Chlorine Too High? Check the Test, Then Let It Fall

High chlorine is the one pool problem that fixes itself. Chlorine gets used up and burned off by the sun every day whether you help or not. So the real questions are how long you want to wait, and whether the number in front of you is even real.

It's rarely a mystery, either. Something is adding more than the pool uses. A salt cell still set for July. A tablet feeder turned all the way up. Boost mode left running. A shock dose worked out for somebody else's pool. Find that, turn it down, and the rest is patience.

Key takeaways

• Check the test before you treat the pool. Above 10 ppm a color-match DPD test can bleach out and read low, and the pH test turns purple. Taylor's FAS-DPD kit reads up to 20 ppm.
• Too high for the water depends on your stabilizer. This site's target is about 11.5% of your CYA, so 8 ppm is right at 70 ppm of CYA and far too much at 30.
• Too high for swimming is 4 ppm. That's the limit on current pool chlorine labels, and they say using the product any other way breaks federal law. This site's target passes 4 once stabilizer is above about 35.
• On a salt pool, the setting is a share of pump time, so longer pump hours make more chlorine. The makers aim for 1 to 3 ppm (Hayward, Jandy) or 2 to 4 (Pentair), and their fix when the lowest setting is still too much is fewer hours.
• Neutralizer is for a real overshoot. Add part of the dose, circulate, and retest, because too much leaves the pool with no chlorine at all.

First, make sure the reading is real

Here's the trap. A test that should say 20 ppm can say zero.

The pink color in a DPD test comes from chlorine reacting with a dye. Push the chlorine high enough and it destroys the dye instead. Taylor, which makes the K-2006 kit, puts the line at 10 ppm: above that, it says the sample can bleach "leading to false-low readings". The sample goes pink and fades, or never turns pink at all.

So an owner who shocked hard tests the next morning, sees clear water in the tube, and adds more chlorine. It's an easy loop to fall into, and it isn't only homeowners. The CDC's model health code caps public pools at 10 ppm partly because operators have kept adding chlorine without realizing the test was bleaching out.

Three ways to get a real number:

• Dilute the sample. Taylor's fix: dilute it and retest, and for a 1:1 dilution "multiply the result by two". Use distilled water for the other half, because tap water can carry chlorine of its own.
• Use an FAS-DPD kit. It counts drops instead of matching colors. Taylor lists the FAS-DPD test in its K-2006 kit as reading 0 to 20 ppm, and with the 10 mL sample each drop is 0.5 ppm. If the pink never shows or disappears, Taylor's instructions say to add more of the DPD powder until it turns pink. That on its own tells you the level is high.
• Use a high-range strip. Common strips stop at 10 ppm. AquaChek's Select and LaMotte's Insta-TEST 6 Plus both do, so a dark pad only means "10 or more". AquaChek sells a high-range strip that reads to 600 ppm, for exactly this.

LaMotte describes the giveaway: a DPD test that's gone colorless next to a dark purple strip means free chlorine is beyond 10 ppm. The strip can't bleach. The DPD test can.

If you want the full walkthrough of the kit, how to test pool water covers it.

How high is too high?

There isn't one number, because the right amount of chlorine depends on your cyanuric acid. Stabilizer holds chlorine in reserve, so the more CYA you run, the more free chlorine it takes to do the same work.

This site sets the target at about 11.5% of your CYA, inside the 10 to 15% band, with shock level at 40%. That's the same rule the balancing guide and the chemistry calculator use.

	Target	More than you need above	Shock level
CYA 30	3.5 ppm	4.5 ppm	12 ppm
CYA 40	4.5 ppm	6 ppm	16 ppm
CYA 50	6 ppm	7.5 ppm	20 ppm
CYA 70	8 ppm	10.5 ppm	28 ppm
CYA 80	9 ppm	12 ppm	32 ppm

So 8 ppm is a problem at 30 ppm of stabilizer and on target at 70. Test CYA before you decide anything else.

Above the band but below shock level, the chlorine is mostly wasted money, and it'll drift back down on its own.

The line for swimming is on the label

Whether anyone swims in the meantime is a different question, and the answer is printed on the chlorine you poured. Current labels on liquid chlorine, cal hypo, trichlor and dichlor from HTH, Clorox, BioGuard, Leslie's and KIK all say not to swim above 4 ppm of free chlorine. HTH's Pool Shock puts it bluntly: "Re-entry into treated swimming pools is prohibited above levels of 4 ppm chlorine for risk of bodily injury." A couple of OxyChem labels still say 3. And every one of them says it's a violation of federal law to use the product in a manner inconsistent with its labeling.

The CDC's model health code lets a public pool stay open up to 10 ppm. But the same code says chlorine levels have to be consistent with the label, and its annex notes that EPA hasn't approved any pool product for use above 4 ppm with swimmers in the water. The annex is candid about the number, too: it found no data suggesting health risks at 4 ppm, "or even substantially above". So 4 is the legal line, not a cliff.

