An air gap is the most effective method for preventing backflow. By creating a physical space between a water supply outlet and any potential source of contamination, an air gap makes it physically impossible for water to flow backward into the clean supply. But it’s not the only option. Several mechanical devices also prevent backflow, and the right choice depends on the type of hazard and the specific plumbing scenario you’re dealing with.
Backflow happens when water flows in the wrong direction through a plumbing system, potentially pulling contaminated water into the drinking supply. This can occur through backsiphonage (when pressure drops in the supply line, creating a vacuum) or backpressure (when downstream pressure exceeds supply pressure). Each prevention method handles these two causes differently.
Air Gaps: The Simplest and Most Effective Method
An air gap is a vertical, unobstructed space of open air between a water outlet and the rim of whatever it’s flowing into. The most familiar example is the gap between a faucet and the top of your sink. Because there’s no physical connection between the supply pipe and the receiving vessel, contaminated water simply cannot travel back up into the supply. No moving parts, no mechanical failure points.
Plumbing codes require the air gap to be at least twice the inside diameter of the supply pipe, with a minimum of one inch, whichever is greater. So a half-inch pipe needs a one-inch gap (since twice the diameter would be less than the minimum), while a one-inch pipe needs a two-inch gap. Air gaps protect against both backsiphonage and backpressure and are approved for both low-hazard and high-hazard situations. They’re commonly used to fill water storage tanks or in commercial applications where maximum safety is needed.
The limitation is practical: you can’t always design a system with an open-air break. Many plumbing connections need to be sealed and pressurized, which is where mechanical backflow prevention devices come in.
Reduced Pressure Zone Assemblies
A reduced pressure zone assembly (RPZ) provides the highest level of mechanical backflow protection. It contains two spring-loaded check valves with a relief valve sandwiched between them. Under normal flow, water pushes through both check valves without issue. If backflow occurs, the check valves snap shut. If either check valve fails or weakens, the relief valve opens and dumps water out of the assembly rather than allowing contaminated water to reach the supply.
The relief valve works by monitoring the pressure difference between zones. It stays closed as long as supply pressure is higher than the pressure in the zone between the two check valves. If that difference drops, indicating a potential backflow condition, the valve opens. In a complete loss of supply pressure, the zone between the check valves equalizes to atmospheric pressure as the relief valve vents to open air, essentially creating a temporary air gap inside the device.
RPZ assemblies protect against both backsiphonage and backpressure and are approved for high-hazard connections, meaning situations where backflow could introduce substances dangerous to health. They’re one of the two most common backflow preventers used in fire protection systems and are also required on connections to chemical feed systems, medical facilities, and industrial processes. They include shutoff valves and test ports for annual inspection.
Double Check Valve Assemblies
A double check valve assembly (DCVA) uses two spring-loaded check valves in series. If backflow occurs, the first check valve closes. If that one fails, the second provides a backup. The assembly includes shutoff valves and test ports so it can be inspected and verified in the field.
DCVAs protect against both backsiphonage and backpressure, but they’re restricted to low-hazard applications, meaning situations where backflow would be objectionable but not a health threat. They’re one of the most commonly installed devices on residential irrigation systems and fire protection connections. They can be installed above or below ground, though codes typically limit burial depth to 24 inches and above-ground height to five feet.
The key difference from an RPZ: a DCVA has no relief valve. If both check valves fail simultaneously, there’s no failsafe to dump water before it reaches the supply. That’s why codes prohibit DCVAs on high-hazard connections.
Vacuum Breakers: Backsiphonage Only
Vacuum breakers protect against backsiphonage but not backpressure. They work by letting air into the line when water flow stops, which breaks the siphon effect. There are three common types, each with different installation rules.
Atmospheric Vacuum Breakers
An atmospheric vacuum breaker (AVB) contains an air inlet valve and a check seat. When water flows, the air inlet closes. When flow stops, the air inlet drops open, letting air in and preventing siphonage. AVBs are the simplest and least expensive mechanical option, but they come with significant restrictions: they cannot be under continuous pressure and may only be pressurized for 12 hours out of any 24-hour period. No shutoff valves can be installed downstream. They cannot be tested in the field, so they’re typically used on individual fixtures like hose bibbs or landscape irrigation zones.
Pressure Vacuum Breakers
A pressure vacuum breaker (PVB) operates on the same principle as an AVB but adds shutoff valves and test ports, making it field-testable. PVBs can handle continuous pressure, unlike AVBs. They must be installed at least 12 inches above the highest downstream outlet or point of water use. This height requirement is critical because the device relies on gravity and atmospheric pressure to function. PVBs are commonly used on irrigation systems where the installation height can be maintained.
Hose Bibb Vacuum Breakers
These small, inexpensive devices screw directly onto outdoor faucets and garden hose connections. They protect against backsiphonage only and are designed for the common scenario where a garden hose submerged in a pool or connected to a chemical sprayer could siphon contaminated water back into the house supply.
Choosing the Right Method
The decision comes down to two questions: what type of hazard does the cross-connection pose, and what’s causing the backflow?
- High hazard, any cause: An air gap or RPZ assembly. These are the only options when backflow could introduce health-threatening contaminants, such as chemicals, sewage, or medical waste.
- Low hazard, backsiphonage and backpressure: A DCVA works here. Common applications include fire sprinkler connections and residential irrigation systems without chemical injection.
- Low or high hazard, backsiphonage only: A PVB or AVB can be used when backpressure isn’t a concern. PVBs are preferred for permanent installations since they can be tested. AVBs suit intermittent-use fixtures.
A barometric loop, which is a U-shaped rise in the piping that reaches about 35 feet high, can also prevent backsiphonage by exceeding the height that atmospheric pressure can support a water column. It’s an uncommon solution due to the space it requires but appears in some industrial settings.
Testing and Maintenance Requirements
Mechanical backflow preventers have moving parts that wear out. Check valves can develop debris on their seats, springs weaken over time, and relief valves can foul. Most local codes require testable assemblies (RPZs, DCVAs, and PVBs) to be inspected annually by a certified tester who verifies that each component is functioning within specifications. Testing requirements, approved device lists, and certification standards vary by state and local jurisdiction.
Air gaps, by contrast, need no testing since there are no mechanical components to fail. They only need to be maintained so the gap isn’t inadvertently bridged by hoses, tubes, or other objects that could create a direct connection between the supply and the receiving vessel. This is the most common way an air gap “fails”: someone runs a hose into a tank and accidentally eliminates the physical separation.