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Understanding the Pool Water Circulation System

Understanding the Pool Water Circulation System

Maintaining good circulation in your swimming pool water is essential to keeping your pool clean. The main engine of your pool’s circulation system is the pump. To keep good circulation, you will need your outlets, skimmer, pump, filter, and returns to work together. When everything is working, your pool will stay clean. But if one part of the pool water circulation system breaks down, it can cause all sorts of problems, including a total failure of the system or its components.

Understanding the Pool Water Circulation System [infographic]

The Parts of The Pool Circulation System

Your pool circulation system is made up of several distinct parts that work together to keep your pool water moving. The heart of the system is your pump, which is the only part of the system that actively moves water. The rest of the system serves the pump or works with it to perform other functions to keep your pool clean.

The main parts of your pool water circulation system, more or less in order of the flow of water, are:

  1. Skimmers
  2. Main Drain
  3. Pump
  4. Filter
  5. Heater (if you have one)
  6. Chemical Feeder (not always present)
  7. Returns

If any one of these components breaks or has problems, it can affect the whole system. At best, you will notice that your pool water is less clean than usual. At worst, it can cause costly and even dangerous damage to one or more components, requiring expensive replacements or repairs.

Skimmers and Main Drains

Skimmers are the first stop on your pool water’s circulation journey. A skimmer is most often a rectangular opening in the pool wall near the top, at the water surface level. Most skimmers also have a floating door called a weir that helps ensure that only the water at the very surface level enters the skimmer. A skimmer takes in water with suction produced by the pump. One of its primary functions is to collect and remove debris from the surface of the water before it reaches the pump.

It is essential to keep the water level consistent with the height of the skimmers. This prevents air from being sucked into the skimmer. If air gets into the plumbing system, it can cause a loss in pressure and reduce the function of the circulation system.

The main drain is another way for water from the pool to reach the pump. It is installed on the floor of the pool, usually at the deepest point. A main drain, although it is called a drain, is actually rarely used to drain the pool. Instead, it is an outlet, where water leaves the pool to reach the pump. The suction from the pump draws water out the drain and towards the pump.

Main Drain Safety

A main drain can be dangerous if not installed properly. Because they create suction, main drains have been known to capture jewelry, hair, or even body parts. In some tragic cases, a main drain has become sealed by a human body part, and the building suction made it impossible to escape, leading to the drowning of the unfortunate victim.

In one such case, Virginia Graeme Baker, granddaughter of former U.S. Secretary of State James Baker, drowned as her mother tried unsuccessfully to pull her away from a drain that was exerting hundreds of pounds of suction per square inch. As a result of that case, and tireless lobbying from her mother Nancy Baker, new laws were passed. The VGB act of 2007 mandated newly designed drain covers that were designed that prevent entrapment. All public swimming pools now require VGB compliant drain covers.

In private pools, it is a good idea to inspect main drains and replace old covers with newer VGB compliant covers. In addition, most new pools are now built with two main drains, which reduces suction. Circulation systems can also be constructed with various safeguards, such as an emergency shut-off when pressure levels rise.

The Pump

After it leaves the pool through the skimmers and main drains, water flows to the pump. The pump is the heart of the circulation system, providing the suction and pressure to keep the water moving. A pool’s plumbing system is generally categorized into two sections, the suction side and the pressure side. The suction side is the part of the plumbing that carries water from the pool toward the pump. The pressure side is the part of the plumbing that carries pressurized water away from the pump and back to the pool.

A pump should be adequate to circulate all of the pool’s water in about 8 hours. To calculate the necessary size for a pump, you need to know how many gallons it pumps per hour and the approximate size of the pool. If the pump can move the number of gallons in the pool in 8 hours, then it is adequate for the pool. On many residential pools, the pumps are overpowered, so it may not even take 8 hours to circulate the pool. However, if you don’t know the power of your pump, running the pump 8-10 hours a day is a good rule of thumb.

The Filter

The next most important part of your pool’s circulation system after the pump is the filter. Just as it sounds, the filter cleans your pool water by removing debris. While the skimmer can remove larger debris, such as hair and leaves, the filter removes tiny impurities. These could be residues from lotions, hair products, and sunscreen, or tiny bits of algae or dust. Any of these could result in cloudy water, which the filter helps prevent.

There are three main types of filter: Sand, DE, and Cartridge.

Sand filters work by running water through specially-sized sand. The debris gets caught in the sand as the water passes through. They are very low maintenance and the cheapest option. However, they only filter down to 20 microns. A sand filter needs to be backwashed and rinsed regularly (every one to two months depending on pool use).

DE filters use a special clay-like powder, diatomaceous earth, to filter out even the tiniest debris. DE filters can capture the smallest debris of any filter, down to 5 microns. The DE powder is spread over a grid covered in a fine mesh of woven polyester or a similar material. As the water flows through, impurities are caught in the DE powder. This is the most expensive type of filter, but by the numbers, it is the most efficient.

The simplest type of filter is a cartridge filter. Much like the air filter on a home HVAC system, the cartridge collects debris and must be removed and washed occasionally. Eventually, a cartridge needs to be replaced.

Heaters, Chemical Feeders, and Other Specialized Equipment

After it passes through the filter, cleaned pool water may flow through any of a number of specialized pieces of equipment. The two most common are heaters and chemical feeders.

A pool heater is usually powered by natural or propane gas. The water passes through a heat exchange where it is heated by the heat of combustion. There isn’t much maintenance on a pool heater. The main concern is just to clean around the heater regularly to ensure nothing combustible is too close to the heater. Clear away leaves and other debris to avoid the risk of a fire.

Many pools have a chemical feeder. A chemical feeder is a tank that may hold pellets or another type of concentrated, time-release chemical, usually chlorine and bromine. By gradually releasing chemicals into the pool water, they reduce the need for manual chemical balancing.


As their name suggests, returns are the last stop on your pool water’s journey, where it returns to the pool. Returns are usually located in at least two spots around the pool, though some older pools may only have one. Typically, returns are small. This is so that the pressure will build and the water will jet out of the returns and reach father into the pool. This improves circulation overall. Returns usually have an adjustable “eye” that lets you direct the flow of the water. It is best to direct the water in a consistent direction to “spin” the water around the pool. For instance, pointing all the returns in a clockwise direction helps the water circulate clockwise. If a pool only has one or two returns, the returns can also be pointed downward to increase circulation.

No matter how many returns you have, there will always be dead sections where water does not circulate. Common dead spots are behind pool ladders or near the pool steps. Any crevices or cracks would also be dead spots. It is important to use a pool brush to clean your pool regularly. This should be a part of your weekly maintenance. Be sure to focus on dead areas where the water isn’t moving well enough to carry away tiny debris. Brushing helps loosen algae and debris before it can build up.

Keeping the Pool Water Circulation System Clean

To keep a pool water circulation system working at its best, it needs to be free of any large debris. Large debris, such as hair and leaves, could plug the plumbing or damage the pump, leading to expensive repairs. Much of this debris should be cleaned out of the pool manually. Smaller pieces can be caught in the skimmer basket. The filter also needs to be cleaned regularly to avoid a pressure buildup and a loss of filtering power. The whole system should be inspected from time to time to preempt any possible damage.

One way to ensure that the pool circulation system is always functioning at its best is to hire a pool cleaning service. The professionals at The Pool Butler can keep your system clean and free of debris, maintain your filter, and perform routine inspections. All this will ensure that your pool is crystal clear and ready for swimming. It will also help avoid larger repair bills. So contact The Pool Butler today to get your service started.

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