An inground swimming pool's main drain is located in the deep end of the pool. A main drain circulates the pool's water by pulling water off the bottom of the pool and sending it to the filter. During winter months or periods of non-use, most pool drains do not need to be plugged, but in some cases plugging a main drain is required or recommended. Plugging a main drain shuts off the suction caused by the drain and prevents water from traveling through the lines connected to the drain.
Lower the water level of the inground swimming pool so the main drain is easily reachable. Drain the water completely during colder months. Ensure all pool equipment is turned off.
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Use a screwdriver to unscrew any screws around the lip of the main drain lid. Pull gently up on the lid until it pops out of place.
Locate the small hole at the bottom of the main drain. This is the plumbing line to the filter. Plug the small hole with an expandable rubber plug or a plastic threaded plug, if the hole is threaded. Replace the main drain lid if needed.