Backflow Prevention Assemblies and Devices
The device family
Know each device, its mechanism, and its approved use.
- Air gap: physical separation between supply and receiving vessel. Highest protection; protects against both mechanisms; requires no testing but needs inspection.
- Reduced-pressure principle assembly (RP): two independently acting check valves plus a hydraulically operated differential relief valve. Protects against backsiphonage and backpressure; health-hazard rated.
- Double check assembly (DC): two check valves, no relief. Protects against backpressure and backsiphonage for non-health hazards only.
- Pressure vacuum breaker (PVB): one check plus an air inlet, for backsiphonage only, non-health and some health uses per local code; cannot be used under continuous pressure or where backpressure is possible.
- Atmospheric vacuum breaker (AVB): air inlet only, no check, no downstream shutoff, no continuous pressure, backsiphonage only.
- Spill-resistant vacuum breaker (SVB): like a PVB but the air inlet stays closed during normal flow, reducing discharge; backsiphonage only.
- Hose bibb vacuum breaker and laboratory faucet vacuum breaker: small, point-of-use, backsiphonage only.
Traps
- Assuming an RP and a DC differ only in test procedure; the relief valve is the defining difference.
- Placing a DC on a health hazard.
- Using a PVB or AVB where backpressure can occur.
- Forgetting that AVBs cannot have downstream shutoff valves.
Rule of thumb
Health hazard plus backpressure possible = RP or air gap. Health hazard plus backsiphonage only = PVB or SVB (per code). Non-health hazard with backpressure = DC. Point-of-use, backsiphonage only = hose bibb or faucet vacuum breaker.
Worked example
A carbonator in a restaurant is fed potable water and can push CO₂-laden water back under pressure. That is a health hazard with backpressure, so an RP assembly or air gap is required, not a DC.