The first sprinkler system valve unit (after the isolation valve) in a lawn irrigation system is usually the backflow prevention device. Choosing the right backflow prevention device for a particular lawn irrigation system requires careful consideration, but need not be cause for confusion. The three most widely approved and used backflow devices are the pressure vacuum breaker assembly, the double check valve assembly, and the reduced pressure zone assembly. All three of these types are installed immediately after the irrigation system isolation valve to protect the entire irrigation system. The fourth common type of backflow device, the atmospheric vacuum breaker, is installed on each zone after the zone control valve.

The reduced pressure zone assembly, also sometimes called a reduced pressure principle assembly, is the most complex, most expensive, and, when working properly, the most secure and reliable of all backflow prevention devices. A reduced pressure zone assembly consists of an inlet shutoff valve, two independently operating spring loaded check valves with a pressure differential relief valve located between the check valves, four test cocks, and an outlet shutoff valve. The first check valve reduces the supply pressure so that the pressure between the check valves is always lower than the supply pressure. The relief valve between the checks—in the reduced pressure zone—adds redundant security by discharging into the atmosphere when a backflow or backpressure condition lowers the difference between the zone pressure and the supply pressure. Because the supply pressure is always greater than the zone pressure, no backflow can penetrate the upstream check even if it leaks. Some localities, such as Honolulu, do not allow reduced pressure zone assembly to be installed below grade in underground lawn sprinkler systems, while others require copper pipe on both ends of the assembly. Local codes may be especially variable and stringent on reduced pressure zone assembly, so have a thorough understanding of local requirements before proceeding. Reduced pressure zone assemblies come in a variety of configurations. Choose an “inline” or “straight” configuration for installation underground. For above-ground installation, consider an “n” configuration assembly for a very small footprint.