PVB Backflow Preventers: Understanding When They Are the Right Choice

Protecting our potable water from contamination is a critical task, and various backflow prevention devices are employed to achieve this. Among the simpler, yet effective, options is the Pressure Vacuum Breaker (PVB).

But it's not a one-size-fits-all solution. Knowing when a PVB is the appropriate choice is essential for effective backflow prevention and code compliance.

So, under what specific circumstances should a PVB be used?What is a Pressure Vacuum Breaker (PVB)?First, let's quickly define it.

A PVB is a mechanical backflow preventer consisting primarily of a single check valve spring-loaded closed and an air inlet valve spring-loaded open. It's designed to protect drinking water systems from contamination specifically caused by backsiphonage.

Key Conditions for Using a PVB

A PVB is the appropriate device only when all the following conditions are met:

1. Threat is Backsiphonage Only: This is the most crucial point. PVBs are designed solely to prevent water from being siphoned backward due to a drop in supply pressure (like a water main break). They do not protect against backpressure (when downstream pressure becomes higher than supply pressure). If backpressure is a potential risk, a different device like a Double Check Valve Assembly (DCVA) or Reduced Pressure Zone (RPZ) assembly is required.

2. Low Hazard Application: PVBs are suitable for applications where the potential backflow involves pollutants (substances affecting water taste or odor) rather than contaminants (substances posing a health risk). They are generally not acceptable for high-hazard situations.

3. Installation Height Requirement: A PVB must be installed at least 12 inches (30 cm) higher than the highest point of water use (e.g., the tallest sprinkler head) downstream of the device. This height difference is critical for the air inlet valve to function correctly and break the siphon by introducing air into the line.

4. No Continuous Pressure: Standard PVBs are not designed to be under continuous water pressure for more than 12 hours in any 24-hour period. This makes them suitable for applications that operate intermittently.

Common Applications for PVBs

Given these conditions, the most common and appropriate application for a PVB is on the main line of a lawn irrigation or sprinkler system where:

• The primary risk is backsiphonage if supply pressure drops.

• The water source (sprinkler heads) could contain fertilizers or pesticides (considered pollutants in this context, though local codes may vary).

• The PVB can be installed 12 inches above the highest sprinkler head.

• The system operates intermittently, not under constant pressure.

When NOT to Use a PVB:

• If there's any risk of backpressure.

• In high-hazard applications (e.g., systems with chemical injectors, medical equipment).

• If it cannot be installed at the required 12-inch elevation.

• If the downstream line will be under continuous pressure.

Conclusion

The Pressure Vacuum Breaker is an effective and economical backflow preventer when used correctly. Its suitability hinges on protection against backsiphonage only, low-hazard conditions, proper installation height, and intermittent pressure use, making it ideal for many residential and commercial irrigation systems.

Always consult your local plumbing codes and a certified backflow professional to ensure you're selecting and installing the right device for your specific situation, safeguarding our precious water resources.