What Does Chlorine Do for a Pool?
The main role of chlorine in a swimming pool is that of a sanitizer. It kills germs and other microorganisms that can cause illness. Chlorinating a pool also helps to get rid of algae and break down nonliving materials in the pool, such as oils and organic waste.
Use a test kit to check the chlorine level of your pool water every couple of days at a minimum. There are several levels to pay attention to:
- Free Chlorine: This is the chlorine that’s available to do the work of sanitizing. Free chlorine is necessary to rid the pool of microorganisms and other contaminants.
- Combined Chlorine: This chlorine has bonded to contaminants and isn’t very effective at further sanitizing. A strong presence of a chlorine smell indicates high levels of combined chlorine.
- Total Chlorine: As the name implies, this is the total level of free and combined chlorine.
The goal is to keep free chlorine levels between 1 and 3 parts per million (ppm) so it can effectively clean the water. You’ll need to replenish chlorine periodically to maintain this level since factors such as sunlight and contaminants in the pool reduce free chlorine over time. Breakpoint chlorination is the point where enough chlorine has been added to the pool to build up free chlorine.
Keep an eye on pH levels as well. pH levels between 7.2 and 7.8 balance water comfort, free chlorine effectiveness and the longevity of pool components.
Different Types of Chlorine
When shopping for pool chlorine, you’ll find that some chlorine is stabilized with cyanic acid (CYA) to maintain the effectiveness of the chlorine in sunlight. Nonstabilized chlorine doesn’t contain this additive, so if you use this type in an outdoor swimming pool, you’ll need to add stabilizer separately.
Two common types of chlorine for residential pools are liquid chlorine and trichlor.
Liquid Chlorine
Liquid chlorine is a nonstabilized form of chlorine. It’s known by several names, including sodium hypochlorite and liquid bleach, and is common in residential and commercial pool maintenance. Liquid chlorine is relatively inexpensive and easy to apply so that it treats the water evenly. However, it has a shorter shelf life than other forms of chlorine, and spills can be difficult to deal with.
Trichlor
Trichlor, or trichloro-s-triazinetrione, is stabilized to resist ultraviolet rays. You’ll often see it in tablet form — 3-inch tablets for pools 5,000 gallons or larger and 1-inch tablets for smaller pools — making it easy to use for your home pool. The high percentage of chlorine in trichlor makes this slow-dissolving sanitizer very effective, but you’ll need to be sure it’s distributed evenly so that the chlorine levels are the same throughout the pool.
Other chlorine products include:
- Calcium Hypochlorite (Cal Hypo): Often used as pool shock, this nonstabilized product is typically available as granules.
- Dichlor (Sodium Dichloro-S-Triazinetrione): Dichlor is a stabilized granular product often used in hot tubs and spas.
- Lithium Hypochlorite (Lithium Hypo): This stabilized granular product has a good shelf life but is one of the most expensive options.
- Saltwater-Generated Chlorine (SWG): Used to sanitize saltwater pools, SWG requires pool salt and a saltwater chlorinator, a device that breaks down the salt to create chlorine.
Never mix types of chlorine, store them together or allow a chlorine product to come in contact with a bucket, bag, box or dispenser used for another type of chlorine. Read, understand and follow all instructions for pool chemicals, including proper use, storage, safety precautions and protective equipment.
Overstabilization
When you use stabilized chlorine, be aware that the stabilizer CYA remains even when the chlorine burns off. As you add more stabilized chlorine, the CYA accumulates. Too much CYA leads to overstabilization, in which the stabilizer makes the chlorine less effective at cleaning the water. If you’re using stabilized chlorine — particularly trichlor and dichlor, which contain a high percentage of CYA — you might need to set up a schedule for diluting the water by draining and replacing some of it periodically.
Liquid Chlorine vs. Tablet Chlorine
Should you use liquid or tablet chlorine? Part of the decision comes down to which of the chlorine types described above you decide to use and how you want to add it to your pool.
Liquid chlorine has several benefits. It’s simple to measure and add to the water, and it’s relatively inexpensive. However, you’ll need to add a separate stabilizer product to make it last in outdoor pools.
