An RPZ, or reduced pressure zone assembly, is a backflow prevention device that uses two independent check valves and a relief valve to keep contaminated water from flowing backward into the public drinking water supply. Testing an RPZ means a certified tester connects a calibrated gauge kit to the assembly's test cocks and verifies each internal component is holding pressure and opening correctly under specific conditions.
What an RPZ Assembly Does
RPZ assemblies are typically installed on connections considered higher-hazard, such as irrigation systems, fire sprinkler lines with chemical additives, commercial boilers, or industrial processes where a cross-connection could let contaminants enter drinking water. The design maintains a zone of reduced pressure between the two check valves; if either check valve fails, the relief valve is designed to open and discharge water rather than allow contaminated water to be pulled backward into the supply. The EPA's cross-connection control guidance covers why these higher-hazard connections are subject to closer oversight than simpler backflow scenarios.
How the Test Works, Step by Step
A certified tester generally follows a sequence like this:
Isolate the assembly. Shutoff valves on either side of the RPZ are closed to isolate it from the rest of the system for testing.
Connect the gauge kit. A calibrated differential pressure gauge is connected to the test cocks located on the assembly.
Check the first check valve. The tester verifies the first check valve holds the required pressure differential.
Check the second check valve. The same process is repeated for the second check valve.
Check the relief valve. The tester confirms the relief valve opens at the correct pressure point, which is what protects the system if either check valve fails.
Restore the assembly. Once testing is complete, valves are returned to their normal operating position and the assembly resumes normal service.
The entire process usually takes well under an hour for a single accessible assembly, though vaults, elevated mounting, or hard-to-reach commercial installations can add time.
Pass vs. Fail
A "pass" means all components performed within the required specifications. A "fail" means at least one component — most often a check valve not holding pressure or a relief valve not opening at the correct point — didn't meet spec. A failed RPZ typically needs repair or rebuild before it can be recertified; see our guide on what happens after a failed backflow test for next steps. Either way, the tester should complete a report documenting the results, which usually needs to be filed with your water utility — see backflow test report filing for that process.
RPZ vs. Double Check Assemblies
RPZ assemblies are generally required for higher-hazard connections, while double check valve assemblies (which lack the relief valve) are often accepted for lower-hazard applications. Which type your property needs is determined by your local water utility or cross-connection control program based on the hazard level of your connection — this isn't something a property owner decides independently. If you're unsure which assembly type is installed or required at your property, your water utility's cross-connection control division can tell you.
A Caution on DIY Testing
RPZ testing requires specialized calibrated equipment and training to interpret results correctly — it is not a DIY task. An improperly performed test can produce a false pass, leaving a failing device in service and undermining the protection it's meant to provide. Most water utilities require the report to come from a tester holding recognized certification, which rules out self-testing regardless of a property owner's comfort level with plumbing.
FAQs
How long does an RPZ test take? Typically well under an hour for one accessible assembly, though access issues or multiple devices extend the visit.
Does RPZ testing require shutting off my water? Yes, briefly — the assembly is isolated during testing, so water to whatever it protects (like an irrigation zone) is interrupted for the duration of the test, though this usually doesn't affect the rest of the building's water supply.
Why does my commercial building have an RPZ instead of a simpler device? Your water utility's cross-connection control program determines the required assembly type based on the hazard level of your specific connection, not on convenience or cost.
How to Choose and Verify a Backflow Testing Company
RPZ testing should only be performed by a tester currently certified — typically to the ASSE 5110 standard or your state's equivalent — and listed on your water utility's approved tester roster. Confirm their gauge kit has a current calibration certificate, since results from uncalibrated equipment can be rejected by the utility.
This directory is an independent, source-backed listing of public records. It does not certify or endorse any backflow testing company. Verify certification and approved-list status directly with your local water utility before scheduling.