DCVA vs. RPZ: Understanding the Key Differences in Backflow Prevention

Protecting our clean water supply is paramount, and backflow prevention assemblies are critical devices in this effort. They stop potentially contaminated water from flowing backward into the potable water system. Two common, testable assemblies you'll frequently encounter are the Double Check Valve Assembly (DCVA) and the Reduced Pressure Zone (RPZ) backflow preventer.

While both prevent backflow, they are designed for different situations and offer varying levels of protection. Understanding the difference is crucial for ensuring water safety and meeting plumbing codes. So, what sets them apart?

Double Check Valve Assembly (DCVA): The Basics

As we've discussed previously, a DCVA consists of two independently acting, spring-loaded check valves in series.

• Mechanism: Water flows through the first check valve, then the second. If backpressure or backsiphonage occurs, both valves are designed to close, stopping the reverse flow.

• Protection Level: DCVAs provide reliable protection against backflow but are typically used in low to medium hazard situations. This means they protect against pollutants (substances that affect water aesthetics like color or taste but aren't a direct health threat) rather than contaminants (substances that can cause illness). Think irrigation systems (without chemical injection) or fire sprinkler lines.

• Key Feature: Redundancy. If one check valve fails or gets debris, the second one acts as a backup.

Reduced Pressure Zone (RPZ) Backflow Preventer: Higher Level Protection

An RPZ (sometimes called an RP, RPPA, or RPPZ) is considered the most reliable mechanical backflow preventer and offers a higher level of protection.

• Mechanism: Like a DCVA, an RPZ has two independently acting check valves. However, it adds a crucial third component: a differential pressure relief valve located in a "zone" between the two check valves. This relief valve is designed to maintain a lower pressure in the zone compared to the supply pressure.

• Protection Level: RPZs are required for high hazard situations where backflow could introduce toxic contaminants or substances posing a significant health risk into the drinking water. Examples include systems with chemical additives, hospital equipment, or commercial boilers.

• Key Feature: The relief valve. If either check valve leaks or fails, or if backpressure/backsiphonage conditions occur, the relief valve will open and discharge water to the outside. This discharge does two things: it provides a visual indication of a problem and, more importantly, creates an air gap, effectively preventing any chance of contaminated water flowing back into the supply.

The Crucial Differences Summarized

1. Hazard Level: DCVA for low/medium hazards (pollutants); RPZ for high hazards (contaminants/health risks).

2. Components: DCVA has two check valves; RPZ has two check valves plus a differential pressure relief valve.

3. Failure Mode: A failing DCVA might allow backflow without immediate external signs (until tested). A failing RPZ discharges water through the relief valve, providing an immediate alert and preventing contamination.

4. Installation: RPZs must be installed in locations where discharge water from the relief valve will not cause damage and can drain away properly.

   
   

Choosing the Right Device

The choice between a DCVA and an RPZ isn't arbitrary; it's dictated by local plumbing codes based on the potential degree of hazard associated with the cross-connection. Using the wrong device can compromise water safety and lead to code violations.Always consult with a certified backflow prevention tester or licensed plumber to determine the appropriate assembly for your specific needs and ensure it's installed and tested correctly.