Atmospheric Vacuum Breaker (AVB) is used to prevent back-siphonage by allowing air to enter the downstream water line. This device must be installed 6-inches above the flood level rim of the fixture it serves. The AVB protects only against back-siphonage and must not have any shut off valves downstream of the device. Under no circumstance shall the AVB be under continuous pressure for more then 12 hours. (ASSE 1001)