Shocking a pool – what is it, why and when to do it.
Despite how it sounds shocking a pool has nothing to do with electricity or with revealing something completely unexpected.
Shocking is the process of adding chemicals (usually chlorine) to your pool to:
- break apart chloramines, also known as combined chlorine
- quickly raise your chlorine level
- kill algae, bacteria or other harmful pathogens
Chloramines Bad, Free Chlorine Good
The most common reason for shocking your pool or spa is to deal with combined chlorine. There are three measurements of chlorine in pool or spa water: free chlorine, combined chlorine, and total chlorine.
Free chlorine is just that, free. Free to interact with other chemicals, algae, bacteria or the like. We like free chlorine because when it interacts with these potentially harmful things, it usually kills them and off-gasses them into the atmosphere. Once it has done that, it continues its carefree “lifestyle” moving on to look for the next thing with which it can interact.
Combined chlorine, on the other hand, is chlorine that is stuck in an unhealthy relationship, often with ammonia or nitrogen. Because it is bound up in this combined state, it is no longer free to interact with other things in the water and is no longer an effective sanitizer. The worst of it is our poor chlorine molecule is doomed to stay in this stinking dysfunctional relationship unless he gets some help from his buddies, free chlorine or some other oxidizer. I say stinking because usually if you encounter a pool with a strong chlorine smell, it is in fact combined chlorine you are smelling.
Total chlorine is simply the combination of free and combined chlorine.
Shocking then releases the combined chlorine and off-gasses the contaminants, increasing the amount of free chlorine in your pool or spa. The question of whether to use a chlorinated or non-chlorinated shock will depend on how much total chlorine you have in your pool or spa. If your total chlorine level is high, you will use a non-chlorine shock; if it is low, you will use a chlorinated shock.
As a rule, you will need to raise free chlorine to 10 times your combined chlorine to hit what is known as “break point.” Therefore, it is good to deal with combined chlorine while it is still small. The formula for hitting breakpoint can get a little complicated, so we suggest you talk to your pool professional.
Zero Chlorine Bad, 3-5ppm Good
The second most common reason for shocking your pool or spa is to raise the sanitizer level quickly.
Sometimes, after a party, for example, your chlorine level will just bottom out. When this happens, you want to quickly get your chlorine level back up to the recommended 3-5ppm (parts per million) before algae or bacteria can gain a foothold. If this is the case, you will want to use a chlorine shock.
Is That Algae I See?
The final reason you might want to shock is to temporarily raise your pool or spa’s chlorine level to deal with an algae bloom or another issue.
How often you need to shock your pool is dependent on many factors such as bather load and frequency, weather factors such as extended periods of rain or sun, and environmental factors like pollen, leaves or dust. All that said, a general rule of thumb for most pool owners is you should shock once a week, knowing, however, that unusual events might require more frequent shockings.
Nighty-night
The ideal time to shock your pool is in the evening after all swimming is complete. In the evening because the sun will not be boiling the chlorine out of your pool, and after everyone is done swimming because shocking is going to bring the chlorine level up to a level that may be irritating to skin and eyes.
Other times that may require an extra dose of shocking:
- After a party - the extra bather load of active swimmers will put additional strain on chlorine in the pool.
- After/during a period of hot sunny weather - Not only does the sun boil the chlorine out of the water but as the water temp increases, algae and bacteria grow at a faster pace. Also, because algae are a type of plant the extra sunlight causes faster algae growth
- After/during a period of excessive rain - Rain usually increases the pH of your pool. When your pH is out of balance, your chlorine does not work as well. Also, as the rain is falling through the air, it picks up contaminants such as pollen and pollutants and dumps them in your pool. Heavy rain may cause ground runoff to empty into your pool.
Shocking 10 Commandments
- Thou shalt shock if thy combined chlorine level is at .5 or higher.
- Thou shalt protect the sanctity of thine face and eyes by only adding shock up-wind so that the wind cannot return unto thine own face or eyes, the shock which can cause damage.
- Thou shalt never add water to shock…only shock to water.
- Thou shalt never add shock to a skimmer (it is BAD for your equipment) only wilt thou broadcast it over the surface of the water.
- Thou shalt quickly disperse and distribute thine shock by brushing and running thine pump for at least 20 minutes after the broadcasting thereof.
- Thou shalt not swim after shocking except that thou art following the wait-to-swim instructions for your particular shock. Neither you, nor your son or daughter, nor any sojourner in your household.
- Thou wilt only cover thy pool or spa when chlorine levels are below 5ppm. Elsewise thou wilt allow thy pool to off-gas
- Thou shalt limit thy shocking to pH levels between 7.2 to 7.4.
- Thou shalt only dispense of thine empty shock bag, after rinsing it in the pool and allowing it to dry.
- Thou shalt never store partial bags of shock…you will only ever use a complete bag.