Cross-Connection Control and Backflow Prevention
Backflow, hazards, and devices
A cross-connection is any actual or potential link between potable water and a nonpotable source. Backflow occurs by backsiphonage (negative pressure pulls water backward) or backpressure (downstream pressure exceeds supply pressure). Backsiphonage commonly follows a main break or high-demand fire flow; backpressure comes from boilers, pumps, or elevated private supplies.
Protection follows a hierarchy. An air gap gives the highest protection and is simple but needs a physical break and proper drainage. A reduced-pressure-zone (RPZ) assembly protects against both backpressure and backsiphonage and is used for high hazards. A double-check valve assembly (DCVA) handles both types for lower hazards but does not protect against a high hazard. A pressure vacuum breaker (PVB) and atmospheric vacuum breaker (AVB) protect against backsiphonage only; the AVB cannot be under continuous pressure. Hose-bibb vacuum breakers protect sillcocks.
Programs distinguish containment (protecting the public main at the service connection) from isolation (protecting fixtures and processes inside a building). Devices must be tested by qualified testers at the required frequency, and records kept. Thermal expansion can result when a backflow device creates a closed system; an expansion tank may be needed.
Traps: assuming a DCVA protects a high hazard; forgetting that a PVB does not stop backpressure; treating an AVB as suitable downstream of a valve; confusing containment with internal isolation.
Rule of thumb: match the device to the hazard and the type of backflow - air gap or RPZ for high hazard, DCVA for lower hazard, vacuum breakers for backsiphonage only.