How to lower chlorine in a hot tub safely

To lower chlorine in a hot tub, take the cover off and run the jets, drain and refill part of the water, or add a chlorine neutraliser. Uncovering is free but slowest, a partial drain works as soon as you refill, and a neutraliser is fastest. Stay out until free chlorine is back to 5 ppm or below.

High chlorine is rarely an emergency, but it can irritate skin, eyes and airways, and over time it wears out covers, pillows and seals. Below you will find how to confirm the reading, which method suits your situation, when it is safe to get back in, and how to stop it happening again.

What chlorine level is too high in a hot tub?

The widely accepted target for free chlorine in a hot tub is 3-5 ppm. Some manufacturers recommend a slightly lower range, so check your owner's manual and follow it if it differs. Anything above 5 ppm is higher than you need, and the water will start to feel harsh.

The abbreviation ppm stands for parts per million. For chlorine in water, 1 ppm is the same as 1 milligram of chlorine in every litre of water, so a reading of 5 ppm means 5 milligrams of chlorine per litre.

Most hot tub guidance says not to use the tub at all once free chlorine passes 10 ppm. Between 5 and 10 ppm, many people notice stinging eyes or itchy skin, so the simple rule is to bring it down before anyone gets in. Our guide to reading hot tub chlorine levels covers the full range of readings.

At very high levels, prolonged skin contact can cause rashes and even chemical burns. People with asthma or sensitive skin are especially vulnerable, so keep them out of the water until the level is back in range.

Free, combined and total chlorine

Free chlorine is the part that is still available to kill germs. Combined chlorine is chlorine that has already reacted with sweat, skin oils and other waste, forming compounds called chloramines. Total chlorine is the two added together.

This matters because a strong chlorine smell often means too much combined chlorine, not too much free chlorine. If your total chlorine is well above your free chlorine, the water needs shocking, not lowering. Lowering it would make the problem worse.

To work out combined chlorine, subtract free chlorine from total chlorine. For example, a total chlorine reading of 8 ppm with free chlorine at 2 ppm means about 6 ppm of chloramines. That water needs a shock dose, not a chlorine reducer, because the free chlorine is already low.

The same applies to skin. The burning or stinging feeling that many owners blame on too much chlorine is usually caused by chloramines (combined chlorine), not by excess free chlorine. Before you lower anything, test free and total chlorine and compare the two numbers.

Signs the chlorine is too high

• Test strip or liquid test shows free chlorine above 5 ppm
• Eyes sting or look red after a short soak
• Skin feels dry, tight or itchy afterwards
• Swimwear fades faster than usual
• A coughing or throat irritation when you lean over the water with the cover off

Coughing and throat irritation come from chlorine gassing off the warm water, and they are worse in an indoor or poorly ventilated hot tub room. If your tub is indoors, open doors or windows while the level is high.

Free chlorine is not the only cause. Chloramines also gas off the water surface, and in a poorly ventilated room they can cause throat irritation, coughing and other breathing symptoms. If a strong chlorine smell comes with breathing irritation, test both total and free chlorine and compare them before you lower anything.

How do you confirm a high chlorine reading?

Before you change anything, make sure the reading is real. Test strips are quick but easy to misread, and a reading taken straight after dosing will always be high because the chlorine has not spread through the water yet.

For this job, the minimum test kit is one that measures free chlorine, total chlorine, pH and total alkalinity. A 5-way or 6-way test strip or a liquid drop kit will do. If you have a digital tester or photometer, use it, as it gives more precise readings than matching colours by eye.

1. Check your test strips first. Look at the date on the pot and replace it if the strips are out of date or have been stored damp, as both give unreliable readings. Keep the lid closed, store the pot in a cool, dry place and never touch the pads with wet fingers.
2. Let the pump circulate the water for a while after the last dose of chemicals, following the timing on the product label.
3. Take the sample from about elbow depth, away from the jets and any floating dispenser.
4. Dip the strip once for only about 2 seconds, then shake off the excess and hold it level so the colours do not run into each other. Leaving it in longer can bleach the pads and give a falsely low chlorine reading.
5. Read it in daylight at the exact time stated on the pot, as colours keep changing after that.
6. Compare free chlorine, total chlorine and pH, and write the numbers down so you can track the drop.