Chlorine tablets are easy to manage and store, and spills are much easier to clean up. This form of chlorine is easy to apply, but it can be more difficult to ensure that chlorine levels throughout the pool are equal. While the presence of a stabilizer in some tablets is convenient for outdoor pools, some forms of chlorine can quickly lead to overstabilization if the pool isn’t managed properly.
You’ll also see chlorine granules, sometimes sold as chlorine powder. Granules have the benefit of dissolving quickly and being easy to spread. Depending on the type of chlorine, granules may be stabilized or nonstabilized.
How Much Chlorine to Add to Your Pool
If testing your swimming pool water shows your level of free chlorine is below 1 ppm, you’ll need to add chlorine to raise the level between 1 and 3 ppm. The product you use should provide information on maintaining free chlorine to the correct level, and there may be different instructions based on how you’re applying the chlorine — for example, using a feeder system versus dropping tablets in a skimmer. Since different products contain different amounts of chlorine, you should always follow the instructions specific to your product.
In determining how much chlorine you need to add to a pool, it’s helpful to have an idea of how much water the pool contains. If you don’t know your pool’s volume, estimating it is a fairly simple calculation. Multiply as follows:
- Rectangular pool volume in gallons = length (feet) x width (feet) x average water depth (feet) x 7.5
- Circular pool volume in gallons = diameter (feet) x diameter (feet) x average water depth (feet) x 5.9
How Often to Add Chlorine to Your Pool
Add chlorine to your pool every couple of days as part of regularly monitoring your pool water chemistry. You can also add it in the form of pool shock when the chlorine in your pool needs a fast boost. Add chlorine after sunset and make sure no swimmers are in the pool. Pay attention to the package instructions on how long you need to wait after adding the chlorine before allowing swimmers back in.
Chlorine Floaters and Other Ways to Add Chlorine
With all types of pool chlorine, start by testing the water to determine the level of free, combined and total chlorine. Use your test results and the product instructions to determine how much chlorine you need to add.
Here are some general methods for chlorinating a pool, but different types of chlorine have different application methods. Follow the instructions for your product.
- Using a pool chlorine floater is a common, easy way to apply tablet chlorine to a pool. Make sure the floater is designed for the type of chlorine you’re using and the tablet size, either 3 inches or 1 inch. Simply drop the required number of tablets in the floater and put the floater in the pool. You can open or close vents in the floater to speed up or slow down the release of the chlorine. To make sure the chlorine gets distributed evenly, you’ll need to ensure the floater doesn’t just drift to a corner or get stuck on a ladder and remain in one place.
- A feeder system or in-line chlorine dispenser tied into your pool pump and filter lines is an efficient way of using tablets to distribute chlorine evenly throughout the pool. Just turn the pump off, add the tablets, close the feeder and restart the pump. Feeder systems are specific to the type of tablets you’re using. Make sure you’re using the correct product with your feeder.
- You can add some chlorine tablets to your pool skimmer.
- You can pre-mix chlorine granules in a 5-gallon bucket that’s 3/4 full of pool water. Add the granules to the water; don’t pour water onto the granules. Pour the mixture around the edges of the pool.
- You can pour liquid chlorine around the edges of the pool. You can also pour it in front of the water return from the pump to distribute it throughout the pool.
Wear eye protection and chemical protection gloves when applying any type of chlorine.
When to Shock a Pool
Regularly adding chlorine to a pool helps maintain the proper level of free chlorine, but it may not generate enough free chlorine or increase the free chlorine levels quickly enough. Adding pool shock, also known as shocking a pool or hyperchlorination, helps to get rid of combined chlorine and generate free chlorine.
In general, you should shock your pool when you open it and close it. Throughout the season, you can shock it weekly or biweekly, but other factors may require more frequent shocking:
- You’ve had lots of people in the pool.
- There’s been a lot of rain.
- The weather has been very sunny.
- Temperatures have been high for several days.
- You see items like leaves, pollen, etc., in the water.
- You notice a strong chlorine odor.
- You notice large amounts of algae in the pool.
As with any pool chemical, read and follow all the instructions when using pool shock.