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.

Step by Step


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


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


(81 to 120 of 132)

InyoPools Product Specialist  Posted: 5/23/2019 

Have you attempted any of the steps listed in the guide?

 Posted: 5/21/2019 

So...we replaced our pool sand with Zeo sand and also replaced our Hayward multi port and laterals. We cannot get our water clear this year!!! It's a hazy light green. You can only see the bottom on the shallow end. We have vacuumed, brushed, backwashed and still don't have clear water. I'm so frustrated!

InyoPools Product Specialist  Posted: 5/21/2019 

Filtering alone is not going to clear algae from your pool; it requires active chlorine. The hazy green is likely a mix of alive and dead algae. I see your list of things you have changed, but have you completed the steps in this guide? You may also want to review this guide: How to Clean a Green Pool?

Anonymous  Posted: 7/14/2019 

I used a clarifier which is the first time I ever used one and it worked.

 Posted: 5/18/2019 

I am struggling with my pool this year. Pool is crystal clear but Free and Total Chlorine is at 0. CH is 250, CYA is 75, TA is at 90, PH is 7.4, Copper and Iron is 0, TDS is 500 and Pho is 0. Looking for any advise. I have spent almost $400 so far and cannot get out of this lock. Pool is a 14 x 28 cresent shape with liner that is approx 15,000. Am i stuck with nothing to do but drain ans start over?

InyoPools Product Specialist  Posted: 5/20/2019 

Have you tried everything mentioned in this guide?

 Posted: 5/17/2019 

Pool is crystal clear but keeps showing 0 free chlorine. Shocking over and over and see algae dying and falling to bottom. FC and TC readings go up about an hour after shocking but flatten back to 0 several hours later. PH is good, stabilizer levels are good. Do I keep shocking? Never had this issue before.

 Posted: 6/18/2019 

Did you ever figure this out? Same thing is happening with my pool. It is clear that showing no Coreen and I have lots in it

 Posted: 5/18/2019 

I have the exact same problem..I have went through 26 bags of shock with no chlorine readings..I've tested with 4 different kits also

InyoPools Product Specialist  Posted: 5/17/2019 

Hello Pete - What is your stabilizer level? Is all the algae dead?

 Posted: 5/17/2019 

Stsbilizer is where it needs to be. Dnt have the ppm handy but it's on the low end of the "good range" I put another bag of TKO in this morningm and more algae was floating up. Water is still clear and that's the perplexing part. Thx

InyoPools Product Specialist  Posted: 5/17/2019 

I'd recommend adding more shock until you show a free chlorine level. Start with doubling what you normally put in.

 Posted: 5/14/2019 

I've been locked before and drained a little more than a third of my pool water. I then ran into hardness of the water issues due to the fact that I have a well. Anyway, I'm not totaly locked at present but the pool is becoming more green by the day and the Ph remains high at 7.8 , the total chlorine 7.8 is high and the free chlorine 2.8 is low. I'm considering trying the second solution and adding 6 pound of the Non chlorine oxidizer. What do you think?

InyoPools Product Specialist  Posted: 5/14/2019 

The best way is to drain and add fresh water but non-chlorine shock has worked for a number of pool owners. The best way to prevent this lock in the future is to keep the stabilizer level under 80 ppm.

 Posted: 5/12/2019 

I have put so Many chemicals in this pool, and my total chlorine and free chlorine don’t show on the test strips, the pool hasn’t improved and of course is green, is this a sign of chlorine lock?

InyoPools Product Specialist  Posted: 5/13/2019 

Yes, those are exactly the symptoms brought up in the guide.

Anonymous  Posted: 5/7/2019 

Hello, I have added chlorine powder which raised the total chlorine. The free chlorine is stuck at zero. PH and hardness are perfect. I haven't added any stabilizer. What gives?

InyoPools Product Specialist  Posted: 5/7/2019 

What is your stabilizer level? What is your alkalinity level? Is your pool water clear?

 Posted: 12/19/2018 

Good day. I have an issue with my pool. (I do not know how many liters or Gallons the pool is) The PH is Balanced, The Alkalinity is balanced, the Acid is balanced, the amount of Back washing is balanced but the Chlorine is non existent. I have shocked the pool,I have added a de flogger, I have vacuumed the pool on waste, I have back washed the pool every second day now and filled up with water and I have added 3kg of chlorine already but nothing comes up on my tester stating that there is chlorine in the pool. We will be replacing the Sand this weekend to see if this will sort out the issue, but the pool is still a misty green blue colour. I cannot keep putting in chlorine into the pool. I have also bought a Stabiliser to see if that would help but I do not just want to put that in the pool and do more harm than good. Please can you help me?

InyoPools Product Specialist  Posted: 12/19/2018 

Hello Surika - To know how much chlorine to put in first we would need to know the pool size in gallons. There are online pool volume calculators that can help with this part. Second, we would need to know the actual number values of your water chemistry; for example, CYA = 80, pH = 7.4, Alkalinity = 100.Also, if your water is bluish-green, then it sounds like you have the beginnings of an algae bloom. Your chlorine is likely being exhausted from trying to combat the algae problem. You need to maintain shock levels in your pool until the green tint goes away. Read: How to Clean a Green Pool?. But again, to know the necessary shock levels for your pool, you need to know the pool size and water chemistry values.

