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


(41 to 80 of 132)

 Posted: 7/2/2021 

Our pool is crystal clear but we can't get the chlorine levels up after shocking or running the chlorinator in the max setting. I do see small spots of green algae but not significant. We have a DE filter and the pool is 22 year old, 30,000 gallon, gunnite pool. I've just cleaned the filter and re-shocked the pool with 4 bags of super-shock (chlorine) and still no luck. Our water test results are: Free Chlorine=.84 ppm, Total Chlorine=.84 ppm, pH=7.3, Hardness=306ppm, Alkalinity=80 ppm, Cyanuric Acid=92 ppm, Phosphate=1127 ppb. We are treating for Phosphates, which were at 1385 ppb.

InyoPools Product Specialist  Posted: 8/3/2021 

Your cyanuric acid is way too high. If you have a traditional chlorine pool, it should be 30-60 ppm. CYA can hinder chlorine if it is out of balance. You need to drain a portion of your water to get it in to check. Shocking the pool will only go so far if the other water chemistry is out of whack.

 Posted: 6/16/2021 

Pool was cloudy had water rested ,chlorine was high so I haven't put any chemicals in for a week let filter running steady and chlorine is still high but water is crystal clear

InyoPools Product Specialist  Posted: 6/17/2021 

If the water is clear and the chlorine is high, I would continue to wait for it to drop. Chlorine gets used up, usually rather quickly this time of year. What are your chlorine levels (free and combined chlorine)?

 Posted: 5/6/2021 

Hello, I think I have chlorine lock as well. My total chlorine is 3 ppm and my free chlorine is reading 0 ppm. The CYA is 100ppm. The pool was in the exact same condition last fall when we closed it. We drained half of the water and the pool was full when we opened it this spring. We back washed and drained a third of the water. We then refilled it. Shocked it. We find ourselves in the exact same position. The water is cloudy, But it has only been a few days since we uncovered it. Any advice? It is a small fiberglass pool holding 8000 gallons.

InyoPools Product Specialist  Posted: 5/13/2021 

I would shock it a time or two more and also use a heavy-duty flocculant to clear up that cloudiness. You may also want to try a fun little product like Pool First Aid which is a good catch-all product for these situations.

Anonymous  Posted: 8/25/2020 

I have a 4,000 gal vinyl pool. Something weird is going on. Cya numbers dropped rapidly over the past two week. Had been up around 100 and they dropped to around 60. Brought the stabilizer back to around eighty and all the other numbers are fine but the pool week not seen to hold chlorine. I literally put in over a gallon of chlorine in to this little pool yesterday and I'm still getting a reading of zero or 0.5 on chlorine. Checked the numbers at the store too in case my test strips were bad, but they're not. I'm stumped. Any ideas? I'm wondering if chlorine lock would tend to give a reading of no chlorine or if you know of something that would drop cya numbers rapidly. Thanks!

InyoPools Product Specialist  Posted: 8/27/2020 

What are your Free Chlorine and Combined Chlorine numbers? Is your water clean, hazy, or milky looking?

 Posted: 8/12/2020 

I recently had an issue with green water, so I decided to shock the pool with liquid chlorine as per instructions for 19k gallon pool. The free & total chlorine came up to 8 over night & within 24 hours the pool was crystal clear. 3 weeks later the pool is still clear, but the free chlorine is 3 & total still 8 & I have added no additional chlorine since the shock. I live in Bali, Indonesia, sun shines every day & the pool gets the sun for at least 8 hours a day. What is my problem

InyoPools Product Specialist  Posted: 9/1/2020 

I suggest using some non-chlor shock to bring down your chloramines. Another option is a liquid shock. But your combined chlorine is so high because all that chlorine you added is getting used up.

 Posted: 8/6/2020 

Free Chlorine, 0.07, Total Chlorine, 0.46, ph 7.1, Cyanuric Acid 141, Phosphates 3152 The local pool place recommended draining about two feet of water out the pool (one foot at a time, refill and recheck). I have yellow algae due to low chlorine but I'm in chlorine lock looks like.