Many strips stop at 10 ppm, so a reading at the top of the chart may mean the true level is higher. Very high chlorine can also bleach the dye in some liquid test kits, which gives a pale or clear result that looks like zero. If the water smells strongly of chlorine but the test shows almost nothing, suspect a very high level and retest a diluted sample.

Rule out the strips as well. If a strip reads zero but the water smells strongly of chlorine, retest with a fresh strip from a newly opened pot before you conclude there is no chlorine.

To dilute a sample, mix one part hot tub water with one part distilled water, test it, then double the result. This gives you a usable estimate when the normal test is off the scale.

What is the quickest way to lower chlorine in a hot tub?

There are three reliable methods. Ranked from slowest to fastest, they are leaving the cover off with the jets running, draining and refilling part of the water, and adding a chlorine neutraliser. The right choice depends on how high the reading is and how soon you want to use the tub.

Speeds vary with sunlight, water temperature and how far the level has to fall, so treat these as rough guides. You can also combine methods, for example leaving the cover off while you work out how much water to drain.

Fastest of all is to combine a drain with a neutraliser. Do a partial drain first, then retest and add a smaller dose of neutraliser based on the new, lower reading. You use less product and have more control over the final level.

For this combined approach, replace roughly 25-30% of the water first. Once the tub is refilled and the water has circulated, retest free chlorine and dose the neutraliser for that new reading, not the original one.

How do you lower chlorine with the cover off and the jets running?

Chlorine breaks down naturally in sunlight and escapes from warm, moving water. With the cover on, both of these slow right down, which is why a covered tub can hold a high reading for days. Opening it up and adding movement speeds the process along without any chemicals.

1. Remove the cover completely or fold it well back so sunlight reaches the water.
2. Turn on the jets and, if your tub has one, the air blower, which pushes more air through the water.
3. Open any air controls on the jets to add extra bubbles.
4. Leave the tub running for a few hours, keeping children and pets away from the open water.
5. Retest free chlorine every 2-4 hours rather than waiting until the end of the day, and keep going until it is back to 5 ppm or below.

Run the jets at full power in sessions of about 30-60 minutes, and keep the circulation pump on its normal filtration cycle in between. Still water loses chlorine more slowly than moving water, so never let the tub sit idle. Many tubs switch the jets off automatically after a set time, so restart them when they stop.

While you wait, do not add any other chemicals, including more chlorine, shock or pH adjusters, until free chlorine is back in range. Extra chemicals make the readings harder to interpret and can push the water further out of balance.

As a rough guide, this is how long free chlorine can take to fall from 5-7 ppm to 3 ppm:

• Cover off, jets on and direct sun: about 6-12 hours
• Cover off and jets on, but no sun: about 12-24 hours
• Cover off, but no jets: about 24-48 hours

From 10-15 ppm, aeration alone can take 48-72 hours. Above about 7-8 ppm, combine it with a partial drain or a neutraliser rather than relying on aeration by itself.

This method works best on a bright day when the reading is only slightly high. On an overcast day, in the evening or in winter, it can take much longer. Expect some heat loss, so the heater will need to work harder to bring the water back up to temperature.

Keep the water temperature at or below 40°C (104°F) as normal. Raising the temperature to burn off chlorine faster is not worth the extra energy cost and makes the water harder to manage.

How much water should you drain to dilute chlorine?

A partial drain lowers chlorine by simple dilution. If you replace a quarter of the water with fresh tap water, the chlorine level drops by about a quarter. Tap water in the UK contains a small amount of chlorine, but far less than an over-dosed hot tub.

A partial drain also dilutes other chemicals that build up over time, such as cyanuric acid (stabiliser) from stabilised chlorine granules. Neither aeration nor a neutraliser removes these, so dilution is the only way to bring them down short of a full drain.

To work out how much to replace, subtract your target from your current reading, then divide by your current reading. For example, going from 10 ppm down to 5 ppm means (10 minus 5) divided by 10, which is a half. In a 1,500 litre (about 400 US gallon) tub, that means replacing about 750 litres (about 200 US gallons).

If you prefer not to drain so much, a smaller partial drain also works. Replacing about a quarter of the water takes 10 ppm down to roughly 7.5 ppm. Then leave the cover off with the jets running to finish the job, and if the reading is still above 5 ppm after that, repeat the partial drain.

Most hot tubs have a drain valve or spigot low down on the cabinet, near the equipment bay. It usually accepts a garden hose, so you can lead the water to a suitable drain. Your manual shows where it is on your model.

