When to use: Every cross-connection between potable water and a source of contamination must be protected. The correct assembly depends on three factors: the degree of hazard (high = toxic contaminant, low = non-toxic pollutant), the backflow mechanism (backsiphonage vs. backpressure), and whether the device is under continuous pressure. Pick an application preset or set conditions manually.
High-hazard backsiphonage under continuous pressure calls for a Pressure (or Spill-Resistant) Vacuum Breaker. An RPZ also works if backpressure could later develop.
An air-inlet (vacuum breaker) plus a single check valve protects against BACKSIPHONAGE only — including HIGH-hazard backsiphonage. Rated for continuous pressure, but it CANNOT be used where backpressure can occur.
Field-tested at installation and annually — verify the air inlet opens and the check valve holds (ASSE 1020).
Install at least 12 in above the highest downstream outlet/head. The air inlet spits water when it opens — do not install indoors over finished space without drainage. Common on irrigation systems.
Cross-connection control and backflow prevention protect the public potable water supply from contamination by non-potable water or other substances that can enter through unprotected cross-connections. A cross-connection is any actual or potential link between the potable water system and any source of contamination — a boiler, irrigation line, industrial process, or even a submerged hose end. Backflow occurs when water flows in the reverse of its intended direction, either by backsiphonage (negative pressure in the supply) or backpressure (elevated downstream pressure). The correct backflow preventer depends on the degree of hazard, the backflow mechanism, and the pressure conditions.
Cross-connections are created whenever potable water piping is connected to equipment or systems that contain non-potable substances. Common examples include: garden hoses submerged in a chemical tank or pool, boiler makeup water lines, medical gas equipment, commercial dishwashers with chemical feed, irrigation systems with fertilizer injectors, and industrial cooling towers.
Backflow from these connections can occur via two mechanisms. Backsiphonage occurs when the supply pressure drops below atmospheric — for example, when a fire department draws heavily from a main, when a water main breaks, or during water service interruption. The negative pressure "sucks" contaminated water back into the supply. Backpressure occurs when the downstream system pressure exceeds the supply pressure — for example, a boiler under steam pressure, a pump boosting circulation pressure, or an elevated storage tank.
The University of Southern California Foundation for Cross-Connection Control and Hydraulic Research (USC FCCCHR) Manual of Cross-Connection Control classifies backflow hazards by the degree of risk to human health. A high health hazard (also called a contaminant) involves a substance that could cause illness, injury, or death if introduced into the potable supply — examples include toxic chemicals, sewage, pathogenic organisms, and radioactive materials. A low health hazard (also called a pollutant) involves a substance that is objectionable or aesthetically unacceptable but not toxic — examples include plain water that has become stagnant, food-grade fluids, or non-toxic industrial water.
Hazard classification governs which backflow prevention assemblies are acceptable. High hazard connections require the most protective devices (air gap or RPZ). Low hazard connections can use less restrictive devices such as a double check valve assembly (DCVA) that would not be acceptable for high hazard conditions.
The backflow mechanism is the second key variable in device selection because certain assemblies protect against only one mechanism. Vacuum breakers (AVB, PVB, SVB) protect against backsiphonage by opening an air vent that breaks the siphon when supply pressure drops, but they are not rated for backpressure — elevated downstream pressure forces the air inlet closed and can drive contaminated water through the device. They must never be installed where backpressure can develop.
Double check valve assemblies (DCVA) and reduced pressure zone assemblies (RPZ) resist both backsiphonage and backpressure because they use spring-loaded check valves. The RPZ adds a hydraulically dependent relief valve between the two checks: if either check valve leaks, the zone pressure drops and the relief valve opens to spill water rather than allowing backflow into the supply. This makes the RPZ the most protective mechanical assembly available for high-hazard, continuous-pressure applications.
Air Gap: The physical separation between the supply outlet and the flood rim of the receiving vessel. Provides absolute protection — no backflow of any type can occur. Requires the downstream water to be re-pumped. Minimum gap = 2× the pipe diameter (minimum 1 inch).
RPZ (Reduced Pressure Zone Assembly): Two independent check valves plus a differential relief valve. Rated for high hazard, both backflow types, continuous pressure. The gold standard mechanical device. Must be installed above grade with drain provisions for the relief valve.
DCVA (Double Check Valve Assembly): Two spring-loaded check valves in series. Rated for low hazard, both backflow types, continuous pressure. Cannot be used for high hazard (toxic) conditions.
PVB (Pressure Vacuum Breaker): Air inlet plus check valve. High or low hazard, backsiphonage only, continuous pressure. Must be installed 12 inches above the highest downstream outlet.
SVB (Spill-Resistant Vacuum Breaker): Similar to PVB but minimizes spillage. Backsiphonage only, continuous pressure.
AVB (Atmospheric Vacuum Breaker): Simplest and least expensive. Backsiphonage only, no continuous pressure (max 12 hours), no downstream valves permitted.
Both the RPZ (Reduced Pressure Zone Assembly) and the DCVA (Double Check Valve Assembly) use two spring-loaded check valves and protect against both backsiphonage and backpressure under continuous pressure. The critical difference is that the RPZ adds a hydraulically dependent relief valve between the two check valves. If the first or second check valve leaks, the zone pressure between the checks drops, the differential relief valve opens, and the assembly spills water to atmosphere rather than allowing the contaminated downstream water to reach the supply. This failsafe mechanism makes the RPZ the required device for high health hazard applications. The DCVA — without the relief valve — is only approved for low health hazard (non-toxic) conditions.
An air gap is required when the hazard is so severe that no mechanical device is considered adequate — for example, connections to raw sewage, highly toxic industrial processes, radioactive materials, or substances classified as immediate health hazards. It is also required when the downstream system cannot maintain pressure (an open atmospheric tank by definition has no backpressure) and the authority having jurisdiction (AHJ) or the local water purveyor mandates it. Many water purveyors in the U.S. require air gaps for all direct connections to cooling towers and medical equipment regardless of whether an RPZ would technically comply with the model code.
A Pressure Vacuum Breaker (PVB) is rated for continuous pressure service — meaning the supply side can be under pressure at all times. The name is sometimes confusing: "pressure" refers to the supply pressure, and "vacuum breaker" refers to the air inlet that opens when supply pressure drops below atmospheric. What the PVB cannot handle is backpressure — elevated pressure on the downstream (protected) side. If downstream pressure exceeds supply pressure, the check valve in the PVB closes but the air inlet may also be forced closed, leaving no protection against the downstream pressure driving contaminated water into the supply. Always confirm that no pump, elevated tank, or steam source exists downstream of a PVB.
Testable backflow prevention assemblies (RPZ, DCVA, PVB, SVB) must be tested by a certified backflow prevention assembly tester (BPAT). Most states and many local jurisdictions require testers to hold a certification from an approved program — common credentials include ASSE 5110, AWWA certification programs, and various state-specific licenses. The tester must use calibrated differential pressure gauge equipment (a DP gauge or test kit) to verify that each check valve holds the required minimum differential pressure and that the RPZ relief valve opens at the proper threshold. Test reports must be filed with the local water purveyor or AHJ and retained on-site.
Annual testing by a certified tester is the standard requirement under IPC Section 608, most state plumbing codes, and local water purveyor regulations. Some high-hazard installations require semi-annual or even quarterly testing (medical facilities, food processing). Testing must also be performed at installation (prior to service), after any repair or relocation, and following any system modification that could affect the assembly. The local water purveyor's cross-connection control program governs the specific testing frequency for each type of installation — always verify requirements with the AHJ and the purveyor before specifying an assembly.
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