Here's where that runs into the table above. At about 35 ppm of stabilizer, this site's target reaches 4 ppm. Above that, water held on target is water the label says not to swim in. The table is about how much chlorine it takes to keep the water clean. The label is the line for getting in. They only agree at low stabilizer, so if you want to be inside both, keep CYA near 30.

Your salt cell's manual disagrees

If you run a salt pool, check its manual, because the makers aim lower than that table:

• Hayward (AquaRite): 1.0 to 3.0 ppm, with stabilizer at 30 to 50.
• Pentair (IntelliChlor and iChlor): 2.0 to 4.0 ppm, which its manual calls the APSP recommended range. It adds that above 4.0 "may cause corrosion of metal components".
• Jandy (AquaPure): 1.0 to 3.0 ppm, with stabilizer at 50 to 75. Its manual warns that levels above 3.0 "may cause corrosion of pool metals", and lists the range among the conditions under which the AquaPure "is warranted to operate properly".

At 70 ppm of stabilizer, this site's table asks for about 8 ppm. That's twice Pentair's top figure, and more than double Hayward's and Jandy's.

This site sets chlorine by stabilizer, because that's what decides how hard the chlorine actually works. But the manual is what a warranty claim gets read against. The two only come close at low stabilizer. At 30 ppm of CYA the target here is 3.5, which sits inside Pentair's range and just above Hayward's and Jandy's.

So if your cell is under warranty, read that clause before you hold 8 ppm. This site's salt water maintenance guide recommends 60 to 80 ppm of stabilizer, which is where the gap is widest.

Find what's feeding it

Lowering chlorine that something keeps adding is bailing water with the tap running. Start here.

A salt cell

The output setting isn't an amount of chlorine. It's a share of the time the pump runs. Pentair spells it out. At 100%, its IntelliChlor makes chlorine for 265 seconds of every five minutes. At 20%, it's 53 seconds. Jandy's TruClear manual puts the consequence plainly: "More pump runtime will result with more chlorine."

That can catch out anyone who switches to a variable-speed pump. The new pump can run 16 or 24 hours at low speed instead of 8 at full. If the cell setting stays where it was, the pool gets two or three times the chlorine.

The other usual causes:

• Boost left on. Pentair's Boost and Jandy's boost mode run at 100% for about a day and then return to your setting on their own. Hayward's older AquaRite super-chlorinates for 24 hours and then asks you to put the switch back to AUTO yourself.
• The weather turned. Cooler water, shorter days and fewer swimmers all cut demand. Hayward's manual says most pools need less chlorine in cold weather and the output "should be lowered accordingly". Jandy warns that running in cold water can over-chlorinate the pool.
• The cover's on. A covered pool loses almost nothing to the sun. Pentair recommends turning output down while the pool is covered, and Hayward's AquaRite S3 can take a signal from an automatic cover and cut output itself.

The makers' fix is the same everywhere. Turn the output down, and Jandy says to do it in 10% steps. If the lowest setting still makes too much, both of Jandy's manuals say to cut the pump's daily run time. Then give it a day before you judge the change.

One more thing. The number on the control panel is what the cell thinks it's making. It has no idea what's in the water. Test with a kit, not the display.

A tablet feeder or floater

A tablet chlorinator set a notch too high is the other usual cause. A floater wedged in a corner, or tablets left in the skimmer, can do it too.

Turn the feeder dial down a step at a time and test daily. Every tablet also adds stabilizer, which is why tablet pools drift up the table above over a summer.

A shock dose

Shock level is 40% of your CYA, not a bag per 10,000 gallons. Two 1-pound bags of 73% cal hypo in 10,000 gallons add about 17.5 ppm. At 30 ppm of stabilizer, shock level is 12. The shock calculator works out the right dose from your own readings.

How to bring it down

1. 1

   Test it properly

   5 minutes

   Use an FAS-DPD kit with the 10 mL sample, where each drop is 0.5 ppm. If a color-match test goes clear or the pink fades, the chlorine is too high for it to read. Get a real number before you do anything.

   Mark done
2. 2

   Stop whatever is adding it

   5 minutes

   Turn the salt cell down, or off if the level is well over, and cancel any boost. Close the tablet feeder or pull the floater. Nothing else works while chlorine is still going in.

   Mark done
3. 3

   Take the cover off and run the pump

   Hours to days

   Sunlight destroys free chlorine. Jandy's manual says most chlorine with no stabilizer is gone within two hours of sun. Stabilizer slows that down, which is its job, so a stabilized pool takes longer. Leave the cover off and keep the water moving.

   Mark done
4. 4

   Retest once a day

   5 minutes a day

   Same time, same kit. Once it's back near your target, set the cell or feeder lower than before, so it stays there.