InyoPools Product Specialist  Posted: 9/3/2020 

Your CYA levels are way too high. The CYA level should be 30-60 for a non-salt pool, and 60-80 for salt pools. Levels that high inhibit chlorine from sanitizing your water. The only way to get that down is to drain a portion of your water and refill with fresh.First, get the CYA down, then add shock, and maintain a healthy chlorine level.

 Posted: 8/6/2020 

we are struggling with CYA levels and chlorine lock. we have a 15 x 48 intex pool about 4000 gallons. Water was clear up until a week ago after some heavy rains. We use strips regularly to test levels and its been fine up until now. Pool store test said CYA was high. to partially drain, refill, partially drain, refill and add some product they sold us to help. Have done the partial drain and refill a few times now. We were told to use stabilizer when we set the pool up so my husband was adding it weekly, not realizing that the chlorine tablets already had stabilizer in it. So we probably built up a high level. We've had heavy rain the past day with the latest tropical storm, and will have more rain today and tomorrow. Test strip shows free chlorine is almost none. Do we now add stabilizer free chlorine? More shock?

InyoPools Product Specialist  Posted: 9/3/2020 

To be able to tell you the course of action, I would need to know the results of the water tests. When it comes to water chemistry, the test results do most of the work for you. Once you know what is low and high, and by how much then you can begin the fun stuff.

 Posted: 8/3/2020 

I have an algae (think green) issue i can't get under control. Last test results were 5.97 Free Chlorine and 5.97 total chlorine (just shocked with cal-hypo), 7.7 PH, CYA 97, calcium hardness 165. I quit using 3" chlorine tabs a few weeks to try and get CYA to start coming down thinking that was issue. But after each shock the algae comes back, even next day sometimes. Feel like I have chlorine lock. Any advice?

 Posted: 2/17/2021 

First importantly? Make sure you use a stiff brush and completely brush the sides and bottom of the pool thoroughly to actually kill the algae. It's a common problem people have with Alge issues. The number one cause is not brushing the pool algae off and then super chlorinate or add algeacide chemicals. It comes back. Also test for phosphates. Phosphates are the food for Alge. It's a bit costly and runs about 70 bucks us dollars for a gallon of phosphate remover. Keeping phosphates down and the cya levels in check will stop the lock and alge growth.

InyoPools Product Specialist  Posted: 9/3/2020 

What was the chlorine reading before you shocked the pool? Taking a chlorine reading right after adding shock can eschew numbers to misleading conclusions. Also, that CYA level is still too high.

 Posted: 8/1/2020 

free chlorine .78 total chlorine 1.18 combined 0.4 PH 7.4 phophates 693 can't get rid of the algae? Any advise?

InyoPools Product Specialist  Posted: 8/21/2020 

Your pool's cyanuric acid (CYA) levels are not listed. Proper CYA levels are essential to the sanitizing efficiency of the chlorine in your pool.What are your CYA levels?

 Posted: 7/29/2020 

I have been fighting a chlorine block for 2 weeks this has cost me to have cloudy water. I've been to the pool store twice. First time told me to add 16oz of algaecide then one gallon chlorine weight 12 hours add another gallon. The water is still cloudy. Went back the second time told me to add pound of pH up and the water is still cloudy. how do I get my water back to crystal clear so I can see the bottom of my pool?

InyoPools Product Specialist  Posted: 9/9/2020 

What are the results of your latest water chemistry test? That info is what we need to determine what steps are needed to correct your issue.

 Posted: 7/20/2020 

i have a 3000 gallon above ground pool with 14 on chlorine and i do not know what to keep chlorine low

InyoPools Product Specialist  Posted: 9/9/2020 

When you say the chlorine is 14, is that the Free Chlorine, Combined Chlorine, or the Total Chlorine?First, let’s make sure you understand the differences between free chlorine, combined chlorine, and total chlorine and how you measure each.Free chlorine (FC) shows the level of disinfecting chlorine available to sanitize your pool. FC is the part of the chlorinated water that hasn’t interacted with contaminants, yet.Once chlorine interacts with compounds such as urine, sweat, bacteria, etc they form chloramines. Chloramines are what we refer to as combined chlorine (CC). The CC levels are chlorine that no longer has disinfecting power. Essentially, it is useless.Total chlorine (TC) is simply your FC plus your CC. Your TC should not be less than the FC level.

