Over the years I’ve gotten many questions about hydrostatic valves. What is a hydrostatic valve? What does it do? And do I need one in my swimming pool? The simple explanation of what a hydrostatic valve does, is that it equalizes pressure between the water in a swimming pool and the ground water under a pool. It is located in the main drain in the bottom of a pool. The valves job is to open and let ground water into the pool if the pressure outside the pool is greater than inside. When the water pressure equalizes the valve shuts so that water doesn’t leak out of the pool. The only swimming pools that usually have a hydrostatic valve are concrete and fiberglass shell pools. These types of pools are more likely to move out of place or heave out of the ground from water pressure because they are a one piece pool. Picture a boat on the water. It floats because it is one sealed unit that water cannot pass through. A fiberglass or concrete pool are similar to a boat if it was burried in the ground. A heavy rain or a lot of melting snow can saturate the ground and cause a pool to try and float. This can cause cracks in the pool shell or the movement can break plumbing on the pool. I have even seen a concrete pool that was drained by a company for some repair work heave out of the ground 6 inches on one side because they removed the hydrostatic valve and replaced it with a plug to stop the water from comming in. If you have a swimming pool with a vinyl liner you probaly dont need a hyrostatic valve. Most liner pools have a sand or semi-hard bottom under the liner. the water flows through both of those surfaces which can cause the liner to float but not the pool. If you own a fiberglass or concrete (gunite) swimming pool verify that there is a hydrostatic valve in the bottom to prevent any poosible damage.