Understanding the Threat: The Two Types of Backflow
Before determining the most effective solution, it's crucial to understand the two ways backflow can occur. Both events can pull contaminated water into your clean drinking supply, but they happen under different conditions.
- Back-Siphonage: This happens when negative pressure, or a vacuum, is created in the water supply lines. A sudden drop in water pressure from a nearby water main break, high water usage during firefighting, or a frozen pipe can cause back-siphonage. This can effectively suck contaminated water from an unprotected cross-connection point back into the main potable water lines.
- Back-Pressure: This occurs when a system connected to your water supply, such as a boiler, pump, or water filtration system, has higher pressure than the municipal supply line. The higher pressure can force water from that auxiliary system backward into the main water lines, carrying potential contaminants with it.
A Closer Look at Backflow Preventer Types
Not all backflow prevention devices offer the same level of protection. Their suitability depends on the type of backflow they prevent, the hazard level of the application, and whether the line is under continuous pressure.
Reduced Pressure Zone (RPZ) Device
The RPZ is the gold standard and is considered the most effective type of backflow preventer. Its complex design allows it to provide maximum protection in high-hazard situations, such as those involving chemicals, medical equipment, or industrial fluids.
- How it Works: An RPZ features two independently operating, spring-loaded check valves in series. Between these check valves is a pressure-monitored relief valve that maintains a reduced pressure zone. If either check valve fails or a back-pressure or back-siphonage event occurs, the relief valve will open to discharge water to the atmosphere, preventing any contaminated water from flowing backward.
- Application: Required for high-hazard situations in both commercial and industrial settings, but can be used residentially where local codes require the highest level of protection, such as for private well systems or large irrigation setups.
Double Check Valve Assembly (DCVA)
This assembly offers a high degree of protection for moderate-hazard applications. It is less complex and expensive than an RPZ but is not suitable for high-risk situations involving toxic chemicals.
- How it Works: A DCVA consists of two spring-loaded check valves acting independently. The device provides a redundant layer of protection, as the second valve will act as a failsafe if the first one fails.
- Application: A common choice for fire sprinkler systems and some irrigation systems where the potential contaminants are considered non-health hazards. It is suitable for continuous pressure lines and can often be installed below ground.
Pressure Vacuum Breaker (PVB)
Simpler and more affordable than both RPZs and DCVAs, a PVB is designed to protect against back-siphonage only and is often used in residential irrigation systems.
- How it Works: It features a single spring-loaded check valve and an air inlet valve. When water flow stops and a vacuum is created, the air inlet valve opens, breaking the siphon and preventing backflow.
- Application: Most often used for outdoor use, such as lawn sprinkler systems. A key limitation is that it does not protect against back-pressure and must be installed at least 12 inches above the highest point of the irrigation system.
Atmospheric Vacuum Breaker (AVB)
This is the simplest and least expensive backflow prevention device, providing basic protection against back-siphonage only.
- How it Works: The AVB has a simple float-check assembly that opens to the atmosphere when the water pressure drops. Unlike a PVB, it is non-testable and cannot be used under continuous pressure for more than 12 hours.
- Application: Suitable for very low-hazard situations like faucets or toilets, but its limitations mean it is not suitable for continuous-pressure applications or fire systems.
Comparison Table: Backflow Preventer Effectiveness
| Feature | RPZ Device | Double Check Valve (DCVA) | Pressure Vacuum Breaker (PVB) | Atmospheric Vacuum Breaker (AVB) |
|---|---|---|---|---|
| Protection Level | Highest | High | Moderate | Basic |
| Prevents Back-Siphonage | Yes | Yes | Yes | Yes |
| Prevents Back-Pressure | Yes | Yes | No | No |
| Typical Applications | Medical facilities, industrial plants | Fire sprinkler systems, commercial irrigation | Residential irrigation systems | Hose bibbs, individual faucets |
| Cost | Highest | Moderate to High | Low to Moderate | Lowest |
| Continuous Pressure | Yes | Yes | Yes (Must be installed 12" above outlet) | No |
| Testable | Yes (Annually) | Yes (Annually) | Yes (Annually) | No |
How to Choose the Right Backflow Preventer
Selecting the most effective device for your property is not a one-size-fits-all decision. Follow these steps to ensure you meet safety regulations and protect your water supply:
- Assess the Hazard Level: Determine the potential risk of contamination. If your system is connected to potential health hazards, an RPZ is required. For moderate risks (non-toxic contaminants), a DCVA may be acceptable.
- Consult Local Codes: Regulations vary significantly by municipality and water district. What is permitted in one area may not be in another. Always check with your local water authority to understand their specific requirements for installation and annual testing.
- Consider the Application: Match the preventer to the specific point of connection. A simple hose bib may only need an AVB, while a complex landscape irrigation system will likely require a PVB or RPZ.
- Consider the Installation: Factors like pressure requirements and whether the device will be installed above or below ground will influence your choice. For instance, RPZs must be installed above ground with adequate drainage, whereas DCVAs can sometimes be installed underground.
- Get Professional Assessment: The most reliable method is to have a certified backflow tester evaluate your system. They can determine the exact hazard level and recommend the correct, compliant device for your specific needs.
For most high-hazard scenarios, the RPZ device is the clear winner for its superior protection. However, for many residential applications with lower-risk situations, a PVB or DCVA may be the most cost-effective and compliant solution. The key is to assess the specific application and local regulations to make the correct, informed choice. Regular testing and maintenance by a certified professional are critical to ensuring the device remains effective over time.
For more information on backflow prevention and compliance, it is wise to consult reputable organizations. The Jordan Valley Water Conservancy District provides excellent resources on backflow prevention and best practices.
Conclusion
While an air gap is the most reliable method of backflow prevention, it is not always practical for every plumbing application. In those cases, the most effective type of mechanical backflow preventer is the RPZ device. Its robust, multi-layered design makes it the industry's gold standard for preventing contamination in high-hazard situations. For residential and moderate-risk applications, other options like DCVAs and PVBs can provide adequate protection if installed correctly and in compliance with local codes. The ultimate effectiveness of any backflow preventer depends on correct installation, regular testing, and choosing the right device for the specific hazard level.
