Backflow Prevention Assemblies and Device Identification
Core Concepts
Know the major devices and their applications. An air gap is the only non-mechanical protection and provides the highest protection when properly sized; it must be at least twice the effective opening of the supply pipe, with a minimum clearance. A barometric loop protects only against backsiphonage and is limited to about 35 feet of head.
Atmospheric vacuum breakers are for backsiphonage only and must be installed above the highest downstream outlet, typically 6 inches minimum. Pressure vacuum breakers add a spring-loaded check and are also backsiphonage-only, installed at least 12 inches above the highest downstream point. Double check valve assemblies protect against both backpressure and backsiphonage for low hazards. Reduced-pressure principle assemblies protect against both for high hazards and include a relief valve. Spill-resistant vacuum breakers are a newer backsiphonage-only device with a check and air inlet.
Traps in Questions
A frequent trap is assigning a device to the wrong hazard class or wrong flow direction. Another is confusing installation height requirements between atmospheric and pressure vacuum breakers. Questions may describe a high-hazard application and offer a double check assembly as a distractor.
Rule of Thumb
Backsiphonage-only devices (atmospheric vacuum breaker, pressure vacuum breaker, spill-resistant vacuum breaker, barometric loop) are never acceptable for backpressure protection. If both backpressure and backsiphonage are possible, choose a double check (low hazard) or reduced-pressure principle assembly (high hazard).