How to Fix Chlorine Demand in a Pool (3 Easy Ways)



Do you have a problem with chlorine demand in your pool?

We’re going to cover what chlorine demand really is, why it can sometimes cause issues with your pool’s ability to sanitize, and how to fix high chlorine demand quickly and easily.

Quick answer: Chlorine demand is how much chlorine a pool needs to keep the water clean. In general, the dirtier the water, the higher the demand. The best way to lower chlorine demand is to prevent unnecessary contamination.

What is Chlorine Demand?

Chlorine demand is the amount of chlorine your pool needs to maintain a stable level of free chlorine in your water, allowing it to stay properly sanitized.

In other words, it’s how much chlorine your pool loses over time. For example, if your pool loses 2 parts per million (ppm) of chlorine every day, your chlorine demand is 2 ppm per day.

Be aware, however, this number isn’t fixed. Your demand will change based on the conditions in and surrounding your pool.

Why is Chlorine Demand a Problem?

Chlorine demand isn’t inherently a problem.

All swimming pools consume chlorine at some rate, so there will always be a demand for it.

Typically, an outdoor pool will lose around 1-3 ppm of free chlorine per day, which is why that’s also the recommended free chlorine level for a pool.

A problem only arises when the chlorine demand exceeds the free chlorine available in the water. For example, if you’re only maintaining a level of 3 ppm where the daily demand is 5 ppm.

Without enough chlorine to meet the demand, your water can’t sustain a residual chlorine level. This leaves your pool unprotected against bacteria, algae, and other harmful microorganisms.

When that happens, the appearance of your water will suffer, eventually turning green when algae begin to take hold.

On top of that, when your chlorine level is too low, it’s unable to fully oxidize the used-up chlorine (chloramines) floating in your water, which soon results in a nasty chlorine-smelling pool.

What Causes High Chlorine Demand?

As you might expect, most of your free chlorine is consumed in the water doing what you expect it to do; fight off contaminants.

That means demand increases as the level of contamination increases, such as more leaves, dirt, and insects getting into your water, or things like sweat, saliva, and lotions being brought in by swimmers.

But that’s not the only way your pool loses chlorine.

In an outdoor pool, some of your chlorine is lost through sun exposure because ultraviolet breaks it down. This is particularly true for pools with little to no stabilizer or cyanuric acid in the water.

The chemical balance of your pool water will also dictate how effective your chlorine is against contaminants, particularly your pH level.

Finally, things like evaporation and splashout can contribute to chlorine loss, though this is often negligible or proportional to water loss.

How Do You Fix High Chlorine Demand?

You can fix high chlorine demand by tackling the root causes.

That means reducing contamination, keeping your water properly balanced, and ensuring your chlorine is protected against UV.

Let’s break it down.

1. Reduce Contamination

The easiest way to preserve your free chlorine level is to reduce its workload — or reduce the amount of “bad stuff” getting into your water.

Again, a lot of this contamination comes from us (sweat, saliva, lotions, etc.), so enforcing showering before entering the pool will make a notable difference in your chlorine consumption rate.

Other sources of contamination include rain, dirt, and insects, which can be heavily reduced by using a pool cover when the pool is not in use. You’ll also want to clean your pool filter as this will serve as the second line of defense against debris.

Finally, shocking your pool will “reset” your water by breaking down organic compounds and oxidizing chloramines. This creates a clean slate for your free chlorine to work with.

2. Correct Your pH Level

A balanced swimming pool will always work more efficiently, and that includes your free chlorine.

In particular, the higher the pH level of your water, the weaker your chlorine. This effect increases chlorine demand because you need more chlorine to achieve the same sanitizing power.

The goal is to keep your pH level low enough for your chlorine to work properly, but not so low that it leads to other issues.

The sweet spot for pH is between 7.2 and 7.8. This is plenty low enough for effective sanitation without causing discomfort to swimmers or making the water corrosive to your pool surfaces and equipment.

3. Monitor Your Stabilizer Level

Chlorine can be easily broken down by sunlight which makes it a huge contributor to chlorine loss in outdoor pools.

Fortunately, chlorine can be stabilized with cyanuric acid to protect it and significantly slow down this process. Some chlorine products even come stabilized off the shelf, including many types of chlorine tablets.

Whether you’re using stabilized or unstabilized chlorine, it’s still worth testing your water to ensure it has enough cyanuric acid.

The ideal level of cyanuric acid in your water is between 30 and 50 ppm for pools with mild sun exposure, and up to 80 ppm for pools with lots of sun exposure (especially for sustained periods).

Just be careful not to overdo it. Too much cyanuric acid can actually weaken your chlorine, which raises your chlorine demand instead of lowering it.

How Do You Prevent High Chlorine Demand?

Much like fixing high chlorine demand, the best way to prevent it is to directly counter the common causes.

The difference here is you can’t wait for your chlorine demand to skyrocket before taking action, you need to be proactive and incorporate the above steps into your regular maintenance schedule.

In this case, we recommend you:

  • Shock your pool once every 1-2 weeks.
  • Test and balance your pH level at least 2 times per week, and after high pool usage, storms, or any signs of algae growth.
  • Test and adjust your cyanuric acid level every month.
  • Always shower before entering the water.
  • Always cover the pool when it’s not in use.

Note: We’ve also put together a complete pool maintenance checklist with our recommended timeframes.

What If Your Chlorine Is Still Ineffective?

If your pool is maintaining a high enough free chlorine level but the water isn’t visibly improving, the problem isn’t chlorine demand.

It’s almost certainly a chlorine lock issue.

While the terms chlorine demand and chlorine lock are often used interchangeably, they’re NOT the same thing.

When your chlorine is “locked”, it means too much of it is bound to the cyanuric acid in your water, where it exists in a far weakened state. As mentioned earlier, this is what happens when your stabilizer levels get too high.

Unfortunately, chlorine lock is harder to fix than high chlorine demand. Too much cyanuric acid in the water can only be solved by draining and refilling your pool with fresh water, even if it’s just a partial drain.

The Bottom Line

Your pool’s chlorine demand will largely depend on the level of contamination in your pool, as well as your pH level and cyanuric acid level.

Therefore, the most effective way to counter a high chlorine demand is to reduce the number of contaminants entering your water, keep your pH level in check, and adjust your stabilizer level to protect your chlorine from the sun.

Categories: Pool Care, Pool Chemistry