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How To Break Chlorine Lock

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Chlorine lock isn't the end of the world but it is a common problem for swimming pool owners. Chlorine lock means that the chlorine in the pool is useless which means the water isn't being sanitized. It can also indicate the presence of chloramines which give off a chlorine smell. Below are a few ways to deal with this issue.



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Step 1

Why Does Chlorine Lock Happen? The most common cause of chlorine lock is an overabundance of cyanuric acid (CYA). Cyanuric acid is commonly referred to as a "stabilizer" and is vital to maintaining healthy chlorine levels in your pool. Stabilizer is found in liquid, granular, and tablet forms of chlorine; and acts as a "chlorine sunblock," preventing burn-off by the sun. 

The recommended range for your pool's CYA level is 30-50 ppm for traditional chlorine pools and 60-80 ppm for saltwater pools. When your CYA creeps above this range, that's when it goes from an aid to an inhibitor.

An imbalance of pH levels can also cause chlorine lock.

Click Here to View Pool Shock

Step 2

How to Test for Chlorine Lock - Determining the severity of your chlorine lock can be done by calculating the Combined Chlorine of your water.  Requires DPD Test Kit

Combined Chlorine = Total Chlorine (TC) - Free Chlorine (FC)
  • Good Result: 4 TC - 4 FC = 0 CC
  • Bad Result: 7 TC - 3 FC = 4 CC
The good result means our water’s chlorine level is healthy; the bad result means there is more “dead” chlorine than active, which causes Chlorine Lock.

Click Here to View the Taylor Water DPD Test Kit

Step 3

Chlorine Lock Prevention Method 1 - Solution by Dilution - the most effective way of breaking chlorine lock is diluting the concentration of CYA in your pool. You can lower CYA levels by draining a percentage of your pool then refilling it with fresh water. This method is the only guaranteed way of lowering pool water’s CYA significantly in one fell swoop.

By draining the pool, you have more control over the amount of CYA more control over how CYA is purged.

Step 4

Chlorine Block Prevention Method 2 - CYA Reducer - CYA reducing chemical agents are a relatively new product that removes the need of draining and refilling your pool. A CYA reducer may be your best choice in preventing or treating chlorine lock if you live in a water-restricted area due to drought or general climate.

A CYA reducer is usually made of the chemical compound Aluminum Sulfate, which acts as a coagulating agent called a flocculant. The flocculant is essentially dirt particle flypaper; everything sticks to it until it becomes a larger mass. Eventually, that coagulated material sinks to the bottom can vacuum to waste.

For in-depth instructions on using Aluminum Sulfate, you can find them here: Lowering Your CYA Without Draining.

Click Here to View a CYA Removal Kit

Step 5

Chlorine Lock Treatment Method 3 - Non-Chlor Shock (Oxidization) - Non-Chlorine Shock, also called Non-Chlor, uses oxidation to break down contaminant compounds in your pool water. Oxidation is different from sanitation, whereas oxidation breaks down chemical compound sanitization to kill bacteria.

You will need to subtract the Free Chlorine (FC) from the Total Chlorine (TC)  and multiply that by the number of gallons in the Pool divided by 10,000, then multiply by 2. 

How to calculate the amount of Non-Chlor Shock Needed:

(TC - FC) ✖ (Pool Size in Gallons ➗10,000 gal.) ✖ 2

The equation for our Pool:

(7 - 3) ✖ (20,000 gal. ➗10,000 gal.) ✖ 2

Becomes,

4 ✖ 2 ✖ 2 ＝16 pounds of Non-Chlor Shock

Click Here to View our Non-Chlorine Shock 

Step 6

Chlorine Lock Treatment Method 4 - Breakpoint Equation - Breakpoint chlorination is the chemical bond that ties nitrogen, chlorine, and ammonia together by using large amounts of chlorine, causing the chlorine residual to drop rapidly. Reaching an actual chlorine breakpoint is vital to breaking a chlorine lock. If a breakpoint is not reached using this method, the problem will only worsen.

Step 7

Chlorine Breakpoint Equation (Cont.) - It takes 7.6 free chlorine molecules to break apart an individual combined chlorine molecule to accomplish a breakpoint. The amount of product you will need will vary depending on the type of shock you are using.

Aside from knowing your total chlorine, free chlorine, number of gallons in the pool, and the weight of a gallon of water (8.34 lbs.), you will also need to know the type of pool shock you are using.

Before attempting this method, the pH needs to be between 7.2-7.4. 

