RPZ Backflow Preventer Explained: How a Reduced Pressure Zone Assembly Works
An RPZ uses two check valves and a relief valve to stop backflow. How it works, why it discharges water, and California's RP installation rules.
Short answer: An RPZ (reduced pressure zone) backflow preventer, called a reduced pressure principle backflow prevention assembly (RP) in California rules, is a valve assembly with two independently acting check valves and a differential pressure relief valve between them. If either check valve leaks, the relief valve can open and discharge water to the atmosphere instead of letting it flow backward into the public water supply. That design is why an RP, along with an air gap, is one of the two methods California accepts to contain high hazard cross-connections (CCCPH 3.2.2(c)).
This guide covers how an RPZ works, why it sometimes drips or dumps water, where California requires one, how it must be installed, and how it is tested.
RPZ, RP, RPBA: the same assembly
You will see several names for the same thing:
- RPZ or RPZA: reduced pressure zone assembly, a common trade name.
- RP: the term used in the State Water Board’s Cross-Connection Control Policy Handbook (CCCPH).
- Reduced pressure principle backflow prevention assembly: the full name in both the CCCPH and the California Plumbing Code.
The CCCPH defines it as an assembly with two independently acting internally loaded check valves, with a hydraulically operating, mechanically independent differential-pressure relief valve located between the check valves and below the upstream check valve. It must have shutoff valves upstream and downstream of the checks and test cocks for accurate field testing (CCCPH 3.1.1). The California Plumbing Code describes the same parts: two check valves, a differential pressure relief valve, four test cocks, and two isolation valves (CPC 603.3.7).
How an RPZ works
Think of the assembly as three chambers in a row.
- Inlet side. Water enters from the public supply through the first shutoff valve.
- The zone. Between the first and second check valves is the “reduced pressure zone.” The first check valve is internally loaded, so water passing it loses some pressure. That keeps the zone at a lower pressure than the supply.
- Outlet side. Water passes the second check valve and the downstream shutoff valve into the property.
The relief valve sits on the zone and senses the difference between supply pressure and zone pressure. While that difference holds, the relief valve stays closed and water flows normally.
If the supply pressure drops (backsiphonage), or if property-side pressure pushes back through a leaking second check (backpressure), the pressure difference shrinks. The relief valve opens and dumps water from the zone to the atmosphere. Water leaves the assembly instead of returning to the main.
Because it protects against both backsiphonage and backpressure, the California Plumbing Code rates the RP for both pollution (low hazard) and contamination (high hazard) conditions (CPC Table 603.2).
Why is my RPZ leaking or dumping water?
The California Plumbing Code notes plainly that an RP “may discharge water” (CPC Table 603.2). Discharge from the relief valve is part of the design, but steady or heavy discharge is a signal that something needs attention. Possible reasons include:
- A fouled or worn first check valve that no longer holds the zone at reduced pressure.
- A fouled or worn second check valve that lets downstream water back into the zone.
- A problem in the relief valve itself.
- Pressure fluctuations in the supply that briefly change the pressure difference.
These are the same components a certified tester checks during the annual test, and the components the CCCPH’s tester performance exam requires testers to diagnose (CCCPH 3.4.1(b)(2)). If your RP is discharging, have it tested. Do not plug the relief port. That defeats the protection.
Where California requires an RP
The CCCPH requires a water system to protect its distribution system from high hazard cross-connections through premises containment with an air gap or an RP, unless the handbook specifies otherwise (CCCPH 3.2.2(c)). A high hazard cross-connection is one that poses a threat to the potability or safety of the public water supply (CCCPH 3.1.1).
The handbook’s Appendix D lists premises that require an AG or RP unless noted otherwise. Examples:
| Category | Examples from CCCPH Appendix D |
|---|---|
| Health care | Medical facilities, kidney dialysis facilities, dental offices with water-connected equipment, veterinarian facilities |
| Commercial | Car washes, dry cleaners, commercial laundries, hotels and motels, gas stations, pet groomers |
| Industrial | Chemical plants, laboratories, biotech, electronics manufacturing, metal plating, petroleum processing or storage |
| Multi-tenant | Business parks on a single meter, mobile home parks, RV parks, campgrounds with RV hookups |
| Other | Mortuaries, cemeteries, fire stations, airports, agricultural premises, temporary hydrant connections for construction |
Some Appendix D items require an air gap only, such as recycled water interconnected with piping that contains water from the public system, unless an alternative is approved (CCCPH Appendix D and 3.2.2(c)).
Fire systems. A high hazard fire protection system, including one that may use chemical additives (wetting agents, foam, antifreeze, corrosion inhibitors) or has an auxiliary water supply, must have no less than RP protection (CCCPH 3.2.2(e)(1)). The California Plumbing Code similarly calls for an RP or an RP detector assembly where chemicals are added to a fire system (CPC 603.5.14.2). On fire lines, an RPDA or RPDA-II may be substituted where an RP is required (CCCPH 3.2.2(a)).
