Understanding Backflow Preventers: Key Types and How They Protect Your Water

Clean, safe drinking water is essential, but unseen plumbing issues can threaten its purity. One major risk is backflow, the unwanted reversal of water flow that can introduce contaminants into your potable water supply.

Thankfully, specialized devices called backflow preventers are designed to stop this. But not all backflow preventers are the same. Let's explore the main types and their roles.

What Causes Backflow?

Backflow happens in two primary ways:

1. Backsiphonage: Occurs when pressure in the supply line drops (like during a water main break), creating a vacuum that sucks water backward from a potentially contaminated source (e.g., a garden hose in a puddle, a sprinkler head).

2. Backpressure: Happens when the pressure downstream from the device becomes higher than the supply pressure, forcing water backward (e.g., from a pressurized system like a boiler or pump).

Different backflow preventers address these conditions based on the level of hazard. Here are the main types:

Atmospheric Vacuum Breaker (AVB)

• How it works: Uses an air inlet valve (poppet) that opens when pressure drops, introducing air to break the vacuum (siphon).

• Protection: Only protects against backsiphonage.

• Limitations: Cannot be under continuous pressure (must be installed after the last shutoff valve) and offers no protection against backpressure. Must be installed vertically, at least 6 inches above the highest downstream outlet.

• Common Uses: Lawn irrigation zones (after the zone valve), hose bibbs, utility sinks.

Pressure Vacuum Breaker (PVB)

• How it works: Similar to an AVB with an air inlet, but includes a spring-loaded check valve, allowing it to be under continuous pressure upstream. Features test cocks and shutoff valves.

• Protection: Protects against backsiphonage.

• Limitations: Does not protect against backpressure. Must be installed vertically, at least 12 inches above the highest downstream outlet. Requires periodic testing.

• Common Uses: Mainline irrigation systems (before zone valves), some industrial applications.

Double Check Valve Assembly (DCVA or DC)

• How it works: Contains two independently acting, spring-loaded check valves in series. Includes test cocks and shutoff valves.

• Protection: Protects against both backsiphonage and backpressure.

• Hazard Level: Suitable for low-hazard situations where the potential contaminant isn't toxic (e.g., preventing stagnant water from a fire sprinkler system from entering potable lines). Requires testing.

• Common Uses: Fire sprinkler systems, food processing equipment (low hazard), irrigation systems in some jurisdictions.

Reduced Pressure Zone Assembly (RPZ or RP)

• How it works: The most complex and protective type. Features two independent check valves (like a DCVA) plus a hydraulically operated differential pressure relief valve located between them in a "zone." This relief valve opens to discharge water if either check valve leaks, preventing contamination even if one fails. Includes test cocks and shutoff valves.

• Protection: Protects against both backsiphonage and backpressure.

• Hazard Level: Designed for high-hazard situations where contaminants could be toxic or harmful to health. Requires testing and can discharge water.

• Common Uses: Systems with chemical injectors (car washes, boilers), medical facilities, commercial buildings, irrigation systems injecting fertilizer/pesticides.

(You might also encounter simple Hose Bibb Vacuum Breakers (HBVBs), small screw-on devices for outdoor faucets providing basic backsiphonage protection.)

Choosing the Right Device

Selecting the appropriate backflow preventer depends on the specific application, the potential hazard level of contaminants, whether continuous pressure is present, and local plumbing codes. Installation and regular testing by certified professionals are crucial for ensuring these devices function correctly and keep our water safe.

Understanding these main types helps appreciate the vital role they play in public health and water safety.