Anonymous  Posted: 12/20/2018 

Thanks for the reply. I Vacuumed the swimming pool last night to get rid of the dust at the bottom, drained quite a bit of water and refilled (Will fill up to full capacity this afternoon). The swimming pool looks much better this morning. We will also replace the sand this weekend and I think then we will be ready to go. What I can tell you is that the pool is 1.3 meters deep, 6 meters in length and 4 meters in width. I will check if the pool goes green and add a Algae remover. Thanks again

 Posted: 5/17/2018 

Hello Blinded - The method of adding more chlorine to break chlorine lock sounds weird but it does work. There is no easy method but draining and refilling is the route that I usually recommend.

 Posted: 5/15/2018 

I am just jammed up on this Chlorine issue. I have a 62,500 gallon pool. ph is 7.5 free Chlorine is .9 and total is 9.8 Hardness is 342 Alk is 241 They told me to put 68 pounds of Calcium Hydrochlorite in. I am just baffled. They keep saying the pool will break. So why put more chlorine in. I double shocked it. Going to hope something changes. I need devine help. Blinded

InyoPools Product Specialist  Posted: 8/29/2017 

Jimmymag - It sounds like you are at the point where you have to replace 18" to 24" of your water. If your nitrates are high that could be causing what appears to be chlorine lock. See "Anonymous" posted on 5/31 for a similar problem and solution.

 Posted: 8/27/2017 

I have a problem similar to the one that Ray posted on 8/23/17. I have 26,500 gal. plaster lined pool. I have been using liquid sodium hypochlorite (10%) which I had to add a gallon every couple of days to keep the level above 2ppm. The ph gradually increased to 7.8. I added some muriatic acid to bring the ph down a little. After doing this, a problem developed with the chlorine. The FC level dropped to zero. To shock, I added 2 gallons and checked the FC after a couple of hours and it was about a three so, I added 2 more gallons late in the day Over night the FC reading was zero. I went to pool supply to have water tested. They said the the cyuratic acid level had dropped to zero. They recommend using 5 lb. of di-chlor to superchlorinate. I did this and the previously clear water turned cloudy like Ray’s did. I ran the pump for about 16 hours but the water was the same. I added some old clarifier that I had and that cleared the water. I superchlorinated with the liquid and again the FC dropped to zero in 12 hours (overnight) . Pool supply recommend something they call “Purge” . Added that and ran pump for 24 hrs and thoroughly cleaned filter. I had water tested and ph still 7.4 and cy acid and all other tested good. I superchlorinated with liquid and again overnight the FC was zero and the water is sparkling clear with no signs of algae. Pool supply retested and all readings were good except that phosphates were 300. They had no answer to the chlorine problem. I shocked again with the same result. Back to pool supply and they ran an additional test for nitrates. Nitrates were high and they said there is no chemical treatment for nitrates and that replacing some of the water was the only way to lower nitrates. There is no chlorine odor My question for you is… could the nitrates be causing the chlorine problem? Thanks, Jim

InyoPools Product Specialist  Posted: 8/25/2017 

Ray - What is your stabilizer level. If it's 0-20, you will have to add more to protect the chlorine from being burned away by the sun. If it's at the 100 level, you may have too much stabilizer in the pool which locks up the chlorine and gives you a 0 reading. The only way to reduce stabilizer is by draining a foot or so of the pool's water.

 Posted: 8/23/2017 

I have a 33,000 gallon in ground plaster pool. I actually have a 2 part question. My first question is regarding chlorine. For some reason my pool zeros out on chlorine weekly. I am not sure why. All my test readings always come back fine except the chlorine. Any knowledge on why that is? My 2nd question is regarding cloudy water. My pool was fine a week ago and then out of nowhere my pool looks like a milky white. I can see the bottom in the shallow end of the pool, but cannot see the bottom in the deep end. I tried non-chlorine shock, water clarifier and ran the filter for 24 hours during this process. Nothing changed. Any other ideas would be great. Thank you very much.

InyoPools Product Specialist  Posted: 8/15/2017 

jjcresci - I'm not sure I have enough information to know what your problem is. If you have drained a significant amount of your pool water, like 1/3, you shouldn't have chlorine lock - if you even did. Your chemical numbers like ok except for your FC. How long have you had no chlorine in your pool? If it's more than a couple of days, your pool should have become cloudy by now. Did your chemical readings come from a local pool store? If not, I would have them check your FC. The function of non-chlorine shock is to oxidize chlorine. If you have no FC, I'd question its use. If your pool is starting to cloud over, I'd add a couple of bags of regular shock to see if that brings your FC up and clears your pool.

