What Is an RPZ Backflow Preventer?

A reduced pressure zone (RPZ) assembly is one of the most widely used backflow preventers for higher-hazard commercial connections. If you've seen "RPZ" on a plumbing inspection report or heard a technician mention it, here's what the device actually does.

How an RPZ Assembly Works

An RPZ assembly uses two independently operating check valves with a relief valve in between them, in a reduced-pressure zone. Under normal operation, the zone between the two check valves stays at a lower pressure than the supply line. If either check valve fails or pressure conditions change enough to allow backflow, the relief valve opens and discharges water rather than letting contamination reach the clean supply.

Why RPZ Assemblies Are Specified for Higher-Hazard Connections

The relief valve is the difference. A double check valve assembly relies entirely on two check valves holding, with no backup if one fails. An RPZ assembly's relief valve gives it a second line of defense: if either check valve fails, the relief valve discharges water rather than allowing contamination back into the supply. RPZ and double check valve assemblies both guard against backpressure (contamination pushed backward by downstream pressure) as well as backsiphonage; devices like a pressure vacuum breaker only guard against backsiphonage. That combination, backpressure protection plus the relief valve's backup, is why water authorities generally require an RPZ, rather than a lower-hazard device, on connections where a failure would pose a serious health risk.

How Is an RPZ Assembly Installed?

Because an RPZ assembly discharges water when its relief valve opens, it needs a code-compliant air gap or drain at the discharge point and clearance for that discharge to go somewhere safe, considerations that have to be planned for at installation, not added afterward. See commercial backflow installation for how that process works.

How Do You Maintain an RPZ Assembly?

An RPZ assembly should be tested on the schedule your local water authority requires, and a relief valve that's discharging outside of a test (continuous or intermittent) is a sign the device needs service, not something to wait out. See commercial backflow testing for what a test involves, or commercial backflow repair if yours is already showing a problem.

How Roto-Rooter Can Help

Roto-Rooter installs new RPZ assemblies, tests devices already in place, and repairs ones that have failed. Visit our commercial backflow services page for the full range of testing, installation, and repair services, or talk to a technician about your RPZ assembly directly.