Do I need to shock my pool?

Why do I have to shock my pool?  Isn’t adding chlorine enough?

When you walk into the pool room of a lesser-kept up hotel you are often greeted with the strong odor of chlorine.  Most people at this point think, “Wow, there is way too much chlorine in that pool,” and would be surprised to learn the pool most likely just needs to be shocked – with more chlorine.

The true culprit of that overwhelming smell is chloramines.  When chlorine gets added to a pool in small doses; through sticks, tablets, automatic feeders, or salt chlorine generators, it gets used up by many sources.  Some examples include algae, phosphates, plant organics or animal additives in the water.

It is essentially dead chlorine that is just clouding and stinking up your pool.

These are chlorine molecules that have already adhered themselves to bacteria and are no longer useful.

Shocking the water gets rid of these contaminants and their no longer usable chlorine counterparts by forcing oxygen into the water and gassing them off out of the pool and into the air.  This is why it is important to always keep your cover off while shocking the water.  If the pool is covered, the gas will hit the cover and fall back into the pool, causing an even larger chloramine level.

If you aren’t sure if you need to shock, the easiest way to find out is to test the water.  Most test strips or test kits show Free and Total Chlorine.  Total Chlorine is the total amount of chlorine in the water.  Free chlorine is the amount that is “free” and still available to kill bacteria and other contaminants.  Combined chlorine, or chloramines, are the difference between those two numbers.

Here are two common examples:

Total Chlorine: 2.1

Free Chlorine: 2.1

Combined Chlorine: 0


Total Chlorine: 1.5

Free Chlorine: .2

Combined Chlorine: 1.3

In the first example, you would not need to shock the water, as it is all free to continue killing bacteria.  In the second example, though, the majority of the chlorine in the pool are chloramines – useless.  You should shock the water to force these out of the water.

As a general rule, you would want to shock the water any time the combined chlorine, or chloramines, are above .5

This generally happens about once a week, giving us the once-a-week shock standard.  Most shocks are chlorine based, just a high dosage of chlorine combined many times with oxygen to help oxidation (gassing off).  The reason for chlorine based shocks are because:

  1. They work well.
  2. Your free chlorine is usually very low when shocking, so it boosts your free chlorine level after shocking. This results in fewer tablets or maintenance chlorine needed to replenish your levels.

Here is one more example that is less common:

Total Chlorine: 7

Free Chlorine: 2.5

Combined Chlorine: 4.5

This is less common but can happen.  Often your pool will be very cloudy with this high of combined chloramines.  Be careful shocking in this situation; if you have shocked less than 24 hours ago sometimes you will get very high levels of chlorine while it is still working to burn off the combined chlorine.  If you have shocked with a non-chlorine shock, this can often skew your readings for as many as five days.  Give it enough time to go down on its own if it has exceeded those times.

If you do need to shock, the best shock in this situation is a non-chlorine shock.  It will break down as many chloramines as it can without raising your free chlorine level, which is already high enough. In addition, high combined chlorine requires exponentially more chlorine shock, and if you do not get enough in all at once, it will just make the combined level larger. Read: Potential Money Pit  Non-chlorine shock will break down what it can and if you do not use enough it will not exacerbate the combined levels.

We see this situation most often on pools with automatic covers.  Pool auto covers work great for retaining heat and preventing contaminants like leaves, bugs, pollen, etc from entering the pool but it is important to remember they also prevent anything from leaving the pool water, including chloramines.  We advise leaving your automatic pool cover off at least an hour a day to allow the pool to breathe.  It is especially important to open the pool after adding chemicals.

One last, extreme example

Total Chlorine: 9

Free Chlorine: 0

Combined Chlorine: 9

In this example, there is an extreme level of chloramines and no free chlorine.  If your test results look like this, you may have a chlorine demand or high phosphates interfering with your ability to keep a chlorine reading.  Check with your local pool company to find the best solution for your individual case.

What shock should I use?


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