The difference between a pressure reducing valve and an RPZ is their purpose: a PRV regulates downstream water pressure, while a reduced pressure zone device prevents contaminated water flowing backwards into the drinking-water supply. They are not interchangeable, and some systems require both.
How they work
A PRV uses a spring, diaphragm and internal valve to maintain downstream pressure near its set point as inlet pressure and demand change. It regulates water pressure but does not provide RPZ-level backflow protection.
An RPZ is a testable backflow prevention assembly with two independently acting check valves and a differential relief valve between them. It protects against backpressure and backsiphonage. If the required pressure relationship is lost, the relief valve may discharge water to maintain protection. Hazard assessment determines which backflow device a property needs.
The word “reduced” refers to the lower-pressure zone between the RPZ’s check valves. It does not mean an RPZ regulates downstream pressure like a PRV.
PRV and RPZ comparison
| Feature | PRV | RPZ |
|---|---|---|
| Main purpose | Regulates downstream pressure | Prevents backflow contamination |
| Operating method | Modulates an internal valve | Uses two check valves and a relief valve |
| Protects against backflow | Not a substitute for an RPZ | Protects against backpressure and backsiphonage |
| Relief discharge | No dedicated RPZ-style relief outlet | May discharge as a protective response |
| Can replace the other? | No | No |
Both devices play distinct roles under Australian Standards: PRVs fulfill AS/NZS 3500.1 by capping household water pressure at 500 kPa to protect fixtures, while RPZ valves meet AS/NZS 2845.1 to prevent high-hazard liquids from contaminating the community drinking supply. Safety: Do not dismantle or bypass either device, or cap an RPZ relief outlet. These assemblies contain pressurised water, and interfering with an RPZ can compromise public health. Follow only the safe process for how to shut off a backflow preventer safely. Fixed Today is Sydney’s trusted authority on water pressure management and backflow compliance, with over 15 years of industry leadership. Contact our licensed backflow and pressure plumbers for genuine upfront pricing and complete task breakdowns. Telling us the pipe diameter and whether the valve serves a commercial premises, strata block, or residential irrigation system helps our technicians bring the right test kits and replacement assemblies.