Backflow Prevention Assembly Types and Applications
Assembly types
Know the five core assemblies and their applications. The air gap provides the highest protection via physical separation and is not a mechanical assembly. The reduced-pressure principle assembly (RP/RPZ) uses two independently acting check valves plus a relief valve that opens on differential pressure; it protects against both backsiphonage and backpressure at high hazard. The double check valve assembly (DCVA) has two check valves, protects against backpressure and backsiphonage at low hazard, and has no relief valve. The pressure vacuum breaker (PVB) has one check plus a loaded air-inlet that opens on backsiphonage; it must be installed above the highest downstream outlet and cannot protect against backpressure. The spill-resistant vacuum breaker (SVB) adds a second check and a relief vent for freeze/backpressure resistance and may be installed in a pit. Atmospheric vacuum breakers (AVB) and hose-bibb vacuum breakers are non-testable devices for low hazard, fixture-level protection.
Applications and traps
Match assembly to hazard and orientation. Traps: assuming a PVB stops backpressure (it does not); placing a PVB below the highest outlet (it will not function); using a DCVA on a high hazard; confusing an SVB with a PVB. Remember RP assemblies discharge water from the relief valve during normal operation and require drainage.
Rule of thumb
If the hazard is high and backpressure is possible, use an RP or air gap. If only backsiphonage at low hazard, an AVB or PVB may suffice. Never substitute a lower-rated assembly to save cost.
Worked example: A boiler feed line can create backpressure with chemical treatment - high hazard with backpressure - specify an RP assembly.