Why is the Chlorine Level at Zero in my Pool?

One of the biggest problems that you can run into with your swimming pool is that it stops holding chlorine. We refer to this in the industry is as the chlorine zeroing out. There are a number of reasons for this and it is more noticeable in the Spring and Summer when the weather starts heating up and the Sun’s UV Rays become stronger. I will touch on the most common reasons why your pool is not holding chlorine week to week.

One of the first things to look at is the pool’s filtration system in run time. This is often overlooked and is crucial to the pool holding chlorine. If the skimmer basket is clogged with debris or the pump basket is clogged the pool will not circulate well. The filter is just as crucial so if it is dirty or the cartridges or grids have tears in them, chances are the pool will have a zero-chlorine reading due to the organic contaminates not being filtered out properly. First, check your pool filtration and your skimmer and pump baskets and make sure they are running efficiently.

Next is the overall pool run time. If you are not running your pool long enough each week, chances are the chlorine level will be low each week. Make sure you are running your pool for at least one cycle each day which all the water in the pool is going into the filter and then back out into the pool. If you are running a Variable Speed pump know that on a lower speed the pump needs to run longer. If you were running your standard pump 8 hours a day it was running at a speed of 3450 RPM. So if you set your VS pump for 8 hours and run it at 1800 RPM that is less than 1/3 the actual run time as before. You may be running your VS pump way too short and at too low the RPM for it to be effective. The same is true for your standard speed pump. A good rule is to run your pump as much as you can until the pool starts holding chlorine.

If you have a Salt Water System (Salt Water Generator) and you do not run your pool long enough, the salt cell cannot produce enough chlorine each day to reach a sustainable level. You can also have the pool running long enough but you may have the salt output set too low. Set the salt output to 100% and then decrease it down if it is producing too much chlorine. Not the other way around.

Algae, Organic Debris, Phosphates, and Nitrates can all affect the chlorine in the pool also. If your pool has visible algae chances are the chlorine is being used up fast as it tries to fight the algae. Phosphates and the less common Nitrates can also quickly destroy the pool’s chlorine level. If the pool water is cloudy or there are visible algae, then there is a water quality issue. One way to solve this is to bring the chlorine level up to shock levels of 20 ppm and run the pool for 12-24 hours. You may also have too little Conditioner (CYA) in the pool. Test the CYA level and make sure it is at least at 30 ppm, otherwise, the Sun will burn off the chlorine within hours.

You could also be using bad chlorine. What I mean by this is chlorine that is expired, especially in the case of liquid chlorine. Over time the power of the chlorine gets weaker. Just like leaving a gallon of bleach in your laundry room and not using it for several months. When you do use it you will notice that it is very weak and does almost nothing when added to the washer. Liquid chlorine also will get weaker over time. So if you purchase it at your local Walmart or Home Depot don’t be surprised if it is ineffective because they failed to rotate their stock. Shock in a bag has a shelf life of about 2 years but that to can get weaker over time. Keep your chlorine fresh by purchasing what you need when you need it and try to buy it from a local pool store versus the big box retail or hardware stores.

These are a few reasons why your chlorine level may be zeroing out each week. There are more but I find these to be the most common.

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