 Posted: 8/13/2017 

my pool was fine until my timer went.had to be off 4-5 days .I replaced timer. My pool was cloudy. So i got it running within 48 hrs it was clear. now pool has 0 fc.
about 10,000 gallons
ph 7.6
Alk 130
stabilizer 80 always been 80
calcium 200
this is a vinyl liner pool that is crystal clear ,but 0 fc will not hold FC and no alge at all
any ideas i did drain water and i'm thinking of useing non chlorine shock.


InyoPools Product Specialist  Posted: 7/25/2017 

Jennhurley59 - Not sure what is going on here. "Chlorine lock" means that the chlorine in your pool is ineffective - cannot eliminate algae. Even if you were able to break the chlorine lock without adding chlorine, I don't know how you avoided growing algae without adding chlorine for two months. If you are now seeing algae growth, I would shock the pool and begin adding regular levels of chlorine plus add whatever chemicals you need to balance the pool.

 Posted: 7/24/2017 

My pool had "chlorine lock", cya levels through the roof we drained and refilled for weeks. Along with adding no chemicals, no algae grew for over 2 months. Now pool is finally turning green, is this a good thing?

InyoPools Product Specialist  Posted: 7/13/2017 

Lifetimelinda - I'd check your CYA reading again. Adding more CYA to your pool if it has a 100+ reading is going the wrong way. Too much CYA can lead to chemical lock. I'd lean more towards draining 20% or your pool (generally 12' to 18") then adding non-chlorine shock (step 2). See "Anonymous" comment 4 comments down.

 Posted: 7/11/2017 

The pool store is saying I have a chemical lock. My readings were chlorine 3, ph below 7, phosphate 1000 and cynaric acid 100++. They suggested I drain 3/4 of my pool and start over. Went for second opinion and they said I had no stabilizer,start with getting it up, then work on alkalinity and then ph. We are pool newbies. So confused so starting with the stabilizer. We have small pool, just 12000 gal. Any thoughts.

InyoPools Product Specialist  Posted: 6/13/2017 

hamnut - It sounds like you have dumped a lot of chlorine in the pool already without much success. I would try draining a foot to 18" of your pool water, then adding shock like "Anonymous" did as stated in the third comment down.

 Posted: 6/13/2017 

I was told I have chlorine lock over the weekend by my pool store. here are the numbers from their testing:
PH: 7.8
Alkalinity: 19
Free Chlorine: 0
Total Chlorine: 6.6
Hardness: 281
CYA: 150

They told me to add 18 pounds of shock. I added 20#s Sunday night and left the pump running all night. I checked the pool water last night again the Free Chlorine is 0 and the Total Chlorine is is about 7 (I use test strips but I do have a reagent test kit.)
PH looked about the same as did the Alkalinity. CYA was still above 100.

I have read about "Chlorine Lock" for the 2 hours and there is a wide variety of opinions. So I am about to add liquid Chlorine. I can get it at the pool store for about $5/gal and get Clorox Bleach at the local food store for about the same price. I am going to try 6 ga. at first. I think I want to stay away from the shock due to my hardness being high.

Does this sound like a good plan?


InyoPools Product Specialist  Posted: 6/2/2017 

Anonymous (chlorine block) - Thank you for your feedback! It's always good to get alternative approaches.

Anonymous  Posted: 5/31/2017 

Thank the Lord, we now have FC! After putting nearly 25 pounds of shock into our pool to no avail, Leslie's pool employee was the answer to our prayer. I told her that our pool was eating chlorine and she said we needed to check for nitrates. The water was positive. She told us to drain the pool about 18 inches to 2 ft. and fill it back up. We did and followed with a super shock. It held and was still 10 the next day!

 Posted: 5/22/2017 

I am draining my pool as I read these comments. Spent a lot of money on chemicals including chlorine free shock and was never able to get ph correct. Finally supply company guy checked my water and gave me the good news...chlorine block. Hope this effort solves my issues now need to determine proper amount of stabilizer when refill is complete. Thanks to all who have gone down this road and provided valued input on finding a solution. Have a great summer.

Anonymous  Posted: 8/15/2016 

ive never ever heard of stabilizer stopping chlorine from sanitizing, and i work at a pool store where our everyday job is to fix these types of problems, testing the water and adjusting the chemical balance and whatnot. do you end up accumulating a larger ppm of chlorine over time because of it? yes. The only thing weve ever found that really stops chlorine from sanitizing is phosphates and nitrates. before you try any of these wacky equations, get a test specifically for phosphates and nitrates. if its either of these, depending on the level, get a phosphate remover and it should take care of the problem. however, there is nothing on the market thats proven to remove nitrates. but we have experimented and found that a strong, commercial grade phosphate remover does help with the nitrates

 Posted: 8/13/2016 

Chlorine lock due to high stabilizer is a heated topic in the pool maintenance industry. I personally believe it is largely a myth and/or overstated. There have been real world studies done on residential pools measuring the amount of stabilizer vs the amount of bacteria and algae present. The studies show that stabilizer levels all the way up to 400 ppm had no impact on chlorine's ability to fight bacteria and algae.