Check valve
A check valve is a one-way valve: it opens to flow in the intended direction and closes when flow stops or tries to reverse. In backflow prevention, check valves are spring-loaded so they close positively, and testable assemblies use two independent checks in series, with the reduced pressure principle assembly adding a relief valve between them. A single check valve on its own is not an approved backflow prevention assembly.
Updated 2026 · Researched from published standards, not professional or legal advice.
Check valves in backflow assemblies
| Assembly | How the check valves are used |
|---|---|
| Double check valve assembly | Two independent spring-loaded checks in series, each tested to hold a minimum differential |
| Reduced pressure principle assembly | Two independent checks with a relief valve between them that opens if the first check leaks |
| Pressure vacuum breaker | One spring-loaded check plus an air inlet valve; protects against backsiphonage only |
| Single check valve | Flow control, not an approved backflow prevention assembly for protecting a potable supply |
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.
Common questions
What are check valves used for?
To keep flow moving one way. In water systems they stop pumps from spinning backward, keep lines primed, and in backflow prevention they are the components that hold reversed pressure out of the potable supply. The spring-loaded checks in an assembly close against reversed flow and hold a pressure difference that a tester measures with a differential pressure gauge.
Relief valve vs check valve: what is the difference?
A check valve closes to block reversed flow. A relief valve opens to discharge water. In a reduced pressure principle assembly the two work together: if the first check leaks and pressure in the zone between the checks rises, the relief valve opens and dumps that water to the atmosphere rather than letting it reach the supply.
Is a check valve a backflow preventer?
Not by itself. Approved assemblies use two independently acting checks, shutoff valves and test cocks so each check can be verified in the field. A lone check valve has no way to be tested in place and no backup if it fouls, which is why water purveyors do not accept one for containment protection.
Sources
- University of Southern California Foundation for Cross-Connection Control and Hydraulic Research, Manual of Cross-Connection Control: field test procedures for RP, double check and vacuum breaker assemblies.
- U.S. EPA, Cross-Connection Control Manual (Office of Water): definitions of backflow, backsiphonage, backpressure and the assembly types.
Related terms
| Double check valve assembly (DC) | A testable backflow prevention assembly consisting of two independently acting spring-loaded check valves, used for non-health hazard applications. |
| Reduced pressure principle assembly (RP) | A testable backflow prevention assembly using two independently acting check valves with a pressure differential relief valve between them, suitable for high hazard applications. |
| Differential pressure gauge | The calibrated field test instrument that measures pressure differences across the components of a backflow prevention assembly to verify they are holding. |
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