Irrigation with pumps. Where a sprinkler or irrigation system has pumps, connections for pumping equipment, or auxiliary air tanks, or can otherwise create backpressure, the California Plumbing Code requires an RP upstream of the source of backpressure (CPC 603.5.6.1).
RPZ vs. double check valve
| Feature | RP (RPZ) | Double check (DC) |
|---|---|---|
| Check valves | Two | Two |
| Relief valve | Yes, between the checks | No |
| Hazard level allowed | High or low | Low only (CCCPH 3.1.1) |
| May discharge water | Yes (CPC Table 603.2) | No, “Does not discharge water” (CPC Table 603.2) |
| Annual field test | Required | Required |
The relief valve is the difference. A DC has nothing to warn you or dump water if both checks foul. That is why the CCCPH allows a DC only for low hazard isolation. Full comparison: double check valve assembly explained.
California installation rules for an RP
The CCCPH sets installation criteria for assemblies that protect a public water system:
- Height above grade: The lowest point of the assembly must be at least 12 inches above grade and, unless the water system approves an alternative, no more than 36 inches above finished grade (CCCPH 3.3.2(b)).
- Side clearance: An RP installed after the CCCPH was adopted needs at least 12 inches of side clearance, and at least 24 inches on the side with the test cocks, unless the water system approves alternate clearances that still allow testing and maintenance (CCCPH 3.3.2(f)).
- Location: As close as practical to the user’s service connection, unless the water system approves another location (CCCPH 3.3.2(g)).
- Access: Accessible for field testing, inspection, and maintenance (CCCPH 3.3.2(h)).
- Approval: Each new or replacement RP must be approved under the USC Foundation for Cross-Connection Control and Hydraulic Research standards or ASSE International standards, including ASSE 1013-2021 for the RP and ASSE 1047-2021 for the RPDA, with the 1YT mark where ASSE is used (CCCPH 3.3.1(b)).
The California Plumbing Code adds that backflow devices with atmospheric vents or ports must not be installed in pits, underground, or submerged locations (CPC 603.4.9), and that drain lines serving backflow assemblies must be sized for the manufacturer’s discharge rates (CPC 603.4.8). Plan for where relief valve discharge will go.
How an RPZ is tested
An RP must be field tested by a certified backflow prevention assembly tester after installation, after repair, after depressurization for winterizing, after permanent relocation, and at least annually (CCCPH 3.3.3(a) and (b)). A water system must receive a passing test before providing continuous service to a newly installed assembly (CCCPH 3.3.3(d)).
The tester uses a differential pressure gauge on the test cocks to confirm that the relief valve opens when it should and that each check valve holds. Testers follow procedures in the USC Manual of Cross-Connection Control, Tenth Edition, or an equivalent (CCCPH 3.4.1(b)(1)(B)(4)). If the RP fails, it must be repaired or replaced within 30 days of notification (CCCPH 3.3.3(e)). See what happens if a backflow test fails.
Frequently asked questions
What does RPZ stand for? Reduced pressure zone. California rules call it a reduced pressure principle backflow prevention assembly, or RP.
What is the difference between an RPZ and a double check valve? Both have two check valves, but the RPZ adds a differential pressure relief valve between them. That allows the RPZ to be used for high hazard connections. A double check may only isolate low hazard connections (CCCPH 3.1.1).
Is it normal for an RPZ to leak water? An RP is designed so that it may discharge water through its relief valve (CPC Table 603.2). Frequent or continuous discharge usually means a check valve or the relief valve needs service, so have it tested.
Where is an RPZ required in California? At high hazard premises, such as medical and dental offices, car washes, laboratories, and sites with an auxiliary water supply, unless an air gap is used instead (CCCPH 3.2.2(c) and Appendix D). High hazard fire systems also need at least RP protection (CCCPH 3.2.2(e)(1)).
Can an RPZ be installed below ground? No, not under the standard rules. The CCCPH requires the lowest point to be at least 12 inches above grade (CCCPH 3.3.2(b)), and the California Plumbing Code prohibits devices with atmospheric vents in pits or underground (CPC 603.4.9).
How often does an RPZ need to be tested? At least once a year, plus after installation, repair, relocation, or winterizing (CCCPH 3.3.3). Your water supplier can require more frequent testing for high hazard premises.
What is an RPDA? A reduced pressure principle detector assembly: an RP with a bypass containing a water meter and a smaller RP, used mostly on fire lines (CCCPH 3.1.1).
Last reviewed: October 30, 2026.
Sources
- State Water Board, Cross-Connection Control Policy Handbook, April 2026 revision (PDF): sections 3.1.1, 3.2.2, 3.3.1, 3.3.2, 3.3.3, 3.4.1, and Appendix D
- State Water Board, Cross-Connection Control Policy Handbook page
- IAPMO, 2025 California Plumbing Code (read-only edition): sections 603.3.7, 603.4.8, 603.4.9, 603.5.6.1, 603.5.14.2, and Table 603.2
Need your RPZ tested or reviewed? Call (805) 991-7333 or request a review.