1. Test and write down the current free chlorine reading.
2. Work out the share of water to replace using the sum above.
3. Mark the target water level on the shell with a strip of masking tape, or, if you know how many litres per minute your drain empties, work out how long to run it.
4. Switch off the hot tub at the isolator so the heater and pump cannot run with low water.
5. Drain the calculated amount using the drain valve or a submersible pump, stopping when the water reaches your mark or the time is up.
6. Close the drain valve, or lift out the submersible pump, before you start refilling. Otherwise the fresh water will run straight back out.
7. Refill with a hose, through the filter housing if your manual recommends it, as this helps prevent airlocks.
8. Once the water is back above the highest jets and the filter, switch the power back on and let it circulate.
9. Let the water mix properly before retesting, as a reading taken within the first few hours of a refill may not reflect the final level. Then retest free chlorine, pH and total alkalinity, and adjust pH and alkalinity before anyone gets in.

Always switch off at the isolator before draining. Running the heater or pump with the water level too low can damage them, and most tubs will not restart correctly until they are properly refilled.

Think about where the drained water goes. Chlorinated water should not run into surface water drains, which often lead straight to streams and rivers. The usual route is a foul drain, and if you are unsure, ask your local water company or council.

When to drain the tub completely

If your sum says you need to replace more than about half the water, a full drain and refill is often simpler. It gives you a clean start and saves you balancing tired water.

• The water is close to its next scheduled change, which many manufacturers suggest every three to four months
• You have used stabilised chlorine granules for a long time and the water no longer responds well to dosing
• Cyanuric acid (stabiliser) is above about 50 ppm, because it locks chlorine into a less active form and makes test readings unreliable
• Bathers keep getting skin or eye irritation even though chlorine tests in range, which points to a wider water balance problem
• The water is cloudy, foamy or hard to balance as well as being high in chlorine
• You have tried a partial drain and the reading is still too high
• Free chlorine is still above 10 ppm after several neutraliser doses, so a full drain and refill is a better choice than adding more product

The age of the water matters too. Water that is three to four months old has built up dissolved solids from chemicals, sweat and cosmetics, which makes it hard to balance. If your water is that old, choose a full drain over a partial one.

How does a chlorine neutraliser work?

A chlorine neutraliser is a chemical that reacts with chlorine and turns it into harmless compounds within minutes. It is the fastest of the three methods and the easiest to overdo, so add it in small amounts and test as you go.

Sodium thiosulfate

Most hot tub neutralisers, often sold as chlorine reducer or chlorine and bromine reducer, are based on sodium thiosulfate. Dosing varies between products, so follow the label for your water volume rather than any general chart.

Neutralisers are also sold under the name dechlorinator. When you shop for one, look for the word dechlorinator as well as chlorine reducer on the label.

1. Check your tub's water volume in the manual so you can dose accurately.
2. Measure out a fraction of the dose the label suggests for your reading. Unless the label says otherwise, dissolve the granules in a bucket of tub water rather than adding them dry.
3. With the jets running and the cover off, pour the solution in slowly near a jet or suction inlet.
4. Let the water circulate for the time given on the label. Sodium thiosulfate usually shows its effect within 15-20 minutes, which is about one circulation cycle.
5. Retest free chlorine after that time and add more only if it is still above 5 ppm.
6. Once the level is right, check pH and adjust if needed.

Overdosing removes all the chlorine, which leaves the water unprotected. Any leftover neutraliser will also react with the next chlorine you add, so you may find the level hard to raise again. If that happens, a partial drain is usually the quickest fix.

Store neutraliser and other spa chemicals in a cool, dry place, in their original containers and away from children. The Health and Safety Executive gives general advice on handling hazardous substances safely, and the same common sense applies at home.

Hydrogen peroxide products

Hydrogen peroxide also reacts with chlorine, and some spa products use it. Only use a product made for hot tubs and follow its label exactly. Household or pharmacy peroxide is not designed for spa water and the strength varies too much to dose safely.

Unlike sodium thiosulfate, hydrogen peroxide-based reducers can raise pH, so check pH once the chlorine is back in range. Wear gloves and eye protection when handling them.

Does baking soda lower chlorine?

