, not CYA. Most UK hot tubs use a dry pH reducer instead.
• Extra pH reducer or acid. Adding more in the hope of lowering CYA can drive pH too low, which can corrode the heater and damage pump seals while leaving CYA unchanged.
• Chlorine shock made from dichlor adds more CYA, so repeated shocking makes the problem worse.
• Non-chlorine shock (potassium monopersulphate) adds no CYA, but it does not remove any either.
• A new filter cartridge improves clarity but does not catch dissolved CYA.
• Topping up after evaporation, because the CYA stays in the tub.

Handling chemicals safely

• Add chemicals one at a time, with the water circulating between additions.
• Never combine dry chemicals, not even in a bucket.
• Never pour dry granular chemicals into the skimmer or filter compartment, or right next to the jets. Add them to the open water, pre-dissolved in a bucket of water where the label allows.
• Handle concentrated chlorine and pH reducer outdoors or in a well-ventilated space, so you do not breathe in fumes or dust.
• Store chlorine products and pH reducers apart from each other, away from heat and direct sunlight.
• Keep children and pets away while you add chemicals and for a while afterwards.
• Never exceed the dose on the label.
• If a chemical splashes onto skin or into eyes, rinse with plenty of water for at least 15 minutes and seek medical help if irritation continues.

Safety: wear gloves and eye protection when handling hot tub chemicals. Add chemicals to water, never water to chemicals, and never mix chlorine products with acids or with each other, because this can release harmful gas. Store them dry, sealed and out of reach of children.

How can you stop stabiliser building up again?

Dichlor is easy to use and widely available in the UK, so many owners stay with it. That is fine, as long as you accept that regular water changes are part of the routine. Testing CYA every two weeks tells you when the next change is due.

If you want CYA to rise more slowly, there are a few options. Each has trade-offs, so think about how you use your tub before you switch.

• Use non-chlorine shock after heavy use instead of extra dichlor, so you add less chlorine overall.
• Shower before bathing, which reduces the oils and sweat that use up chlorine.
• Keep the cover on whenever the tub is not in use.
• Switch to an unstabilised chlorine once CYA is in range, but note that calcium hypochlorite raises calcium hardness and liquid chlorine raises pH.
• Change to bromine, which does not use CYA. Most manufacturers advise a full drain and fresh fill when you switch.

The type of shock matters when stabiliser is already high. Calcium hypochlorite shock, like non-chlorine shock, adds no CYA, whereas dichlor shock adds more with every dose.

Bromine suits covered tubs well. It stays more effective than chlorine at hot tub temperatures and across a wider pH range, and non-chlorine shock reactivates spent bromine so it can keep working.

For an uncovered tub in a sunny spot, unstabilised liquid chlorine plus a small, measured stabiliser dose lets you hold CYA at a level you choose. Dichlor cannot do this, because it always adds stabiliser in a fixed ratio to chlorine.

What day-to-day care looks like after switching

A typical bromine routine is simple: a floating dispenser topped up with bromine tablets as needed, plus a weekly non-chlorine shock. There is no stabiliser to monitor, so CYA testing drops out of your routine.

Expect pH to move in a different direction after switching. Bromine tablets tend to lower pH slightly over time, so you may need a pH increaser now and then. Liquid chlorine raises pH, so you will use pH reducer more often.

Bromine usually costs more to run than chlorine. Many owners find its smell less sharp than the chloramine odour that chlorine water can develop when high CYA slows the chlorine down.

Liquid chlorine is usually dosed in a measured amount after each soak. That means dosing more often than with granules or a floater, but you keep full control over the chemistry and add no stabiliser.

If you stay with chlorine, our guide on how to use chlorine in a hot tub covers dosing, timing and how to keep levels steady without overdoing it.

What if your cyanuric acid reading is too low?

Low CYA is rarely a problem in a covered hot tub. It usually happens after a fresh fill, or when you use bromine or an unstabilised chlorine. If your tub is covered most of the day and your chlorine holds steady, you do not need to add any.

In an uncovered tub, splash-out and rain gradually dilute the water, so CYA can fall over time even without a full drain. This is another reason to test rather than assume.

An uncovered tub run on unstabilised liquid chlorine starts at zero CYA after every drain and refill. Nothing in the liquid chlorine replaces it, so the small stabiliser dose has to be added again to the fresh water each time you refill.

For an uncovered tub in strong sun, CYA below about 10-15 ppm lets chlorine break down noticeably faster. A modest target of about 20-30 ppm is enough to slow that loss. The higher levels often suggested for outdoor swimming pools are not needed in a hot tub.

If your tub is often open in strong sun and chlorine drops fast on bright days, a small amount of stabiliser can help. As a guide, 10 g of pure cyanuric acid raises 1,000 litres by about 10 ppm. Products vary in strength, so always check the label for the correct dose.

As a worked example, raising a typical 1,500-litre tub from 0 to 30 ppm takes about 45 g of pure cyanuric acid. A 1,000-litre tub needs about 30 g for the same rise.

