Which statement correctly describes the operation of a reduced pressure principle backflow assembly?

• A. It relies on a single check valve
• B. It has no relief valve
• C. It prevents backflow solely through gravity
• D. The relief valve will open and drain if a pressure differential occurs between the two checks

Answer: (D)

A reduced pressure principle backflow assembly operates with two check valves in series and a relief valve positioned between them. Under normal conditions, the checks block backflow while allowing flow toward the building. If a pressure differential develops between the two checks—such as backpressure or back-siphonage—the relief valve between the checks opens and drains to the atmosphere, dropping the downstream pressure and preventing any backflow into the supply. This vent-and-drain action is what reliably isolates the public water supply from a contaminated downstream system. The other ideas don’t fit: a single check valve doesn’t provide the needed relief path; there is a relief valve in an RPZ; and backflow prevention isn’t accomplished by gravity alone but by the coordinated operation of the two checks and the relief valve.