Anti-Siphon valves are installed above ground and will prevent back flow if properly installed. Anti-Siphon valves should always be installed at least 6" above the highest head on the valve line, and should never have another valve installed further down the line from the main valve. Our Anti-Siphon valves also feature our patented vacuum breaker–available for automatic or manual applications. All of our Anti-Siphon valves work with any standard sprinkler timer.