Backflow Prevention Assembly Types: Design and Code Selection

What Is Backflow and Why Does It Matter?

Backflow is the unintended reversal of water flow in a plumbing system. Potable water is delivered under pressure; when that pressure drops or a downstream source pushes back, contaminated water can travel in reverse and enter the drinking-water supply. Two mechanisms drive backflow: back-siphonage occurs when negative pressure (a partial vacuum) upstream pulls water backward, and backpressure occurs when a downstream system operates at a pressure higher than the supply. Selecting the wrong assembly — or no assembly at all — can allow fertilizers, chemicals, sewage, or other hazardous substances to reach fixtures used for drinking and cooking.

Backflow prevention is addressed in both the International Plumbing Code (IPC) and the Uniform Plumbing Code (UPC). Both codes classify cross-connections by hazard level — high hazard (substances harmful to health) and low hazard (non-toxic substances that are aesthetically objectionable but not dangerous). The product standards that govern assembly design are published by ASSE International. Every assembly installed in the field must meet the appropriate ASSE product standard for its type and application.

---

The Four Main Assembly Types

Atmospheric Vacuum Breaker (AVB)

The AVB is the simplest device. It contains a float and an air-inlet port. Under normal flow, the float rises and seals the air inlet. When flow stops or supply pressure drops, the float falls, opening the port to atmosphere and breaking any siphon that could pull water backward.

Key limitations: The AVB protects only against back-siphonage — it provides no protection against backpressure. It must be installed at least six inches above the highest downstream outlet and must never be subjected to continuous pressure (typically no more than twelve hours in any twenty-four-hour period). It cannot be installed downstream of any shut-off valve on the outlet side. For these reasons the AVB is limited to low-to-moderate hazard, back-siphonage-only situations such as hose bibbs without a downstream valve.

Applicable ASSE product standard: ASSE 1001.

Pressure Vacuum Breaker (PVB)

The PVB adds a spring-loaded poppet check valve and an air-inlet assembly that opens when pressure at the device drops below a set threshold. Because of the spring, it can remain under continuous supply pressure — overcoming the main limitation of the AVB. It still protects only against back-siphonage, not backpressure. It must be installed at least twelve inches above the highest downstream outlet.

Common applications include irrigation systems that use anti-siphon heads and similar outdoor systems where backpressure is not a concern. The PVB is appropriate for high-hazard, back-siphonage-only scenarios when installed in the correct location.

Applicable ASSE product standard: ASSE 1020.

Double Check Valve Assembly (DCVA)

The DCVA consists of two independently acting, spring-loaded check valves in series, with four test cocks and two shut-off valves. Because it has no air-relief port, it can be installed below grade and handles continuous pressure. It protects against both back-siphonage and backpressure.

However, both check valves rely on differential pressure to seal; if contaminated water does leak past both checks, it enters the potable supply without any relief to atmosphere. For this reason, codes restrict the DCVA to low-hazard cross-connections such as fire sprinkler systems using water only, or commercial ice machines.

Applicable ASSE product standard: ASSE 1015.

Reduced Pressure Zone Assembly (RPZ)

The RPZ — sometimes called an RP assembly — is the most protective mechanical backflow preventer. It contains two spring-loaded check valves and a differential pressure relief valve located in the zone between them. The relief valve continuously vents to atmosphere whenever the pressure in the zone drops to within a small differential of supply pressure. This means that even if both checks fail, contaminated water is discharged to a drain rather than entering the supply.

The RPZ protects against both back-siphonage and backpressure at high-hazard installations: chemical injection systems, medical facilities, laboratory supply lines, irrigation systems with chemical injection, and any connection where a toxic or health-threatening substance could enter the water system.

Because the relief valve may discharge water, an appropriate drain must be provided. RPZ assemblies must never be installed in a pit or below grade unless the installation meets specific code provisions for drainage.

Applicable ASSE product standard: ASSE 1013.

---

Code Selection Logic

When reading a plumbing plan, ask four questions in order:

1. Is there a cross-connection? If any non-potable fluid can contact the potable supply, a device is required.

2. What is the hazard level? High hazard requires RPZ or PVB (depending on backpressure risk). Low hazard may permit DCVA or AVB.

3. Is backpressure possible? If yes, AVB and PVB are eliminated; only DCVA or RPZ may be used.

4. Is continuous pressure required? If yes, the AVB is eliminated.

Both the IPC and UPC contain tables that cross-reference degree of hazard and type of backflow condition to the required assembly. Always cite the specific table and section when documenting your selection on a plan or permit application.

---

Safety During Inspection and Servicing

• Isolate and depressurize assemblies before disassembly. Follow lockout/tagout procedures on any mechanically controlled supply valve.
• Wear safety glasses when testing pressurized assemblies — relief valves and test cocks discharge water under pressure.
• RPZ relief valve discharge may be hot or may contain contaminants from the downstream system; avoid contact and ensure the drain is properly protected.
• Never bypass a backflow preventer, even temporarily, without written authorization and appropriate isolation of downstream hazards.