Kentucky Journeyman Plumbing Practice Test

The idea being tested is how a backflow prevention device creates and controls pressure to stop contaminants from being drawn back into the drinking water supply. The reduced pressure principle/backflow preventer does this by forming a small, controlled zone between the supply and any potential backflow that is kept at a lower pressure than the incoming supply. It accomplishes this with two check valves and a relief valve between them. If downstream conditions try to push water back or if the upstream pressure drops, the relief valve opens just enough to vent to the atmosphere and maintain the zone pressure below the supply pressure. That reduced-pressure zone is what blocks backflow, because any attempt for contaminated water to flow back would have to overcome the maintained differential, which the device is designed to resist.

Air gaps rely on a physical separation rather than maintaining a pressure difference, so they prevent siphonage by distance but don’t continuously regulate a reduced-pressure zone. A backflow preventer with atmospheric vent uses venting to atmosphere but doesn’t establish or maintain the same continuous reduced-pressure condition as an RPZ. A simple check valve stops flow in one direction but doesn’t protect against backflow caused by pressure fluctuations in the system or backpressure downstream.