1. Test CYA first and confirm it is below about 30 ppm.
2. Work out your tub's volume from the manual and calculate a dose to reach about 30 ppm.
3. Measure out about 80% of the calculated dose, so you can add the rest later without overshooting.
4. Dissolve the stabiliser in a bucket of warm water if the label allows, as it dissolves slowly. Use water that is warm but not hot, and stir for about 2-3 minutes before adding it to the tub. Never tip dry granules onto the acrylic shell, because undissolved granules can bleach or mark the surface.
5. Add it with the pump running and the cover open, pouring slowly near a return jet rather than into the skimmer or filter, since undissolved granules can damage filter media.
6. Run the jets for at least 30 minutes, then leave the water circulating.
7. Retest after 24-48 hours and add the remaining dose only if the reading is still low.

Be patient with the retest. Stabiliser dissolves and mixes slowly, so a test within the first day often gives a falsely low reading. That low number tempts owners to add more, which can push the level past 50 ppm once everything has dissolved.

Add stabiliser in small steps. It is easy to add more later, but the only way to take it out again is to drain water.

Mistakes to avoid and when to get help

• Adding stabiliser to a tub that already uses dichlor granules.
• Shocking with dichlor again and again when the water stays dull.
• Raising the chlorine dose to fight high CYA instead of diluting.
• Trusting one strip reading without retesting or diluting the sample.
• Leaving the same water in the tub for many months without testing CYA.
• Adding stabiliser to a bromine hot tub.
• Doing a partial drain, then going straight back to heavy dichlor dosing, which brings CYA back to high levels within weeks.
• Draining the tub in a way the manual does not allow, or using chemicals it does not approve. Either can affect your warranty cover, so check the documentation before you drain.

Speak to your dealer or a hot tub technician if the water stays cloudy after a fresh fill and correct balance, if chlorine disappears within hours, or if your tub is under warranty and you are unsure which chemicals are allowed.

If CYA is extremely high, around 150 ppm or more, and bathers have had rashes, eye irritation or breathing discomfort, keep everyone out of the tub. Ask a hot tub technician to assess the water before anyone uses it again.

Get help too if CYA climbs back to high levels within 4-6 weeks of a fresh fill even though you have cut back on dichlor. It may mean someone else in the household is still dosing dichlor, or that a product you use contains stabiliser without saying so clearly on the front of the pack.

A rarer cause of a fast rise is the fill water itself. If CYA climbs back within 4-6 weeks and you are sure no stabilised products are going in, test a sample of the water you fill from. A contaminated source would explain why each fresh fill starts higher than expected.

If anyone gets an itchy rash, sore eyes, or a cough or fever after using a hot tub, stop using it until the water is sorted. Speak to a pharmacist or GP, or contact NHS 111 if you are worried about symptoms.

Keeping stabiliser under control

Dichlor granules add cyanuric acid with every dose, and that CYA stays in the water until you replace it. Aim for about 30-50 ppm, test every two weeks, and plan a partial or full water change when readings pass 50 ppm or approach 100 ppm.

Do not rely on shock, baking soda, tennis balls or acid to bring it down. Fresh water is the fix, followed by careful rebalancing. Your hot tub manual always has the final say on the ranges for your model.

Frequently asked questions

What is another name for cyanuric acid?

Cyanuric acid is sold as stabiliser, conditioner or CYA, and chemists also call it isocyanuric acid. Chlorine granules that contain it are labelled sodium dichloroisocyanurate, often shortened to dichlor, while slow-dissolving chlorine tablets are usually trichloroisocyanuric acid, called trichlor. If a chlorine product's chemical name contains isocyanurate or isocyanuric, it adds stabiliser to your water each time you use it.

Can you lower cyanuric acid without draining the hot tub?

Not reliably. Enzyme and bacterial reducer products exist, but reported results are mixed and most are designed for cooler swimming pool water. Some pool companies offer mobile reverse osmosis filtering, but it is rarely practical for a domestic hot tub. Because a hot tub holds so little water, replacing part of it is usually the quickest, cheapest and most predictable option.

Does cyanuric acid affect a bromine hot tub?

Cyanuric acid does not stabilise or protect bromine, so it brings no benefit in a bromine tub and should not be added. If you are switching from dichlor to bromine, most manufacturers recommend a full drain, clean and fresh fill first. This gives you a clean start without leftover stabiliser and makes your bromine readings easier to trust.

Is high cyanuric acid harmful to bathers?

At the levels found in hot tubs, the main concern is not the CYA itself but the slower action of your chlorine, which gives germs more time to grow. If anyone develops a rash, sore eyes, a cough or a fever after using the tub, stop using it until the water is corrected, and speak to a pharmacist or GP.

Does shocking a hot tub remove cyanuric acid?

No. Shocking oxidises sweat, oils and other waste, but it does not break down CYA. A dichlor shock actually adds more stabiliser, so repeated chlorine shocks raise the level further. Non-chlorine shock, based on potassium monopersulphate, adds no CYA, which makes it a useful choice for clearing the water after heavy use without making the build-up worse.

How often should you test cyanuric acid in a hot tub?

If you use dichlor granules, test CYA about every two weeks, because it rises steadily with every dose. If you use bromine or an unstabilised chlorine, a monthly check is usually enough. Always test after a refill to record your starting point. Keeping a simple log of dates and readings shows you how quickly your tub builds CYA and when to plan the next water change.