   Mark done
5. 5

   Use neutralizer only if you can't wait

   1 hour

   Sodium thiosulfate is the usual one, and the published doses don't agree. HASA's directions work out to 4 oz per 10,000 gallons for each ppm. The pool-operator table that Indiana's health department reprints says 2.6 oz. So use the lower figure, dose for half the excess, broadcast it with the pump running, and retest after an hour, which is when HASA says to check. Repeat if you need to.

   Say a 20,000-gallon pool reads 12 ppm and its target is 4.5. That's 7.5 ppm over. Half of that is 3.75 ppm, which at 2.6 oz per 10,000 gallons comes to 19.5 oz, a little over a pound. Some neutralizers are sodium sulfite instead of thiosulfate, and those have their own label dose. Either way, overshoot and the pool has no chlorine at all.

   Mark done

What doesn't work

Draining. To take 10 ppm down to 5 by dilution, you'd have to replace half the pool. The sun does the same job for free, usually faster.

Adding acid. Acid moves pH, not chlorine. If you added it because the pH test came out purple, that was the chlorine talking.

Adding stabilizer to soak it up. More CYA does make the chlorine you have less active. But it doesn't lower the reading, and it never leaves except by draining. You'd swap a problem that fixes itself in a few days for one that needs a hose.

Is it doing any harm?

For a few days, mostly to comfort. Stinging eyes, dry skin, faded swimwear. How long after shocking you can swim covers when it's all right to get back in.

Held high for weeks, it starts to cost you. The salt cell makers' corrosion warnings above are about metal: ladders, handrails, light rings and heater exchangers sitting in strong chlorine for a season.

Vinyl liner makers set their own limits. GLI's care guide says to keep free chlorine between 1.5 and 2.5 ppm, warns that liner fading and wrinkling may occur above that, and says never to cover the pool while it's higher. Merlin's line is 10 ppm: no running the pool or closing it for winter above that for any length of time. Merlin also says not to put tablets in the skimmer or use a floating chlorinator at all, because either can bleach the liner. GLI's figure sits under every target in the table above, the same gap as the salt cell makers. On a vinyl pool, that's one more reason to keep stabilizer near 30.

Tablets left in the skimmer are their own problem. They sit in still water while the pump's off, and when it starts, the first rush carries very strong chlorine to the heater. That's why the heater makers want a feeder last in line, with a check valve.

If it happens every fall

If it's October, this is where I'd look first. The setting that held the pool in July is too much for September. The water's cooler, the days are shorter and nobody's swimming, so demand drops and the cell keeps going.

Turn the cell down a step every time the weather turns, and test twice a week through the change. The cells stop on their own once the water gets cold: Hayward's AquaRite stops below 50°F, and Pentair's IntelliChlor at about 52°F. Until then, it's on you.

If a freeze controller will run your pump through the winter, Jandy says to turn an AquaPure down to 10 to 20%. Otherwise, it says, production will overshoot its range.

Go deeperWhy the right chlorine level depends on stabilizerThe chart behind the table above, and how CYA creeps up on a tablet pool.

Frequently asked questions

How do I lower the chlorine in my pool fast?

Stop whatever is adding it, take any cover off, run the pump and let the sun work. That is free and it is usually enough. If you can't wait, a sodium thiosulfate neutralizer is faster. Published doses disagree, so use the lower one, treat half the excess, and retest after an hour, because overshooting leaves the pool with no chlorine at all.

Is it safe to swim when chlorine is high?

Not above 4 ppm, by the label. Current pool chlorine labels from HTH, Clorox, BioGuard, Leslie's and KIK prohibit swimming above 4 ppm of free chlorine, and a couple of OxyChem labels say 3. Each label says using the product any other way is a violation of federal law. Below that, high chlorine mostly costs comfort: stinging eyes and dry skin.

Why is my salt water pool chlorine so high?

Usually because the cell is making the same amount while the pool needs less. The output percentage is a share of the time the pump runs, so longer pump hours make more chlorine at the same setting. Cooler water, a cover and fewer swimmers all cut demand. Turn the output down in small steps, and if the lowest setting is still too much, run the pump fewer hours.

Why does my test show no chlorine right after I shocked?

The chlorine may be too high to read. Taylor says levels above 10 ppm can bleach a DPD sample and give a false-low reading. Dilute the sample half and half with distilled water, test again and double the result, or use an FAS-DPD kit, which reads to 20 ppm.

Can high chlorine damage my pool?

A few days of it mostly costs comfort: stinging eyes and faded swimwear. Held high for weeks is different. Pentair's salt cell manual says above 4 ppm may corrode metal parts, and Jandy's says the same above 3 ppm. GLI, which makes vinyl liners, says fading and wrinkling may occur above 2.5 ppm.