The Main Drain has a unique grille design and is available for concrete or liner pools. The drain is used to draw the base water for filtering and is also used as a principal method of emptying the pool. There is a collection tube which is used to collect water from the underside of the pool floor as well as a hydrostatic relief valve which helps to avoid possible damage to an empty pool if the water rises above the level of the pool floor.
Current recommendations state that every pool should have at least two main drains, therefore at least one main drain on every pool will need the bottom port sealed off - that's why the Certikin main drain body has a knock out blanking plate in the bottom port to save you time on site.