Reduced pressure principle assembly (RP)
An RP (reduced pressure principle) assembly, also called an RPZ, is the testable backflow preventer used for high-hazard connections. Two independent check valves hold a zone between them at lower pressure than the supply; if either check fails, a relief valve dumps water from that zone to the atmosphere. It protects against both backsiphonage and backpressure and is listed to ASSE 1013.
Updated 2026 · Researched from published standards, not professional or legal advice.
RP assembly key facts
| Item | Detail |
|---|---|
| Standards | ASSE 1013; AWWA C511 |
| Hazard level | High (health) hazard |
| Protects against | Backsiphonage and backpressure |
| Parts | Two check valves, relief valve, two shutoff valves, four test cocks |
| Installation | Above grade with room for the relief discharge, never in a pit that can flood |
| Testing | Typically annually by a certified tester with a differential pressure gauge |
A reduced pressure principle assembly, commonly called an RP or RPZ, is a mechanical backflow preventer built around two independently acting, spring-loaded check valves with a hydraulically operated relief valve located between them. The assembly also includes shutoff valves on each end and test cocks for field testing. In normal operation the zone between the checks is kept at a lower pressure than the supply, which is the principle that gives the assembly its name.
The RP is regarded as the highest level of mechanical protection because of how it fails safe. If either check valve fouls or leaks, or if pressure conditions reverse, the relief valve opens and discharges water from the zone to the atmosphere rather than allowing it to pass backward. That visible discharge is both a safeguard and a signal that something needs attention. Because it protects against both backpressure and backsiphonage, the RP is the standard choice for health hazard connections.
Testers and property owners should plan for the RP's practical needs. It must be installed where the relief port can discharge freely and drain safely, which usually means adequate drainage and protection from flooding and freezing. Occasional relief discharge can be normal during pressure fluctuations, but continuous discharge indicates a fouled check or relief problem that warrants service. Like other testable assemblies, it requires periodic testing, typically annual where required, set by the water authority.
Common questions
What is an RP valve?
People often call the whole assembly an RP valve, but an RP is an assembly: two check valves, a relief valve between them, shutoff valves and test cocks. It is the highest level of mechanical backflow protection.
RP vs double check: what is the difference?
A double check assembly has two check valves but no relief valve, so it is rated for low (non-health) hazards only. An RP adds the relief valve that discharges water if a check fails, which is why it is required for high hazards.
Sources
- ASSE International product standards (1001, 1013, 1015, 1020, 1056)
- AWWA C511, Reduced-Pressure Principle Backflow Prevention Assembly
- USC Foundation for Cross-Connection Control and Hydraulic Research, Manual of Cross-Connection Control
- U.S. EPA, Cross-Connection Control Manual (EPA 816-R-03-002)
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. |
| Relief valve | The hydraulically operated valve between the checks of a reduced pressure principle assembly that discharges water to atmosphere when internal pressure conditions become unsafe. |
| Check valve | A valve that allows water to flow in one direction and closes to prevent flow in the reverse direction. |
| Degree of hazard | The classification of a cross-connection by the severity of harm its contaminant could cause, typically as a health hazard or a non-health hazard. |
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