A hydrostatic valve is a pressure relief device installed on the bottom of a swimming pool which prevents ground water pressure beneath the pool damaging the liner or even raising it out of the ground. The hydrostatic valve achieves this by allowing ground water to flow into the pool thereby relieving the pressure on the liner. This process typically only occurs when the swimming pool is empty and does not have the necessary weight to counteract the effects of ground water pressure. A hydrostatic valve or hydrostatic relief valve (HRV) relies on a spring loaded, one-way valve that keeps the pool water in but opens to allow high pressure ground water into the pool. These devices are often included as an integral part of main drains and preclude the need for dual fittings on the pool floor.
A build up of ground water can exert surprising amounts of hydraulic pressure on structures such as swimming pools. It is not uncommon for a swimming pool liner to be pushed several feet out of the ground or even fractured by ground water pressure. One of the best ways of avoiding these costly mishaps is the installation of a hydrostatic valve in the bottom of the pool which will open when the pressure of the ground water exceeds a set point and allow it to flow into the pool. Some dirty water in the pool or a slight overflow is then the only result of the excess pressure.
The flow of high pressure ground water into the pool typically occurs when the pool has a large amount of water drained out of it or is totally empty. A sudden surge in ground water levels such as those experienced after heavy rains will then cause the HRV to open and allow for the equalization of the ground water pressure. If this happens, the valve should be checked immediately for obstructions as grit and small stones are usually carried through it when it opens. Foreign objects are a common cause of HRVs being unable to close properly with a resultant loss of pool water.
A hydrostatic valve may be bought as a stand alone fitting or as part of an integrated master drain unit. These compact units allow for a single pool floor fitting to be installed. This type of combination fitting typically consists of a standard drain body with the relief valve and valve stem in the center. The master dress ring and grate then provide a cover for both units. Hydrostatic valve parts do degrade with time and causing leak; as a result, valve checks should be a standard part of any pool maintenance regimen with standard replacements carried out at approximately five year intervals.