No. Baking soda (sodium bicarbonate) raises total alkalinity and nudges pH up, but it does not remove chlorine. At a higher pH, chlorine works less actively, so the water may feel slightly gentler, but the ppm reading stays the same and the tub is just as over-chlorinated.

When is it safe to get back in the hot tub?

Get back in only when free chlorine is at 5 ppm or below and the rest of the water is balanced. Test again just before you use the tub, rather than relying on a reading from earlier in the day.

Do not rely on your nose. High chlorine cannot always be detected by smell, so test before getting in even when the water looks and smells normal.

• Free chlorine: 3-5 ppm, or the range in your manual
• pH: 7.2-7.6
• Total alkalinity: 80-120 ppm
• Calcium hardness: 150-250 ppm
• Water temperature: no hotter than 40°C (104°F)

After shocking, stay out until free chlorine is back to 5 ppm or below, even if the shock product says a shorter wait. The test result matters more than the clock.

Check pH and alkalinity after lowering chlorine

Each method can move your pH. Fresh tap water changes it during a partial drain, and neutralisers can push it slightly down. Chlorine also works best within the 7.2-7.6 range, so a correct pH means you need less of it.

Free chlorine above about 10 ppm can bleach the pH pad on a strip, or the dye in a liquid kit, and give a falsely low pH reading. Lower the chlorine first, then retest pH before making any big pH or alkalinity adjustments.

The type of chlorine behind the overdose matters too. Dichlor granules are acidic and trichlor tablets even more so, so an overdose from either has probably pushed pH down as well as raising chlorine.

Low pH is worth fixing promptly. Below 7.2, the water feels harsh and causes the same skin and eye irritation as high chlorine, even when the chlorine reading is correct. It also speeds up corrosion of the heater and other metal parts.

If you did a partial or full drain, always test the new water. Depending on your local supply, fresh tap water can bring total alkalinity well below 80 ppm or well above 120 ppm.

Fix total alkalinity first, as it acts as a buffer that holds pH steady. Then adjust pH, let the water circulate, and retest before anyone gets in.

1. If total alkalinity is below 80 ppm, raise it with sodium bicarbonate (alkalinity increaser).
2. If total alkalinity is above 120 ppm, lower it with small doses of pH decreaser (sodium bisulfate) with the jets running.
3. Once alkalinity is in range, if pH is still below 7.2, raise it with sodium carbonate (pH increaser) in small steps.
4. Retest after each addition, allowing the circulation time on the label, before adding any more.

If someone has already reacted to the water

If your eyes or skin are irritated after a soak, get out, rinse off in the shower with clean water and change into dry clothes. Mild irritation usually settles on its own once you are out of the water.

If symptoms do not settle, speak to a pharmacist or your GP, or use NHS 111 for advice. If someone has trouble breathing, call 999. The US Centers for Disease Control and Prevention also publishes general guidance on healthy hot tub use.

Why did the chlorine go so high, and how do you stop it happening again?

Most high readings come down to dosing by eye instead of by volume. Knowing the cause stops you from fixing the water one week and repeating the problem the next.

• Adding granules by guesswork rather than measuring for your water volume
• Assuming the tub holds more water than it does, which leads to bigger doses
• Testing straight after dosing, getting a low reading, and adding more
• Getting in after a shock dose before the level has dropped
• A floating dispenser set too open, or holding too many tablets
• Fewer people using the tub, so less chlorine is used up between doses
• Leaving the cover on for long periods, which keeps chlorine in the water
• A chlorine generator set higher than the tub needs

Dosing habits that keep chlorine steady

• Always test before adding chlorine, and only dose when free chlorine has dropped below your target, as topping up a tub that is already in range is a common cause of overdosing
• Find your tub's exact water volume in the manual and dose for that figure
• Use a proper measuring scoop or spoon rather than the bottle cap, as caps are not standard measures, and add a little at a time rather than one large dose
• Add chlorine with the jets running so it spreads evenly, and wait about 30 minutes before testing again
• Test two or three times a week, and before every use if the tub is busy
• Close a floating dispenser down when the tub is not being used much
• Reduce your dose when you go away, and test before the first soak on your return

Product labels give doses for a set volume of water, so scale the dose to your own tub. For example, if your tub holds half the volume shown on the label, use half the stated dose.

Our guide to how much chlorine to put in a hot tub by litres gives a straightforward way to work out each dose. If you are new to chlorine, start with how to use chlorine in a hot tub for the basics of granules, tablets and routines.

