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Chlorine Lock and Chlorine Demand: Here’s the Simple Cure

Your pool is a vampire.

No, not like the Smashing Pumpkins song but actually a chlorine vampire that can never quench its perpetual thirst. As a responsible pool owner, you test your chlorine levels twice weekly and keep adding more when needed… but it never seems to be enough.

It’s not your fault. You’re doing everything right. The need for more and more chlorine is just part of having a swimming pool. Sometimes, despite your best efforts, chlorine doesn’t even register on your test strips. There’s a reason for that: chlorine lock.

What is Chlorine Lock?

Well, sort of. “Chlorine lock” is a controversial subject in the pool industry and has been found to actually be a misnomer — a term that some pool companies will use interchangeably with the term “chlorine demand”. These are actually two different things. 

Chlorine lock is supposedly a condition that affects your pool when there is too much cyanuric acid present in the water. The idea is that the excess cyanuric acid, which is a chlorine stabilizer, causes the chlorine to be unusable, even if your chlorine levels are where they need to be.

The term chlorine “lock” is playing on the notion that an abundance of cyanuric acid is locking up your chlorine, holding it hostage, and making it unusable. Unuseable chlorine? Not so much. 

Have no fear. There is actually no real evidence for this situation and no proof that any amount of cyanuric acid in your pool water can make the present chlorine ineffective (or even less effective).

What’s really going on here is that you just don’t have enough available chlorine in your pool to register on a test.

What You Really Have is Chlorine Demand

So if you’re not getting readings on your chlorine test strips or liquid test kit, it probably means you have chlorine demand. All this means in a nutshell, is that your pool needs to slake its everlasting thirst for blood— I mean, chlorine.

True “Chlorine Lock”

In case you were wondering, there is actually a pool plight that is truly a situation of chlorine lock. In this case, cyanuric acid levels have nothing to do with it, but the phenomenon happens for quite a different reason.

The real culprits behind chlorine lock are those pesky chloramines. You remember, the byproduct of chlorine breaking down all of the nasty stuff in your pool like plant debris, urine, and (hopefully not) fecal matter? Well, in case you forgot…

As chlorine sanitizes your pool, it works to decompose all of these nitrogen-based waste products in your pool and the resulting byproducts are chloramines. Chloramines are those unpleasant compounds that cause eye and skin irritation for swimmers. They are also responsible for the chlorine-like smells near pools.

Since chloramines have no real or functional purpose as sanitizers whose mere existence reduces the levels of available chlorine in your pool, it stands to reason that they are the actual culprits of “chlorine lock”.

What Causes Chlorine Demand?

Despite chlorine demand being a fairly rare issue (as pool issues go) it can happen. When it does happen, it’s usually in the spring when you’re opening up your pool for the first time after a long winter of being closed up.

This situation can also crop up any time your pool has been sitting for long periods of time, untreated and uncirculated. If your pool has been exposed to the elements, it may have collected all sorts of debris like leaves, dirt, grass, dust, and other organic matter that will deplete your chlorine supply like crazy.

If you’ve recently done a test after a heavy downpour or monsoon and notice that your chlorine isn’t even registering on the test, there’s a good chance you have chlorine demand in your pool. 

Since the purpose of chlorine is to sanitize your pool water, any foreign particles, bacteria, or non-water-related materials are going to keep your chlorine hard at work, doing its job. All of these factors like organic debris, rainfall, and disuse can quickly devour what chlorine you do have in your pool, causing a low or invalid reading on a routine test.

The Simple Cure For Chlorine Demand

Unfortunately, despite the vampiric nature of your pool, garlic and a wooden stake aren’t going to cut it here. So, what’s the solution? 

You guessed it: add more chlorine! Give your pool what it wants.

The strongest and fastest-acting variety of chlorine is pool shock. Your best bet for getting your levels back up to normal quickly is going to be calcium hypochlorite pool shock. The rule of thumb is approximately 3 pounds of pool shock per 10,000 gallons of pool water. But always make sure to follow the manufacturer’s instructions when adding chemicals to your pool.

Also, make sure to test your pool water for pH and other chemical levels before adding the shock. We’re specifically looking for the levels of cyanuric acid here. The healthy range for this chlorine stabilizer is 30-80ppm (parts per million) so ensure those levels are where they need to be before adding the shock.

If debris, rain, and stagnation weren’t enough – you have to deal with the sun too. The UV rays from the sun will decimate chlorine almost as soon as you add it. This means it’s important to always shock your pool at night to actually give the stuff a fighting chance. See? Sun is bad. 

I told you your pool was a vampire.

Supply & Demand

Although you may have heard the terms interchangeably for years, they are in fact, two different things with the same quick fix.

Now that you know the difference between chlorine lock and chlorine demand you can rest assured you know the causes of any chlorine issues you and your pool may face. Either problem has an easy solution, which is simply to give your pool water more of what it wants: that sweet, sweet chlorine.

With renewed peace of mind and a steady supply of chlorine, you can keep your pool clean, sanitized, and satisfied all year long. 

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