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(1 to 40 of 132)

 Posted: 6/30/2022 

FC=0, Chlorine=0, Stabilizer=100, PH=7 3, Alkalinity=85, Calcium=200. Pool is 21,120 gallons Pool is crystal clear, but we cant get the chlorine levels to budge. I did reduce the stabilizer from 110 so far, by draining some water, not lower than the skimmer, twice. And adding cyu reducer 8 oz on Sat. Can you advise?
 Reply

InyoPools Product Specialist  Posted: 6/30/2022 

Your CYA is still about twice what it should be. Maybe I am reading the comment wrong, but if you have 0 free chlorine and 0 combined chlorine, it sounds like you haven't put any chlorine in your pool. If that is the case, I would start with pool shock and a stabilized chlorine like trichlor; but only after the CYA is brought under control. If you have added chlorine and the FC is still 0, I'd start with the non-chlorine shock mentioned in Method 3.As for things to try, any of the three methods I mention in the guide would be.
 Reply

 Posted: 6/11/2022 

FC=0, Chlorine=1, Stabilizer=20, PH=7, Alkalinity=100, Calcium=150 Pool is 26,700 gallons Pool is crystal clear, but we cant get the chlorine levels to budge. Also have a suds when splashing, Can you advise? Pool company recommendation is to increase PH 2lbs, and then add 3 gallons pf Chlorine shock.
 Reply

InyoPools Product Specialist  Posted: 6/15/2022 

Your pH is low, your stabilizer is too low. Increase your pH, and add cyanuric acid. Chlorine does not operate in a vacuum, it needs balanced pH and CYA to operate effectively. Their recommendations seem fine, but you'll still need a stabilized chlorine to maintain levels in your pool.
 Reply

 Posted: 6/1/2022 

I have a 33,000 gallons concrete pool. I opened around 5/20 - dealing with virtually no free chlorine: FC=0.76, TC=6.56, pH=7.7, Total Alkalinity=110, Calcium=236, Cyanuric Acid=5, Iron=0.1, Coper=04, Phosphates=1191, TDS=500. Every time I go to the local Leslie's I get a new number of shock pack I need to through in - nothing increased free chlorine yet. Please help - how do I break this "lock"?
 Reply

InyoPools Product Specialist  Posted: 6/2/2022 

Your cyanuric acid level is 5 when it should at least be 30. That is why chlorine is not staying in the pool. Chlorine will be burned off by the sun without enough CYA to protect it.
 Reply

 Posted: 6/3/2022 

I already poured 3Gal of Instant Conditioner - I ran the pump overnight and measured the result: ACID=28, FC=0.61, FC=5.6 - the rest is the same - I came home after the test and added 4pounds of Oxidizer Shock
 Reply

InyoPools Product Specialist  Posted: 6/9/2022 

That is not enough non-chlorine shock. Did you follow the formula in Step 5 for calculating how much non-chlorine shock you'd need to add? Because I calculated 38.28 pounds of non-chlorine shock. You need to follow the steps and suggestions of the guide. Guessing at amounts is not going to get you out of the water chemistry hole.
 Reply

Anonymous  Posted: 5/30/2022 

Hi there. 18k in ground pool, vinyl liner. FC=1.22, TC=14.21 PH=7.7, ALKALINITY=98, CALCIUM=224, CYA=15(low), phosphates are high=814, TDS=400. Pool store is recommending a huge shock treatment of of 12-24lbs. Water looks great. We opened the pool 2 weeks ago and just can't get this gulf between FC & TC dialed in. The shock treatment seems counter intuitive to me. Is this a chlorine lock situation? Is draining some the best course? thanks
 Reply

InyoPools Product Specialist  Posted: 6/2/2022 

You've already pointed to the problem in your test results; your CYA is half of what it should be. CYA's nickname in the pool industry is chlorine sunscreen. It is summertime, and chlorine is burned off by the UV rays of the sun. CYA in the pool isn't optional; it is a must.Also, the guide provides the formula for calculating how much non-chlorine shock is necessary.
 Reply

 Posted: 5/18/2022 

I have a clear pool but my FC is 0 and TC is 3.0. PH, Alk and CYA all in good range. 17,000 gallon pool. The store print out recommended 6 lbs of regular shock, I was thinking oxidizing nonchlorine shock would be better. What do you think is best option?
 Reply

InyoPools Product Specialist  Posted: 5/23/2022 

Try the non-chlorine oxidizing shock first.
 Reply

 Posted: 5/17/2022 

I could use some help. Persistent CC problem. 8500 gallon inground plaster fresh-water pool. Combined Chlorine: .72 ph: currently 7.8 (but I keep it between 7.2 and 7.6) Hardness: 295 Alkalinity: 60 CYA: 29 Copper: 0.3 Iron: 0.1 Phosphate 505 (currently, but it's been near-zero and hasn't mattered) I drained a third of the pool. No difference. I shocked with 1 gallon of Sodium Hypo (hi-concentrate). CC doubled. I then shocked with 3x the recommended dosage of non-chlor shock. CC doubled again. I run the filter 18 hours a day. What if I add chlorine eliminator? Would it remove all chlorine, and I could start from scratch? Many thanks for your thoughts.
 Reply

