Turnover of Pool Water - Further Reading

Turnover of Pool Water

You will see it said in the TFP Forums that turnover is a myth. What is meant by that shorthand saying is that there is no science behind the pool water needing any number of "turnovers" and that 100% water turnover is not achievable.

Trying to achieve a turnover goal gets pool owners to focus on the wrong metrics when selecting a pump and then running the pump faster then necessary if it is a variable speed pump (VS).

There are times, such as during the SLAM Process clearing algae, when you need maximum filtering and running your pump at high speeds 24/7 is recommended. For normal pool operation that is unnecessary and is further discussed in the Pool School Determine Pump Run Time article.

This is supported by the conclusions of the study that Florida Power & Light commissioned Florida Atlantic University to perform on Swimming Pool Circulation System Energy Efficiency Optimization Study in 1984.

You need to run your pump for a reason, and all we are saying is that "turnovers" are not a reason by itself.

Running the Pump to Achieve Turnovers is Unnecessary

If you have a SWCG, then you need to run the amount of time it takes to generate the amount of chlorine your pool needs. That is a real reason to run the pump. You might only have to run my pump 6 hours, because you have a large SWCG and a small pool, or you might have to run your pump for 18 hours, because you have a large pool with a small SWCG. The point being that we believe in running the pump to achieve some result, and not just to meet some magical number that someone pulled out of their rear.

The main reasons to run your pump are:

  1. To generate chlorine if you have a SWCG or tab feeder.
  2. To skim your pool.
  3. To mix chemicals if you manually add them.

Turnover of 100% Cannot be Achieved

Pools have very poor hydraulic efficiency and so even when you run multiple turnovers, the pool equipment does not "See" all of the water. Even if you assumed "perfect" mixing of the pool water, you would see the following dilutions:[1][2]

  • 1 turnover: 63.2%
  • 2 turnovers: 86.5%
  • 3 turnovers: 95.0%
  • 4 turnovers: 98.2%
  • 5 turnovers: 99.3%
  • 6 turnovers: 99.75%
  • 7 turnovers: 99.91%
  • 8 turnovers: 99.966%
  • 9 turnovers: 99.988%
  • 10 turnovers: 99.995%

You never get to 100% and again, the above assumes perfect near instantaneous mixing in the bulk pool water.

Thus, in a theoretical pool with perfect mixing (so basically, no pool ever in existence), it takes 7 turnovers of the water to get the pool equipment to "see" 99.9% of the water. Therefore, any sanitizer that doesn't establish a residual concentration (ie, UV, Ozone) will be very ineffective at sanitizing.

In summary, any number of turnovers of pool water are not necessary for normal filtration or sanitation of a residential pool.

Are Turnovers Needed to Distribute Chemicals?

With the pump on low speed, full chemical distribution when adding liquid chemicals such as chlorine or muriatic acid occurs at about the 30 minute mark.[3]

Hard to dissolve solid chemicals like salt and stabilizer/CYA may require continuous pump running to dissolve the solid chemicals, not to distribute it in the water.

Are Turnovers Needed to Filter the Water?

You run the pump and filter for the time necessary to remove stuff that falls into the pool. If your pool has a continuous load of dust, etc falling into the pool, then you will need to run your system most of the time to clear it. Or, if the stuff is dirt, it will fall to the floor of the pool as it is more dense and a robot cleaner will very efficiently remove it.

Particles that remain suspended in water are generally too small (<2 microns) to be filtered out the water with a standard pool filter. Larger particles will eventually sink to the pool floor which can then be removed by a pool cleaner. This is far more efficient energy wise than trying to filter these temporarily suspended particles using multiple turnovers since each turnover only filters about 64% of the water.[4]

Filtration is used to clear physical debris from your pool and keep it clear. Every pool has its own unique requirements for the amount of daily filtration which is not related to any turnover requirement.

Is There a Required Number of Turnovers of Water in A Pool?

There is no "official" regulation or specification on the number of turnovers for residential pools.[5]

Pool Builders will look to 3 turnovers in 24 hours as a common design goal, though anything from 2 to 6 turns in 24 hours is common enough. In practice, the fewer turnovers your system is capable of the more energy efficient the pump will be.

Commercial pools can have turnover requirements in their local regulations. Note that commercial pools have very different sanitation requirements due to higher bather load then a residential pool.