Cross-Connection Control and Backflow Prevention
Backflow Mechanisms
Backflow occurs by backsiphonage (negative pressure in the supply) or backpressure (downstream pressure exceeding supply pressure). Learn to identify which mechanism applies to a scenario, because device selection follows from the mechanism and hazard level.
Hazard Levels and Device Selection
Know the difference between a pollution hazard (taste, odor, or color) and a contamination hazard (health risk). An air gap provides the highest protection and is the only device suitable for the highest hazard. A reduced-pressure principle assembly protects against both backsiphonage and backpressure and is used for high hazards. A double check valve assembly protects against backpressure only for lower hazards. A pressure vacuum breaker protects against backsiphonage only and must be installed above the highest downstream outlet.
Traps in Questions
Watch for devices installed at the wrong elevation, test cocks omitted, and assemblies installed below grade without proper drainage. Questions often ask which device is required for a specific connection such as a boiler feed, irrigation system, or carbonator.
Rule of Thumb
Match the device to two variables: the direction of flow risk and the hazard level. Worked example: an irrigation system with chemical injection requires an air gap or reduced-pressure principle assembly, not a double check valve, because chemicals create a health hazard.