PRV vs RPZ valve: what's the difference?

A properly selected and functioning PRV reduces inlet pressure toward its outlet setting within its rated operating range. Outlet pressure can vary with demand, and a regulator cannot compensate for inadequate inlet pressure. The plumbing code sets the ceiling. Section 604.8 of the 2024 International Plumbing Code, one of the two plumbing codes Phoenix lets designers use, says: "Where water pressure within a building exceeds 80 psi (552 kPa) static, an approved water pressure-reducing valve conforming to ASSE 1003 or CSA B356 with strainer shall be installed to reduce the pressure in the building water distribution piping to not greater than 80 psi (552 kPa) static."

The EPA's WaterSense program explains why that matters. It says water pressure in supply mains "can reach 100 pounds per square inch (psi) or more," and that high pressure "can damage plumbing piping, fixtures, and appliances, and can lead to increased water consumption, premature equipment failure, and risk of leaks." WaterSense recommends setting the PRV "to limit pressure to between 45 and 60 psi."

Inside, most PRVs work the same way. WaterSense describes the common direct-acting valve as "a globe-type body with a spring-loaded, heat-resistant diaphragm." Turning the adjusting screw on top changes the spring tension, and that changes the outlet pressure. Two other code details are worth knowing:

• The code specifies a failure design. Section 604.8.1 says the valve "shall be designed to remain open to permit uninterrupted water flow in case of valve failure." That requirement does not diagnose every malfunction. Debris, wear or a damaged regulator can cause pressure creep or restricted flow; measure inlet and outlet pressure before deciding what failed.
• It can be serviced in place. Section 604.8.2 requires that PRVs and strainers be built and installed "to permit repair or removal of parts without breaking a pipeline or removing the valve and strainer from the pipeline."

For more on the home side, see whether you need a pressure reducing valve and what home water pressure should be.

An RPZ stops backflow, which is water reversing direction and pulling contamination from inside a building into the drinking water system. Backflow happens two ways. Backsiphonage is when street pressure drops, from a main break or a hydrant opened nearby, and pulls water backward. Backpressure is when something in the building, like a boiler, pump, or pressurized tank, pushes harder than the city supply.

Phoenix City Code defines the assembly this way: "An assembly containing two independently acting approved checkvalves together with a hydraulically operating, mechanically independent pressure differential relief valve located between the checkvalves and at the same time below the first checkvalve." In other words:

1. 1Water passes through a first check valve and then a second one.
2. 2The space between them, the zone, is kept at lower pressure than the supply side.
3. 3The relief valve responds to the pressure difference between the inlet and the zone. It can discharge when that difference falls too low, including some check-valve or supply-pressure conditions. A failed assembly must be tested and repaired; discharge is not proof that every component works.

Watts' spec sheet for its LF009 series describes the same layout, "two in-line independent check valves, captured springs, and replaceable check seats with an intermediate relief valve." Its specification says an RPZ "shall be installed at each potential health hazard location to prevent backflow due to backsiphonage and/or backpressure."

That relief valve is why an RPZ sometimes spits water, and why it needs a drain. The IPC says in Section 608.14.2 that the RPZ's "relief opening shall discharge by air gap and shall be prevented from being submerged." If yours is dripping or dumping, see why an RPZ backflow preventer spits water.

The one-line version: the PRV is about how much pressure comes in, the RPZ is about which direction the water goes.

A PRV is required by pressure. An RPZ is required by hazard.

PRV. Any building whose static pressure is over 80 psi needs one under IPC 604.8. The code has one exception, for "service lines to sill cocks and outside hydrants, and main supply risers where pressure from the mains is reduced to 80 psi (552 kPa) or less at individual fixtures." In practice that means a building can leave the hose bibs on full street pressure while the pressure inside is reduced. WaterSense adds that you should "CONFIRM that service pressure does not exceed 80 psi by consulting with the public water supplier."

RPZ. The IPC's Table 608.1 lists the reduced pressure principle assembly for "High or low hazard" and for both "Backpressure or backsiphonage," in sizes from 1/4 inch through 16 inches. Phoenix City Code 37-144 then lists facilities that need approved backflow protection at the water service, and many of them allow only an air gap or an RP. Examples from that list:

• "Supermarkets, butcher shops, and restaurants (air gap or RP)."
• "Carwash facilities (air gap or RP)."
• "Hospitals, medical buildings, sanitariums, morgues, mortuaries, autopsy facilities, nursing and convalescent homes and clinics (air gap or RP)."
• "Retail shopping centers and strip malls; retail and industrial shell buildings (when one service supplies more than one tenant) (RP)."
• Irrigation "using a chemical injection system (RP)."

A double check assembly is the lighter option for low-hazard connections. Our page on RPZ vs double-check backflow assemblies explains that choice.

### Who has to test the RPZ?

In Phoenix, the building owner does. City Code 37-143 says the customer "shall test all backflow prevention assemblies at least once a year," that "Assembly testing must be performed by a certified tester," and that repairs "must be performed by a certified repairer." The IPC agrees in Section 312.11.2: RPZs "shall be tested at the time of installation, immediately after repairs or relocation and at least annually." A PRV has no test requirement, but a pressure gauge on a hose bibb tells you if it is still holding its setting. See what happens if you miss a backflow test deadline in Phoenix.

Because a building can have high street pressure and a hazard that needs backflow protection. Take a restaurant on a high-pressure main. It needs the RPZ for its kitchen hazards. It needs the PRV so the dish machine, ice machine, and fixtures stay at or under 80 psi.

For the order on the service line, Phoenix City Code 37-144 wants the backflow assembly "as close as practicable to the point of service delivery," though the customer may ask for a different location. The PRV typically sits downstream of the backflow assembly, ahead of the building's distribution piping. Your permit drawings and the city's approved detail control the exact layout.

### What about thermal expansion?

An RPZ's check valves prevent reverse flow. A PRV can also restrict reverse expansion flow, although some models have a bypass that works only under specified pressure conditions. Other checks or backflow devices can still close that path. Have the plumber assess the actual layout and the regulator instructions. The IPC addresses this in Section 607.3: "Where a storage water heater is supplied with cold water that passes through a check valve, pressure reducing valve or backflow preventer, a thermal expansion control device shall be connected to the water heater cold water supply pipe." WaterSense flags the same point for PRVs, noting thermal expansion "can occur from installing a PRV since it creates a closed system." Provide the required expansion control according to the applicable code and equipment instructions. Uncontrolled expansion can raise system pressure, sometimes causing relief-valve discharge, but a drip alone does not establish the cause and not every heating cycle produces the same symptom.

### How do you tell which valve you have?

Look at the shape and the fittings:

• An RPZ is usually a long, level assembly with a shutoff valve on each end. It has several small test cocks (little brass plugs or ball valves) along the side. A relief port sits at the bottom, often over a funnel-shaped air gap drain. Most commercial RPZs sit above ground near the meter, some in a cage.
• A PRV is a single bell-shaped valve, often with an adjusting bolt and locknut on top. It has no test cocks and no relief port.

If the valve has a tag, the ASSE number tells you for sure: 1003 is a PRV, 1013 is an RPZ.

HQ Plumbing & Air installs and replaces pressure reducing valves and backflow assemblies for businesses and homes across the Phoenix area, and adds thermal expansion control where the code calls for it. HQ also tests and certifies domestic and fire backflow assemblies and files the results with the city. These assembly services are separate from fire-sprinkler-system work. We're licensed and insured, with free estimates and upfront pricing. See our commercial plumbing services.

The PRV sets how hard the water pushes. The RPZ makes sure it only goes one way.