InyoPools Product Specialist  Posted: 5/25/2022 

Your alkalinity should be in the 80 - 120 ppm range; you list it at only 60. Is this a salt pool or traditional chlorine? The CYA range for traditional chlorine is 30 - 50 ppm, and salt is 60 - 80 ppm. Adding a chlorine exterminator will lower your chlorine levels, it won't help in this situation. Chlorine needs the pH, alkalinity, and CYA levels to be in check for chlorine to be efficient and effective.
 Reply

 Posted: 5/15/2022 

18 years old 20,000 gallon Gunite pool in Maryland which I’ve been working with for about six years since I bought the property. Two years paying a pro. And the last 4 years opening closing and doing the chemicals myself. The first year was a bit of a disaster and the 2nd year at opening also as I learned. Last 2 also have a dophlin robot that gets everything off the bottom of the pool. Last year went pretty well without much or any combined chlorine, and this year it seems to be even going better as I open. But what I’m worried is that I’m missing something, I really didn’t shock my pool all that much last year, but using the tailor test, and doing it generally very regularly sometimes every day, combined with occasionally strips as a check in between, I seem to be able to maintain 1 to 4 ppm of free chlorine in my pool with little to no combined chlorine that ever registers. At most it registers at .1 of combined chlorine. I keep my CYA generally between 40 and 50, once you get it there you’ll leave it there, and I use liquid chlorine. PH is always in the range. The pool doesn’t have any funky smell and stays clear, but I’m told that I’m supposed to shock every week to break chlorine lock if you have combined chlorine. I know another website says you don’t have to shock often unless you have combined chlorine registering. My question is, do you think I’m really missing something, because when I add chlorine, free chlorine goes up as it should and it goes down over time especially when hot as it should and it registers on both the Taylor liquid tests and even the test strips indicator about the same, it doesn’t seem like it’s locked… my chlorine occasionally registers as I said very little combined chlorine but it just takes one drop (Of the agent) and it turns right back to clear indicating it’s .1 % or less… Is it possible that I’m able to actually run my pool without really generating combined chlorine pretty consistently? And would you advise shock?
 Reply

InyoPools Product Specialist  Posted: 5/25/2022 

It sounds like your pool is working just fine. Pool shock is not a requirement for a weekly chemical regimen: use it when you need it. Keep doing what you're doing; it's working.
 Reply

Anonymous  Posted: 5/14/2022 

Opened pool three days ago. Shocked three times and still green and cloudy. Stabilizer reading was a 5. Is this why the shock is not changing the free chlorine level. I don’t want to be wasting shock.
 Reply

InyoPools Product Specialist  Posted: 5/25/2022 

Yes, that's precisely why the chlorine is not working. By throwing shock into a pool with essentially no chlorine, you're throwing money down a well. Your pool chemistry needs to be balanced (pH, alkalinity, CYA) for chlorine to work effectively.
 Reply

 Posted: 5/13/2022 

Our pool is clear but we are having a chlorine lock. We’ve been getting different suggestions from our local pool place but nothing seems to work. Free chlorine is .21, total chlorine is 2.45, ph 6.7, alkalinity is 84, calcium hardness is 140, cyanuric acid is 17, iron is 0.1, copper is 0.4, phosphates are 613, tds is 500 Thanks for any suggestions!
 Reply

InyoPools Product Specialist  Posted: 5/23/2022 

Your pH is too low, calcium is too low, and cyanuric acid is low. Chlorine is not independent of these results. The chlorine needs all of these other elements to work efficiently. The only real suggestion here is to balance your pool water. For the basics of pool chemistry and the correct ranges, look at this guide: How To Maintain A Swimming Pool Part 1 (Chemicals)
 Reply

Anonymous  Posted: 5/9/2022 

Like many others, I believe my pool has a chlorine lock problem. It is a 20 year old 10K gallon inground fiberglass pool. Total chlorine is 4.8, free chlorine is .3, pH is 7.3, total alkalinity is 115, calcium hardness 153, stabilizer 66. Water is hazy/cloudy. Local pool place suggested 9 lbs. of shock (Tidal Wave 73). This sounds like an extreme amount of shock. Do you agree that it needs that amount? Thanks for your help
 Reply

