How to Break Chlorine Lock

Chlorine is your pool’s immune system, without it, your pool would become ill, wretched and die. OK, that was a little overdramatic, but without chlorine, your pool would turn green, scummy, and become your very own backyard mosquito hatchery. And for this reason, we must keep our chlorine in check.

But what happens if your chlorine is “too” in check? By that, I mean too much cyanuric acid (CYA), and not enough chlorine.

What Is Chlorine Lock?

The phenomenon of “Chlorine Lock” occurs when a pool’s free chlorine (FC) is overpowered by its cyanuric acid (also known as CYA or Stabilizer). Though CYA is vital in stabilizing (hence the name) chlorine for sanitation, too much of it decreases the effectiveness of chlorine. The sudden drop in your chlorine’s ability to kill bacteria, allows bacteria and algae to take over your pool quickly. Chlorine Lock can put your pool in a sort of limbo, where there is not enough chlorine to sanitize your pool properly, but not too little where a carnival of mosquitoes and algae come to party.

There are some pool boffins, and chemical whizzes that say Chlorine Lock does not exist their reasoning is that “Chlorine Lock” is more of an is an umbrella term that overstates an issue.


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Why Does My Pool Need Cyanuric Acid?

Chlorine hates sunlight, because sunlight can burn off chlorine almost as fast as an algal bloom. Let’s look at it from a human perspective, what do you do to prevent sunburn? You use sunscreen. And that is where the cyanuric acid comes in, because CYA is chlorine’s sunscreen. The cyanuric acid shields chlorine from sunlight allowing it to seek and destroy any bacteria or virus in your pool. This relationship keeps everyone healthy.

Side note: If you want a really in-depth look at the relationship between chlorine and cyanuric acid, read our man Charlie’s article:  The Relationship Between Chlorine and Cyanuric Acid

But When Does a Healthy Amount of Stabilizer Become Too Much Stabilizer?

Traditional Chlorine

CYA LevelMinimum FCTarget FCShock FC
20*23 - 510
3024 - 612
4035 - 716
5046 - 820
6057 - 924
70*58 - 1028
80*69 - 1131
90*710 - 12 35
100*811 - 1339
*Outside of recommended level


CYA LevelMinimum FCTarget FCShock FC
*Outside of recommended level

Because CYA protects chlorine, if there is too much of it in your pool it blocks the chlorine from working. Unlike chlorine, CYA does not dissipate, the only way to lower your stabilizer levels is to drain and refill a portion of your pool water. Chlorine Lock generally happens in older pools that use dichlor or trichlor tabs. These tabs can contain up to 50-60 percent CYA. Eventually, this CYA builds up to levels that must be compensated for.

And thanks again to Trouble Free Pool (TFP) and their pool chemistry experts for creating this handy-dandy Free chlorine to CYA use chart. The first columns show your CYA level, the following columns list Minimum FC and Target FC level needed to maintain proper levels. And the last column denotes the level of FC needed to shock your pool.


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How Do You Fight Chlorine Lock?

An easy way to fight Chlorine Lock is to add more non-stabilized chlorine or cal-hypo to up your FC levels. This can be done by adding high percentage pool shock or bleach (yes, the kind you find at those big box stores). But not the splashless bleach, because additives within the formula foul your water.

For you skim readers out there, here is a quick and easy process to break Chlorine Lock:

1)  Test your water’s CYA Levels.

2) a) If CYA is low, use the TFP calculator chart to add the correct CYA dosage to pool.

b) If high, drain 10 percent of your water, then SLAM (Shock, Level and Maintain).

3)   Retest your water and repeat step 2 if necessary.

If you have another way on how to avoid or break chlorine lock, please leave a comment down below.

18 thoughts on “How to Break Chlorine Lock

  1. NST Tabs , Poolife are Awesome! No Cya and Only Go In the Skimmer, A Must Know, Only Skimmer.. Until Lonza sells the Exact Chlorinator To Usebwith Them.. Natural Chemistry can help cut down the CYA, Bio-Active did not work well at all for me.

