Pool Circulation: How Does it Work?

Whether you’re looking to build your own pool or already have a pool and are just curious about its filtration, you need to learn about pool circulation as it is responsible for filtration, even distribution of pool chemicals, and freshness of the water in your swimming pool.

A pool circulates water by using circulation lines and a water pump alongside a well-positioned drain. The water goes towards the drain, encountering skimmers along the way. It is then fed by the water pump to the primary filter, then passed to the jet nozzles through the circulation lines.

In this article, you’ll learn more about pool circulation and discover what causes poor circulation and what happens when a pool doesn’t sufficiently circulate its contents. Above all, you’ll learn in detail about each stage of pool circulation, including what happens when

  • Water goes through a skimmer
  • The filter panel on the drain removes larger impurities
  • The water pump moves the water through the main filter
  • Circulation lines take the water to the jet nozzles

The Skimmer Retains Larger Objects

This is the first stage of the water filtration cycle responsible for the circulation of water in non-wavemaker variety pools. In smaller pools, this step is compounded with the next step because filtration pipes are easier to replace. 

In larger pools that you find at hotels and apartment complexes, this step is essential to ensure the pool doesn’t end up closed for the public due to leaves and other items clogging the circulation pipes. The skimmer is located at the water level on the farthest end of the pool. This is the rectangular window-like device you’ll notice on the pool wall. In some instances, it will be reminiscent of Venetian blinds.

The Water Reaches a Filter Panel

The filter basket is a second skimmer which is relatively finer but isn’t anywhere close to the main filter. The fact that it too has the word “filter” in its name can be confusing. The filter basket or grill simply makes sure that materials that are too big for the finer filter do not get through and clog the smaller pipes. The initial skimmer retains leaves and chocolate wrappers, among other things that may slip in the pool. The filter basket then picks out even smaller matter like hair.

Relatively Cleaner Water Gets Drained

The filter panel is located directly over the drain, usually at the lowest point in the pool. A drain is present in pretty much every pool with circulation, regardless of how big or small the pool is. In some pools, the filter pump and drainage are managed by a single contraption, but this practice is far from standard. 

The pool drain works the same way as a bathtub drain. It simply serves as an outlet from which water passes due to gravity. Inground pools often rely on gravity for drainage because of two reasons: firstly, they can, and second, because relying on a pump alone to pull that much water would be cost-exhaustive. Above ground pools use a pump to pull the water through the skimmer instead of having a drain that automates the process through gravity.

The Water Pump Pushes the Water to the Filter

The water pump is also called the filter pump in smaller pools where the single device houses both the filter and the circulation-inducing pump. Ultimately the same thing happens at this stage: the water is pulled by the pump from the lowest point post-drainage and passed through the filter. 

The filter retains impurities on a granular level. Here particles like pollen, sand, and dandruff are held while cleaner water is pushed to the cockstops. If any objects larger than a button make it to this point, it is only a matter of time before the entire circulation system fails.

Filtered Water Is Pushed to the Jet Nozzles

The filtered water is pumped to the opposite side of the skimmers. In the smaller pools, the burden of circulating the water is on a single pump, but with standard swimming pools, the jet nozzles take up some of that burden because otherwise, the water pressure towards the release-end of the pool would below. 

While this would be effective for the practical purposes of lowering hydrostatic pressure and preventing stagnation, it would not be fun. Jet nozzles make circulation more fun by reducing the surface area of the outlet. As a result, the water pressure is higher despite the outlet being farther away from the pump. The nozzles themselves don’t pump water.

Why Do Pools Need Circulation?

As you have learned more about water circulation in pools, you may have noticed that this setup is quite expensive, especially in larger pools. Why do pool builders invest time and money in setting up such an elaborate circulation system? Because in the long run it is wiser and saves money. Here are a few reasons why your pool needs circulation.

Circulation Is the Difference Between a Pool and a Swamp

Without a circulation system, your pool would still have circulation, but it would be natural circulation. Natural circulation occurs when the sun’s heat evaporates the water, which then rains back onto the body of water. 

Not only is this inefficient for an organically inert (bacteria-free environment), but it is also incredibly slow. It is no coincidence that the word “swap” conjures up images of bugs, mosquitoes, and dirt because all of those are a result of stagnant still water. Circulation is the biggest differentiator between a large-scale puddle and an actual pool.

Circulation Is Needed for Filtration

Though algaecide and chlorine can kill smaller microbes, much of the pool’s impurity comes from bigger dirt particles and matter like leaves and pollen. These can build up and make the water unappealing to swim in. Circulation keeps the water moving through increasing layers of finer filters starting with skimmers and going all the way to the main filter, which in most cases is capable of fishing out even a single grain of sand in water.

Neutralizing Hydrostatic Pressure

Hydrostatic pressure is the pressure that builds up when water is stationary. It leads to “popped pools” when the still water is in an inground pool. An above ground pool simply collapses under the pressure of a large quantity of water if it goes through a long enough period of zero activity. 

Filter pumps for each variety usually have significant enough circulation capacity to allow minimize hydrostatic pressure. Even if your pool isn’t in use, it is advisable to keep the circulation process semi-regularly active so that the water gets sufficiently cleaned and the pool doesn’t pop or collapse.

What Causes Poor Pool Circulation?

If your pool’s circulation is poor, it will still neutralize the threat of hydrostatic pressure, but almost every other advantage of circulation will be adversely affected. Filtration and distribution of pool chemicals will be impacted the worst. That’s why it is crucial to know the cause of poor pool circulation.

Poor pool circulation is caused by a leak in the circulation lines or a bad connection between the circulation lines and the filter. Either way, a pressure leak will lead to water getting released prematurely. Upon spotting the leak, one can use a gasket and improve circulation.

Final Thoughts

Pool circulation is essential for water health and the removal of impurities from the pool’s contents. It is also a must-have for every pool, and in its absence, your pool is only a large bathtub and may require draining within 48 hours. Here’s a recap of the post covering the steps of pool water circulation.

  • Skimmers filter out the large matter as water moves towards the drain
  • The water goes through the drain because of gravity
  • Filter pump channels the water through the filter
  • Filtered water is fed to the circulation lines
  • Jet nozzles build pressure and release the water into the pool

Jed Arnold

Jed spent every year from the ages of 15 - 22 as a Lifeguard (Red Cross) and ages of 17 - 22 as a Certified Pool Operator (CPO). Between working for over a dozen facilities and owning a pool, he carries over a decade of pool experience.

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