Backflow Prevention and Cross-Connection Control
Two mechanisms, one remedy
Backflow happens by backsiphonage (negative or reduced supply pressure pulling contamination into the system) or by backpressure (downstream pressure exceeding supply pressure). The remedy is a properly selected and installed device, matched to the hazard. High-hazard connections - anything that could introduce a toxic or health-threatening substance - require an air gap or a reduced-pressure zone (RPZ) assembly. Low-hazard connections may use a double-check valve assembly or a pressure vacuum breaker. Individual fixtures use vacuum breakers or integral check valves.
Device rules that get tested
An air gap is a physical separation of at least twice the effective opening of the supply pipe, and it is the only non-mechanical protection. An RPZ has two check valves plus a relief valve and requires test cocks, accessibility, no bypass except for fire protection, and protection from freezing and flooding - never in a pit. A pressure vacuum breaker must sit at least 12 in. above the highest downstream outlet and cannot be used where backpressure can occur. An atmospheric vacuum breaker cannot be under continuous pressure and cannot have a downstream shutoff. Double-check assemblies are for low hazard only. All testable assemblies are tested at installation and annually thereafter by a certified tester.
Traps
Specifying a double check for a high hazard; burying an RPZ; installing an AVB with a downstream valve; omitting thermal expansion control downstream of a backflow preventer; and forgetting that boiler makeup, irrigation, carbonators, and chemical dispensers all need protection.
Rule of thumb
Ask two questions: what is downstream (hazard level), and can pressure reverse (backpressure allowed?). High hazard forbids the double check; any risk of backpressure forbids the vacuum breaker.