How often does an RPZ assembly need to be tested?

RPZ assemblies must be tested annually by a certified backflow prevention tester. Testing verifies that both check valves are functioning properly and that the relief valve opens at the correct differential pressure. Test results must be submitted to the governing water authority.

What happens when an RPZ assembly fails a test?

If an RPZ assembly fails a field test, it must be repaired or replaced before being returned to service. The certified tester will issue a failure report identifying the specific component(s) that failed. Most water authorities require repair and retest within 30 days of the failure report.

Reduced Pressure Zone Assembly (RPZ) — Backflow Prevention

Q: When is an RPZ assembly required?

RPZ assemblies are required by water authorities for severe-hazard connections where contamination of the potable water supply could pose a health risk. Common applications include medical and dental offices, food processing, chemical injection systems, car washes, industrial process water connections, and irrigation systems with fertilizer or pesticide injection.

Technical Specification

CSA Standard: CSA B64.4 — Reduced Pressure Principle Backflow Preventers
Also listed as: ASSE 1013 (US); USC FCCCHR (Foundation for Cross-Connection Control)
Protection Level: Highest — protects against both backpressure and back-siphonage
Hazard Classification: Severe-hazard connections
Testing Standard: Field test per manufacturer procedure with calibrated differential pressure gauge; results reported to water authority
Testing Frequency: Annual minimum; more frequently if the water authority requires or after repair
Installation Requirement: Must be installed above grade, accessible, with full-bore shut-off valves and test cocks

CSA B64.4 — Canadian Standards Association; ASSE 1013 — American Society of Sanitary Engineering

How the RPZ Works

The RPZ assembly contains three key components operating in series:

First check valve — Spring-loaded; closes to prevent backflow from the downstream side
Reduced pressure zone (middle zone) — A hydraulically isolated zone maintained at a pressure lower than the supply pressure by at least 2 psi (14 kPa)
Relief valve — Mechanically independent; opens and discharges to atmosphere if the pressure in the zone rises to within 2 psi of supply pressure (indicating check valve failure), preventing contaminated water from reaching the supply
Second check valve — Downstream spring-loaded check valve providing an additional barrier

The relief valve is the critical safety feature. If the first check valve fails while the second check valve holds, the relief valve will open and discharge rather than allow backflow. If the relief valve is discharging continuously during normal operation, this indicates a failed first check valve and the assembly requires immediate service.

Required Applications

Water authorities require RPZ assemblies for any connection classified as severe-hazard, where the substance connected to the water system could cause illness, injury, or death if it contaminated the potable supply. Common commercial applications include:

Medical and dental offices — Autoclaves, sterilizers, dental chair water lines, procedure room equipment
Restaurants and food service — Carbonated beverage dispensers (CO₂ connections), commercial dishwashers, pre-rinse stations
Industrial / manufacturing — Process water connections, chemical storage connections, cooling towers with biocide treatment, boiler feed with chemical additives
Car washes — Detergent and chemical injection systems
Irrigation systems with chemical injection — Fertilizer, herbicide, or pesticide injection elevates irrigation from minor to severe hazard
Healthcare facilities — Laboratory water connections, medical gas systems
Funeral homes — Embalming room connections
Veterinary clinics — Surgical suite and grooming water connections

Annual Testing Procedure

RPZ assemblies are tested using a calibrated differential pressure test gauge connected to the test cocks installed on the assembly. The test verifies:

Both check valves are seated properly and not leaking
The relief valve opens at the correct differential (typically 2 psi / 14 kPa or greater difference between supply and zone pressure)
The relief valve closes tightly when the differential is restored
The shut-off valves are operational

A passed test is documented with specific pressure readings for each check valve and the relief valve opening differential. This documentation is submitted to the governing water authority and provided to the property owner.

Check valve seats, disc assemblies, and O-rings degrade over time with no visible external sign. An assembly that looks intact and has not been disturbed may no longer hold the required differential pressure — a condition undetectable without a field test. Annual testing with a calibrated gauge is the only reliable method to confirm that internal components remain functional between inspection cycles.

Common Failure Modes

Failure	Symptom	Action Required
First check valve fails closed / leaking	Low differential across first check; relief valve discharges	Repair or replace first check module
Second check valve fails	Low differential across second check	Repair or replace second check module
Relief valve fails to open	Relief differential test fails — won't open at required threshold	Replace relief valve module
Relief valve fails to close (leaking)	Continuous dripping from relief valve during normal supply conditions	Clean seat or replace relief valve module
Debris fouling	Erratic test readings; relief valve cycling	Disassemble, clean, inspect seats and discs

Frequently Asked Questions — RPZ Assembly

What is a Reduced Pressure Zone (RPZ) assembly?

A Reduced Pressure Zone assembly is a high-protection backflow preventer consisting of two independently operating spring-loaded check valves with a hydraulically operated, mechanically independent pressure differential relief valve located in the zone between the two check valves. It is listed under ASSE 1013 and is required for severe-hazard cross-connection control applications.

When is an RPZ assembly required instead of a DCVA?

Water authorities require an RPZ when the connection presents a severe cross-connection hazard — where contaminants could cause illness or injury if they entered the potable supply. DCVAs are used for moderate-hazard connections where non-health-threatening substances (like system water in a multi-unit residential building) present the risk. The authority makes the hazard classification decision.

Can an RPZ assembly be installed indoors?

Yes, but the relief valve discharges water when activated, so indoor installations require a drain connection capable of handling the full flow of the assembly. Floor drains must not be connected to the sanitary system in a way that would create a cross-connection at the drain. Many water authorities have specific installation requirements for indoor RPZ assemblies.

How often does an RPZ need to be tested?

Annual testing is the minimum required by all Canadian municipal water authorities. RPZ assemblies should also be tested after any repair, after any backflow event or suspected contamination, and after the assembly has been shut off and drained (as this can displace debris).

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Reduced Pressure Zone Assembly (RPZ / RPZA)