An anti-siphon valve is essentially a valve & atmospheric backflow prevention device combined into one. In irrigation, these valves serve two purposes. It prevents water from the irrigation system from entering the main line and it keeps water in the system under pressure. The controller is wired to the anti-siphon valve which sends a signal telling it to allow water to pass through into the sprinkler system. Sprinkler Warehouse carries a variety of anti-siphon valves from top manufacturers such as Rainbird, Hunter, Irritrol and Champion.