InyoPools Product Specialist  Posted: 5/11/2022 

We break down the math pretty simply in the article...Combined Chlorine = Total Chlorine (TC) - Free Chlorine (FC)4.8 - .3 = 4.5, Which is bad. How to calculate the amount of Non-Chlor Shock Needed: (TC - FC) ✖ (Pool Size in Gallons ➗10,000 gal.) ✖ 2 ... (4.8 - .3) ✖ (10,000 gal. ➗10,000 gal.) ✖ 2 = 9 pounds. 
 Reply

 Posted: 4/27/2022 

Can't get free chlorine to come up. Free is .74, Total is 5.48 (combined 4.74) per test at pool center. Advised to add 6 bags of non chlorine shock. Doesn't seem to be causing any change per my test strips.
 Reply

InyoPools Product Specialist  Posted: 5/2/2022 

Please post the full results of your latest water chemistry test. Chlorine does not operate in a vacuum. The other chemistry categories have an effect on how well chlorine can do its job.
 Reply

 Posted: 3/22/2022 

Help! My test stripes keep showing yellow for chlorine yet I have shocked the tub and added chlorine tablets. I think the tub is locked. The PH is low - barely orange. What shall I do? Shall I add something else to reduce it so I can start again? I really dont want to empty the pool and start again.
 Reply

InyoPools Product Specialist  Posted: 3/22/2022 

Unfortunately, those color ranges don't tell me much about the actual numerical value of the results. The color shades used in pool test strips are not uniform and vary from kit to kit. Please provide numerical values for the water chemistry you are referencing. You can also get a free water test printout from a local pool store, which you can then provide.
 Reply

 Posted: 9/9/2021 

TC = 0 FC = 0 pH > 7.8 Alk > 180ppm What is the best route of treatment? Also, average ambient air temperature in our area is 100 degrees during summer (inland SoCal). Thanks!
 Reply

InyoPools Product Specialist  Posted: 9/10/2021 

What is your CYA level? Is your water hazy or turning green?
 Reply

 Posted: 8/17/2021 

I don,t see any chlorine granueles for sale ??
 Reply

InyoPools Product Specialist  Posted: 8/18/2021 

There is a chlorine shortage due to damage to a major chlorine manufacturer's factory. A lot of distributors are out of tablets and stabilized forms of chlorine.
 Reply

 Posted: 8/1/2021 

FC = 12 ppm (too high) CC = 1.9 ppm TC = 13.9 ppm pH = 7.7 Cyanuric Acid = 68 ppm I need to bring free chlorine to 2-4ppm range, turned my salt cell all the way down, but free chlorine staying stubbornly high
 Reply

InyoPools Product Specialist  Posted: 8/3/2021 

The two routes I would use are draining a portion of your pool, refilling with fresh water, or using a chlorine neutralizer to wrangle in the chlorine level.
 Reply

 Posted: 8/3/2021 

Thank you
 Reply

InyoPools Product Specialist  Posted: 8/4/2021 

You're welcome! Glad to help.
 Reply

 Posted: 7/25/2021 

Cloudy water and can't raise free chlorine level. Have followed advice of local pool store but problem persists. Current test strip measurements are Total Hardness 250-300, Total Chlorine 3-5, Free Chlorine 0.5, pH 7.0, Total Alkalinity 100, Cyanuric Acid 0. What would you do if this were your pool? Thanks for your suggestions.
 Reply

InyoPools Product Specialist  Posted: 7/27/2021 

No wonder there isn't any chlorine in your pool. You have cloudy water, low pH, and no stabilizer. You HAVE to balance those other categories before chlorine can efficiently clean your water. How to Balance Your Pool WaterYour local pool company should have told you that throwing chlorine into a pool with those numbers is useless.
 Reply

 Posted: 7/17/2021 

just triple shocked our pool (cal hypo) last night hoping to raise free. total is 7 (from 3) free is 2 (from 1). so looks like chlorine lock. water is crystal clear, perfect. no signs of bacteria etc. run filter 12 hours a day. shocked it at dusk. est method to get thigs ack to normal? should i consider having water tested at local pool place? using the Remington copper solar shock device, would that e playing any role? pool near alot of trees\pants\dirt yet root cleaned 1-3 times a day, o visale dirt etc. thank you for your time.
 Reply

InyoPools Product Specialist  Posted: 8/4/2021 

What are the results of your latest water chemistry tests? In order to know what to fix, we have to know what is broken.
 Reply

 Posted: 7/8/2021 

My pool looks like milky water. Have had water tested several times and adjusted chemicals accordingly. Could it be chlorine lock? Last pool test showed normal everything. Most frustrating pool season to date!!!
 Reply

InyoPools Product Specialist  Posted: 7/14/2021 

If you show normal free chlorine levels then you don't have chlorine lock. You may want to check you aren't mixing two or more chemicals that are causing the cloudiness. See this guide - Bad Pool Chemical Combinations
 Reply