Check valve

A check valve is a one-way valve. It opens when water flows in the intended direction and closes when flow stops or attempts to reverse. In backflow prevention assemblies, checks are typically spring loaded so they close positively and hold a pressure differential rather than relying on gravity or flow alone. The internal parts are simple: a disc or poppet, a seat it presses against, a spring, and guides that keep the moving parts aligned.

Check valves are the fundamental building blocks of the testable assemblies. A double check valve assembly is essentially two of them in series, and a reduced pressure principle assembly adds a relief valve between two checks. The quality of protection depends on how tightly each check seals under real conditions, which is why assembly standards require each check to close independently and hold against reverse pressure, not merely to swing shut when flow stops.

Testers and property owners should understand that check valves fail quietly. Debris, scale, worn rubber, or a weakened spring can hold a disc slightly off its seat, and water service continues normally while the reverse-flow protection is silently gone. This is the core justification for differential pressure testing: a gauge can measure whether each check is actually holding, something no visual observation of an assembled valve body can confirm.

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