Sure Flow Automatic 3/4\” Anti-Siphon Valve. Professional Grade Anti-Siphon Valve with Flow Control. Its unique reverse-flow design helps prevent flooding in the event of internal diaphragm failure, keeping the valve closed and saving water. Anti-Siphon valves should always be installed at least 6 inches above the highest head on the valve line, and should never have another valve installed further down the line from the main valve. After installation and before startup, always the flush system of dirt and debris manually using the valve\’s external bleed screw. Equipped with the additional advantage of adjustable flow control, this model helps prevent wasteful misting when water pressure is too high. A top mounted, easy turn flow control knob allows you to optimize flow to the zone, permitting sprinklers to water at their most efficient rate for more uniform coverage.