Backflow Prevention and Cross-Connection Control
Cross-connections and the devices that stop them
A cross-connection is any actual or potential link between potable water and a contamination source. Backflow happens two ways: backsiphonage, driven by negative pressure in the supply (main break, hydrant use, nearby fire flow), and backpressure, when downstream pressure exceeds supply pressure (booster pump, elevation, chemical injection).
Match the device to the hazard. Health (toxic) hazards require an air gap or a reduced-pressure principle assembly (RPZ). Non-health hazards may use a double check valve assembly (DCVA) or a pressure vacuum breaker (PVB), depending on local code and whether backpressure is possible. A PVB cannot be used where backpressure exists and must sit at least 12 in. above the highest downstream outlet. An atmospheric vacuum breaker (AVB) may not be under continuous pressure and cannot have a valve downstream of it. Hose-bibb vacuum breakers protect sill faucets only.
Traps to watch: a DCVA is never acceptable for a toxic hazard; an AVB installed downstream of a solenoid valve is a violation; the assembly must be accessible and protected from freezing. Most jurisdictions require annual testing by a certified tester.
Rule of thumb: if you can picture a chemical injector, pond, boiler, or irrigation system on the other side of the connection, treat it as a health hazard and specify an RPZ or air gap.