Hydrostatic relief valves

The Hydrostatic relief valve is located in the deepest part of the pool.  It may be under the main drain cover or in its own box situated on the bottom of the swimming pool. In vinyl and fibrglass pools, it is integrated into a main-drain pot. The valve allows water into the pool but not out, opning when there is too much groundwater pressure under th pool  This allows the ground water to enter the pool and prevents the pool or its liner popping up.