Backflow prevention devices protect drinking water by stopping contaminated or nonpotable water from reversing into the clean water supply. Atmospheric vacuum breakers (AVBs), pressure vacuum breakers (PVBs), and reduced pressure devices each provide different levels of protection. The correct device depends on the type of cross-connection, whether backsiphonage or backpressure can occur, the level of hazard, and applicable local requirements.
Backflow prevention devices are an important part of protecting drinking water from contamination. You may see one connected to an irrigation system, commercial plumbing system, or another location where potable water could potentially come into contact with a nonpotable source.
But not every backflow preventer works the same way. An atmospheric vacuum breaker, pressure vacuum breaker, and reduced pressure device are designed for different applications and levels of protection. Installing the wrong type—or failing to maintain an existing assembly—can leave a plumbing system inadequately protected.
Understanding the differences can help homeowners and property owners recognize what they have, why it’s there, and when professional testing or service may be required.
Quick Facts
What Do Backflow Prevention Devices Do?
Backflow prevention devices help keep contaminated or nonpotable water from reversing direction and entering a potable water system.
Water normally travels from the public water supply into a property. Under certain conditions, however, that direction can reverse. This unwanted reversal is known as backflow.
Two primary conditions can cause it:
- Backsiphonage: A drop in supply pressure creates a vacuum or negative pressure that can pull water backward.
- Backpressure: Pressure downstream becomes greater than the pressure in the potable supply and pushes water backward.
Cross-connections involving irrigation systems, chemical equipment, boilers, and other plumbing configurations can create contamination risks if appropriate protection isn’t installed.
What Is an Atmospheric Vacuum Breaker?
An atmospheric vacuum breaker (AVB) is a relatively simple backflow prevention device designed to protect against backsiphonage.
An AVB uses an air inlet that opens when water pressure drops. Allowing air into the system helps prevent contaminated water from being siphoned backward into the potable supply.
Atmospheric vacuum breakers are commonly associated with applications such as certain irrigation connections. However, an AVB does not protect against backpressure.
AVBs also have important installation limitations. They generally cannot remain under continuous pressure, and downstream shutoff valves cannot be installed in a way that keeps the device continuously pressurized.
When Might an AVB Be Used?
AVBs may be appropriate where the primary concern is backsiphonage and the installation can meet the required elevation and pressure conditions.
Because these devices have specific installation restrictions, a professional should determine whether an AVB is appropriate for a particular application.
What Is a Pressure Vacuum Breaker?
A pressure vacuum breaker (PVB) protects against backsiphonage while allowing the plumbing system to remain under continuous pressure.
A PVB uses a spring-loaded check valve and air inlet valve. If supply pressure falls, the air inlet can open and help prevent contaminated water from being siphoned backward.
Unlike an AVB, a PVB is a testable assembly and can accommodate downstream shutoff valves. That makes PVBs useful in applications such as certain irrigation systems where the line remains pressurized.
However, a PVB still does not provide protection against backpressure. If backpressure is possible, a different assembly may be necessary.
What Is a Reduced Pressure Device?
A reduced pressure principle backflow prevention assembly provides protection against both backsiphonage and backpressure and can be used for higher-hazard applications when properly specified.
Often called an RP, RPZ, or reduced pressure assembly, this design uses two independently operating check valves with a relief valve positioned between them. The relief valve helps maintain a lower-pressure zone between the check valves.
If one of the check valves fails or pressure conditions become unsafe, the relief mechanism provides another layer of protection.
This makes reduced pressure assemblies one of the more protective options used in cross-connection control.
We provide professional backflow services for property owners who need assistance identifying, testing, maintaining, or repairing their systems.
AVB vs. PVB vs. Reduced Pressure Device: What’s the Difference?
The biggest differences involve the type of backflow each system prevents, whether it can operate under continuous pressure, and the level of hazard it can address.
| Type | Backsiphonage | Backpressure | Continuous Pressure |
|---|---|---|---|
| Atmospheric Vacuum Breaker | Yes | No | No |
| Pressure Vacuum Breaker | Yes | No | Yes |
| Reduced Pressure Assembly | Yes | Yes | Yes |
The appropriate device isn’t simply a matter of choosing the most expensive or protective option. The correct solution depends on the cross-connection, hazard level, plumbing configuration, local requirements, and other installation conditions.
How Do You Know Which Backflow Device You Have?
A plumber or certified backflow professional can identify your device and determine whether it matches the requirements for its application.
Property owners may also be able to locate manufacturer information, model numbers, or certification markings directly on the device.
Never modify, remove, bypass, or replace a backflow preventer simply because you’re unsure why it was installed. It may be required to protect both your property and the public drinking water system.
If you’re uncertain about your plumbing configuration, our broader plumbing services can help identify potential cross-connections and determine whether professional backflow service is appropriate.
Do Backflow Prevention Devices Need to Be Tested?
Testable backflow prevention assemblies may require periodic testing according to California and local water supplier requirements.
California’s Cross-Connection Control Policy Handbook establishes statewide requirements for public water systems and cross-connection control. Individual water suppliers also administer programs that determine which service connections require protection and how applicable assemblies are tested and documented.
Testing requirements can therefore depend on the property, hazard, device type, and water supplier.
During professional testing, a qualified tester checks whether the assembly’s internal components operate within required parameters. A failed assembly may need repair and retesting before it can return to compliant operation.
Why Professional Selection and Installation Matter
Backflow protection must match the specific hazard and hydraulic conditions of the plumbing system.
An incorrectly selected or installed device may not provide the protection a property needs. Height requirements, drainage, accessibility, pressure conditions, downstream valves, and the type of potential contaminant can all affect which solution is appropriate.
The U.S. Environmental Protection Agency’s guidance on cross-connection control and backflow prevention describes the different protection capabilities of AVBs, PVBs, and reduced pressure assemblies.
Professional evaluation helps property owners avoid guessing and ensures the selected protection matches the actual plumbing conditions.
Backflow Compliance and Local Expertise in California
California updated its statewide approach to cross-connection control when the Cross-Connection Control Policy Handbook took effect in 2024, with additional revisions adopted in 2026. Local public water systems implement cross-connection control programs under these requirements, so property owners should follow instructions provided by their applicable water supplier.
At P&M, we help property owners understand and maintain their backflow prevention devices while addressing related plumbing concerns. For homeowners and businesses that receive a testing notice, inherit an unfamiliar backflow assembly, or suspect an existing device needs service, professional evaluation can clarify what equipment is installed and what should happen next.
Frequently Asked Questions About Backflow Prevention Devices
What is the main purpose of a backflow prevention device?
What is the difference between an AVB and a PVB?
Does a pressure vacuum breaker protect against backpressure?
When is a reduced pressure device used?
Do all backflow prevention devices require annual testing?
Can I replace one type of backflow preventer with another?
Understand Your Backflow Device Before Problems Develop
Atmospheric vacuum breakers, pressure vacuum breakers, and reduced pressure assemblies all serve the same broad purpose—protecting potable water—but they accomplish that job in different ways. Knowing which type is installed on your property can help you understand its limitations, maintenance needs, and potential testing requirements.
If you’re unsure what type of backflow prevention device you have or whether it needs professional attention, contact us to learn about your backflow device and get guidance based on your property’s plumbing system.
Further Reading & References
- Backflow Services
- Plumbing Services
- Contact a Professional
- 5 Signs You Might Need a Leak Detection Service
- U.S. EPA – Cross-Connection Control and Backflow Prevention