    1. I cringe anytime I see tablets in a skimmer. Putting tablets in a skimmer will quickly destroy the pumps shaft seal leading to leaking and shortened motor life.

  2. Seem to be having the same issue with my chlorine but before I take action I want to ad that I did a stain removal last week. Water was crystal clear then over a couple days went to a bluish cloudy color and now I can’t get my chlorine to come back up. I’m getting 0 FAC, 0 TAC, Cya is 80, TA 110 and pH is 7.6.

    1. Your CYA is above the normal level, get that in check. Your chlorine is nonexistent, so that haze could be a algae problem that is blooming/dying out. Drain water to get CYA down, add pool shock and regular chlorine to kill any lingering nasty stuff, then use a clarifier to get it out of your pool.

  3. My CYA has been high all summer. I have drained some and refilled some for a few months. It has come down some but not at normal level yet. It has been exhausting trying to keep chlorine in the pool without using my chlorinator. Can you put a NST chlorine tablet in skimmer? The beginning of your article says NO but manufacturer of the NST chlorine tablet says to ONLY put tabs in skimmer. Can I use a floater instead?

    1. Yes, use a floater.

      If your CYA levels aren’t dropping, you may not be draining enough water. Draining, refilling and then draining again is not as effective as doing a longer initial drain.

  4. I have a CYA reading of somewhere around 100, I cannot get a much more accurate reading due to test strips only reading 0, 30-50, 100, 150, 240 and cannot find anybody local who can analyse my water. Also locally all are tabs and shock are trichlor or dichlor.
    I am assuming I have to drain off water to fix CYA issue but with trichlor tabs will it just go back up?

    1. First, I would get a different testing kit because the ranges on that kit are way too wide to be able t figure out what needs to be done. There are tons of options out there, the best-reviewed manufacturer of test kits are Taylor. And again, you can buy shock, chlorine, and other types of chemicals online as well. The trichlor will eventually bring up the stabilizer level but it won’t boost it to where it will be a problem right away. You may also use cal hypo shock to boost the chlorine levels.

      1. We have recently had an algae bloom
        Did the SLAM only to have it return within two days. My chlorine is reading very high and the cYA level is between 225-275. I think I have a chlorine lock, any suggestions.

        1. Becka, just a quick glance at the chart in the article should tell you your CYA level is like three times higher than it should be. You need to drain a large portion of your water and fill with fresh to try to get the CYA levels in check. With CYA levels that high you may as well not put any chlorine in there until you drain and fill.

  5. How much liquid chlorine or shock do I put in to break the lock after draining/refilling? I have a 10000 gal pool and have battled this lock all summer. I assume this has to be corrected before we close the pool for the summer?

    1. Most pool shock package instructions state you would need 1 pound of shock per one pound 10,000 gallons. How much chlorine you would need after draining and refilling is dependent on the water chemistry (just like any other time you are balancing chemicals.) First, test the water then added the necessary amount of chlorine. If you need a precise measurement, I suggest visiting the TFP PoolMath Calculator.

  6. I definitely have chlorine lock. CYA is well over 200. I thought I should drain the pool and re-fill with fresh water. Local pool store tells me not to do that and to open the pool as normal, then they’ll help me re-balance it. I feel like it would be a waste of time and money to open the pool as normal (paying to re-fill to normal level) just to turn around and drain it again. Thoughts?

    1. It sounds like you are leaving a big part of the background out of your question. How low is your water? If your pool is three-quarter empty, then filling it with fresh water could make the CYA balance out. If it is a third empty, then you need to drain more water to add more of the fresh.

      It could be a greedy local pool store trying to get you waste money on a futile “rebalance.” Or they could be on the right path to fixing the issue. It all depends on the pool water level.

      1. When we closed it, we drained it just below the slimmer and lights. Maybe a foot. It’s not even half empty. It’s an above ground 10,000 gallon pool for reference.

        Knowing this, you’d drain more before filling it for summer? I’m afraid draining just half will only bring the CYA level to 100. I really think it needs to be almost completely drained. Or am I on the wrong track?

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