The hydrostatic valve is an integral part of an in-ground pool's plumbing system. Located in the deepest part of your pool, the valve allows water to enter the pool from underground, but it should not allow water from the pool to drain out. The purpose of a hydrostatic valve is to release pressure from underground water when necessary so that the pressure does not damage your pool. A faulty or blocked hydrostatic valve is often the reason for an unexplained loss of water in a swimming pool.
Fill the pool with water. Mark the water line with a pencil on the faceplate of the skimmer or other easy-to-read location.
Turn the pool system on and let it run for 24 hours. Recheck the water level.
Fill the pool again, then turn the pool system off. Let the water sit for 24 hours before rechecking the level.
Compare the two tests. If your pool lost water when the system was running but not when the system was off, the problem is likely somewhere in the plumbing system, such as the hydrostatic valve. If the pool lost virtually the same amount of water during both tests, the source of the leak is likely somewhere other than the plumbing system.
Examine the valve itself to see if there is any blockage. The valve is generally located under the main drain, but it also may be in a separate location near the drain. If something is stuck in the valve, such as a pebble or other debris, water will escape. Remove anything that's blocking the valve.