Slow-release options

If granule overdosing keeps happening, a slow-release option such as tablets in a floating dispenser gives a steadier level. This suits owners who use the tub infrequently, as long as the dispenser is set low and checked with regular tests.

If overdosing still happens despite better habits, you could consider switching to bromine or to a salt water chlorine system. Both give a steadier sanitiser level than dosing granules by hand. Check your manual first, as some manufacturers limit which sanitisers or systems can be used with their tubs.

When to ask for a professional water test

If chlorine stays high after several neutraliser doses, or keeps coming back, ask your hot tub dealer for a professional water test. A faulty test kit, a stuck feeder or a malfunctioning chlorine generator may be the cause, and these need checking rather than more chemicals.

Does a tennis ball help with chlorine?

Some owners float a clean tennis ball in the water to soak up skin oils and lotions, which can reduce scum on the waterline. It does nothing to lower chlorine. If you try it, use a new ball and replace it often, and remember that a clean filter does the same job far more reliably.

There is a longer-term benefit, though. Removing skin oils, lotions and cosmetics, whether with a tennis ball or a scum absorber, means less waste for chlorine to react with, so fewer chloramines form. Your chlorine then works more efficiently over time, even though the reading itself does not drop.

Common mistakes when lowering chlorine

• Adding pH reducer to lower chlorine: it lowers pH, which makes chlorine act more harshly, not less
• Pouring in a full bottle of neutraliser at once and stripping out all the chlorine
• Lowering chlorine when the real problem is chloramines, which need a shock dose
• Getting in to use the chlorine up, which only exposes you to the high level
• Leaving the tub uncovered and unattended where children or pets can reach it

If the strong smell came from chloramines, follow our steps on how to shock a hot tub instead. If you use bromine, the same three methods apply, and our guide to lowering bromine in a hot tub covers the differences.

Lowering hot tub chlorine: the short version

• Confirm the reading first with fresh strips, and check free against total chlorine
• Above 5 ppm is too high; above 10 ppm, keep everyone out
• Slowest and free: cover off with jets and air running, retesting every 2-4 hours
• Faster: drain and refill the share of water your sum tells you to
• Fastest: a neutraliser, added a little at a time, or a partial drain followed by a smaller neutraliser dose
• Get back in only at 5 ppm or below with pH at 7.2-7.6
• Prevent a repeat by testing first and dosing for your exact water volume

Frequently asked questions

Is 10 ppm chlorine safe in a hot tub?

No. At 10 ppm free chlorine, most hot tub guidance says to keep everyone out. The level is about double the top of the normal 3-5 ppm range, and it is likely to irritate eyes, skin and airways. Bring it down with one of the three methods and only get in once your test shows 5 ppm or below.

Does vitamin C lower chlorine?

Ascorbic acid, the chemical name for vitamin C, does react with chlorine and is used to remove it in some water treatments. However, it also lowers pH, and dosing it from tablets or powder is guesswork. A neutraliser made for hot tubs is easier to measure and has clear instructions for your water volume, so it is the safer choice.

Does hot water lose chlorine faster?

Yes. Warm water loses chlorine more quickly than cold water, which is one reason hot tubs need more frequent testing than swimming pools. This does not mean you should turn the heat up to lower chlorine. Keep the temperature at 40°C (104°F) or below and use aeration, dilution or a neutraliser instead.

Can I leave my hot tub uncovered overnight to lower chlorine?

You can, but it is not very effective, because sunlight does most of the work and there is none at night. You will also lose a lot of heat and collect leaves and insects. An open tub is a drowning risk for children and pets, so only leave it uncovered when someone can keep an eye on it.

Can high chlorine damage my hot tub?

Water that stays well above the normal range for a long time can wear out the parts that touch it. Covers can fade and break down on the underside, pillows can crack, and rubber seals can harden. Over long periods it can also break down the acrylic shell and corrode metal fittings, especially in the heater when pH is low. A short spike that you fix quickly is unlikely to cause lasting harm.

Should I neutralise the chlorine before draining my hot tub?

It helps if the water is very high in chlorine and you are draining it anywhere other than a foul drain. Letting the level fall first, or using a neutraliser, reduces the risk of harming plants and wildlife. Never send chlorinated water into a surface water drain, and check with your local water company or council if you are unsure.