Shocking a pool is part of the regular pool maintenance that needs to be carried out to keep the water quality in good condition. It is important to know when is it safe to go back in the pool after shocking.
In general, after shocking a pool, no-one should swim in the pool for a minimum of 6 hours and up to 24 hours. This time can vary depending on the type of shock used and the free chlorine level which you should test to make sure it is lower than 5ppm before anyone swims.
For this reason, I normally shock my pool in the evening and then leave the pool pump running overnight. That way the pool is normally safe to swim in by the next morning.
When to shock a pool
There are a number of different reasons and events that will make it necessary to shock a pool. These include:
- Start up or opening
When you first start up the pool or open it at the beginning of the season.
- After heavy rain
Rain can bring with it contaminants and perhaps also lower the pH level (as rain is often quite acidic).
- Heavy use
After the pool has received heavy use, perhaps after a children’s pool party or some similar event.
- Free Chlorine level low
If you test your pool water and the free chlorine level is zero or just above then shocking is a good idea to increase the free chlorine levels.
- Water temperature
If the temperature of the pool water increases due to a spell of very hot weather this can increase the growth rate of bacteria.
- End of season
Shocking when you close your pool in winter can make life easier when you reopen it in spring.
Types of shock
Different types of shock can make a difference to the time required before swimming so the first piece of advice advice would be to read the manufacturers instructions on the packaging. This should tell you how long after shocking a pool before swimming can recommence.
There are two main types of Chlorine shock (that is, shock containing Chlorine) which are Calcium Hypochlorite (often referred to as Cal-Hypo) and Dichloroisocyanuric acid (often referred to as Di-Chlor).
Cal-Hypo is the one you will see for sale most often, primarily I guess because it is not only the cheapest but strong too. It is very quick to dissolve (always in a bucket or container pre-filled with water before being added to the pool). A very popular Cal-Hypo shock available on Amazon is the In The Swim Chlorine Pool Shock which comes in 24 1 pound bags.
Di-Chlor is a granulated, stabilized chlorine. It is slower dissolving than Cal-Hypo and the stabilizer will raise the cyranuric acid level in your pool. In the Swim Sodium Di-Chlor Chlorine Shock is available in 24 1 pound bags on Amazon.
You can also buy a liquid chlorine shock which doesn’t require pre-mixing or dissolving.
ALWAYS READ THE INSTRUCTIONS ON THE PACKAGING!
Non-chlorine shocks are known as Oxidizing shock. They contain no chlorine but instead uses oxygen to to remove contaminants from the pool. With this type of shock you can usually swim in as little as 20 minutes after adding it to your pool. However, Oxidizing shock will not get rid of algae so you will need chlorine shock for that.
One of the most popular on Amazon is the In The Swim Chlorine-Free Oxidizing Pool Shock which comes in 24 handy 1 pound bags.
Testing the water after shocking
You can test the water using test strips, liquid test or an electronic water tester to establish when it is safe. The general consensus is that the free chlorine levels should be below 5 ppm and ideally below 3ppm before swimming resumes.
You should leave the chlorine shock to work for at least 4 hours before you first test the water.
You should keep the pool pump running after shocking.
Pool Maintenance Course
When I first bought my house with a pool I knew nothing about how to clean and maintain it. I was recommended Swim University’s Pool Care Handbook so I bought it and have never regretted it. It was probably the best money I spent that year as I have saved thousands by doing it myself.
What happens if you swim in a shocked pool?
Swimming in a pool that has just been chlorine shocked can be potentially dangerous (as opposed to oxidizing shock – see above).
Swimming before the levels described above are reached can potentially result in one or more of the following problems:
- stinging eyes
- skin irritations
- potentially worse conditions such as lung irritation.
If in doubt, don’t swim! Test the water and only go in when the levels are correct.
Can you over shock a pool?
Well in a way you can but in practice it generally just means that you will have to wait longer for the chlorine levels to reduce to a safe level before you can swim in the pool again. Perhaps up to 48 hours.
If you have severely over shocked and have a vinyl liner the strong chlorine solution may bleach it.
You can help reduce the chlorine levels by adding sodium thiosulfate granules to the pool. You can buy this on Amazon – Pool Dechlorinator Sodium Thiosulfate Pentahydrate 15 lbs by Cesco Solutions
So the lesson is, follow the instructions and ensure that you only shock to the level recommended.
As with adding any chemicals to a pool, always add the chemicals to water, NOT WATER TO CHEMICALS.
In other words, if you will be mixing the shock solution in a bucket always fill the bucket with water first and then sprinkle in the shock. Do not put the shock into the empty bucket and then fill with water.
How long should you wait when adding other chemicals?
Although some of the other chemicals you may add to your pool are relatively harmless, at least when compared with chlorine, it is generally not a good idea to swim immediately after adding them.
It is a good idea to wait at least 15-30 minutes for the chemicals to mix with the pool water so there are no “strong” areas for chemicals such as algaecide, muriatic acid, pH plus or minus or clarifier.
When adding calcium Chloride it is normally recommended to wait longer than 30 minutes, perhaps 2 to 4 hours. Follow the guidelines on the pack to be safe.
My top 3 pool cleaning tools
These are the pool cleaning tools I have found the most useful since I have had my pool.
Step and corner vacuum brush
This is a really useful tool for getting into the areas that a standard vacuum head simply cannot reach. Aquatix Pro Pool Step & Corner Vacuum Brush
Leaf rake net
If, like me, you get plenty of leaves at the bottom on your pool then a good leaf rake/net is a must. The Stargoods Pool Skimmer Net gets under the leaves easily.
Robotic pool cleaner
These are quite expensive and it was a number of years before I bit the bullet and bought one. I have never regretted it. The Dolphin Nautilus CC Plus is the most recommended pool cleaning robot on all of the pool forums. It not only cleans the bottom of the pool but also the sides and the waterline.