In-ground swimming pools are basically holes in the ground in which water is held. Of course, an in-ground swimming pool is a bit more complicated than just a hole full of water. For example, some types of in-ground pools feature plumbing systems able to process and return huge amounts of water. A vital part of how some types of in-ground pools handle their water with relative ease has to do with what's called a main drain.
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Swimming pool main drains are one of two mechanisms used in many pools to circulate and filter their water, the other being skimmers. Skimmers sit near a swimming pool's waterline, taking in water via gravity and letting it flow down to the filter. Main drains in swimming pools sit in their lowest points. A swimming pool main drain uses suction to pull pool water through it. Heavy particulate matter that sinks to a pool's bottom is often sucked away through a main drain.
Swimming pool main drains don't actually drain anything. Rather, a swimming pool's main drain, if it has one, is a suction device. A pump is located underneath or near a swimming pool equipped with a main drain and it's used to pull water down into the drain. Once water is sucked into a swimming pool's main drain, it's whisked via piping to the pool's filtration unit. To avoid main drain suction entrapment of a swimmer, main drains are typically equipped with anti-suction covers.
Laws regulating safe main drain operation in swimming pools are abundant. For pool main drain safety, single main drains must be equipped with suction interruption devices. The best way to limit overwhelming suction with a pool main drain is to have at least two of them to lessen each drain's suction. Another method to prevent a main drain from creating a suction entrapment situation is to have a vacuum release that stops suction if a blockage is detected.
Many new in-ground swimming pools use different methods for circulating water that don't require main drain usage. If your swimming pool has a main drain, ensure, at minimum, that it has a suction entrapment-proof cover. The federal 2007 Virginia Graeme Baker Pool and Spa Safety Act regulates swimming pool main drains. Since 2007, federal and state laws require new pools to have either multiple main drains, unblockable drains or no main drains at all.