 Posted: 5/30/2020 

I would like to use the chlorine break point method, but I have no free chlorine or available chlorine. If I test it right after I add chlorine, there is a little, but if I test 30 minutes later, there is none at all.

InyoPools Product Specialist  Posted: 10/23/2020 

What are the results of your most recent water chemistry test?

 Posted: 4/7/2020 

CYA is at 187 with free chlorine at 0.69 and total chlorine at 1. PH at 7, calcium hardness at 469 and phosphates at 343. Told to drain and refill to bring CYA down but I am also seeing yellow stains on walls and black spotting on floors. Will draining help? Or should I be doing smth else? Thanks

InyoPools Product Specialist  Posted: 4/8/2020 

The staining is likely black and mustard algae due to no chlorine in the water. You know the CYA is too high and the guide covers why that is bad. Lower the CYA to a reasonable level, then add the chlorine so you can begin the cleaning process. Unfortunately, there really isn't a workaround to this issue; balance your water chemistry, add chlorine, and brush your walls to get rid of the stain and algae.

 Posted: 3/25/2020 

28K GAL POOL. Free Chlorine .43 PPM, Total Chlorine 1.33 PPM, Combined Chlorine .9 PPM. PH, Hardness, Alkalinity, Cyanuric Acid, Cooper Iron, PHosphate are all good. My pool stays cloudy. How to fix?

InyoPools Product Specialist  Posted: 3/31/2020 

First, it doesn't seem like you have very much chlorine in the pool. Have you been treating your pool for an algae bloom? If so, have you added algaecide that contains copper? If so, that could be a cause of the milky water.Second, are you just now opening the pool and have to work to get it clear, or has the pool been blue and only recently turned cloudy? Have you added any clarifiers, flocculants enzyme cleaners to work on clearing the cloudiness?

 Posted: 3/20/2020 

So, my free chlorine is 0.83 and my total chlorine is 6.35. Stabilizer is at 36 so it’s not high. How do I fix this?

InyoPools Product Specialist  Posted: 3/23/2020 

Add cal-hypo shock or non-stabilised chlorine to raise the free chlorine levels

Anonymous  Posted: 6/27/2019 

I think its easier to backfill the pool and a lot cheaper

 Posted: 5/29/2019 

Is it ok to use town water to refill your pool when it is chlorine locked or do I need fresh non-processed water like from a well or lake etc. I wasn't sure if draining and refilling with town (chlorinated) water would fix the problem or make it worse.

Anonymous  Posted: 6/5/2019 

I just solved the mystery of the chlorine lock in my pool. After 9 days of intensive care and chemical balancing, shocking, and cleaningI was ready to give up. What i did to solve this problem is that i shocked my 10000 gallons with 4 pounds last night, then after 5 hours turned off the pump 12 hours until today after noon no result yet but that i guess was part of the solution. Then today aftrr noon i filled a dry bucket with 6 pounds of shock and dumbed them all togather in the pool instay in the deepest side of ghe pool at the same spot. and right away started brushedthe pool ground and

InyoPools Product Specialist  Posted: 5/30/2019 

It is ok to use city water to refill your pool.

 Posted: 5/22/2019 

How much water would you recommend draining if you are in a Chlorine lock? Our pool is 15,600 gallons?

InyoPools Product Specialist  Posted: 5/23/2019 

Drain a third and refill with fresh water. As step two covers, there isn't a set amount; but if you want to put the odds in your favor, draining more will be better than less.

 Posted: 5/21/2019 

My pH is 7.4, my stabilizer is low, my free chlorine is .5 and total is!!

Anonymous  Posted: 5/23/2019 

Yes, I have added 22 gallons of chlorine with no luck

 Posted: 5/24/2019 

If 22 gallons of chlorine is not budging it, then you need to drain a third or a little more of your water then refill with freshwater. Your pool is to the point where you just have to press the reset button.