Assembly Guide

Types of Backflow Testing

Different hazards call for different assemblies — and each one is tested a different way. Here's what's out there and how we test it.

Reduced Pressure Principle Assembly

Testable — annual

Where it's used: Highest-hazard applications: chemical injection, boilers, car washes, medical and dental, irrigation with fertilizer injection.
How it works: Two independent spring-loaded check valves with a pressure-differential relief valve between them. If either check fouls or pressure reverses, the relief valve opens and dumps water to atmosphere rather than letting it flow backward.
How it's tested: Four test cocks are used to verify check valve #1 and #2 hold tight against backpressure and that the relief valve opens at the required differential (typically 2 psi or more below the #1 check).

DCVA

Double Check Valve Assembly

Testable — annual

Where it's used: Non-health (pollutant) hazards: fire lines without additives, some irrigation, food-grade process lines.
How it works: Two spring-loaded check valves in series, each providing independent protection against backpressure and backsiphonage. No atmospheric relief.
How it's tested: Each check valve is isolated and verified to hold at a minimum 1 psi differential, plus shutoff valves are checked for tightness.

PVB

Pressure Vacuum Breaker Assembly

Testable — annual

Where it's used: The most common lawn irrigation assembly. Must be installed at least 12 inches above the highest downstream outlet.
How it works: A spring-loaded air inlet valve plus a single check valve. Protects against backsiphonage only — not backpressure.
How it's tested: The air inlet must open at 1 psi or higher as pressure drops, and the check valve must hold tight at 1 psi.

SVB

Spill-Resistant Vacuum Breaker

Testable — annual

Where it's used: Indoor or sensitive locations where a standard PVB spilling water during operation is unacceptable.
How it works: Same principle as a PVB but designed so it does not discharge water when the air inlet opens.
How it's tested: Air inlet opening point and check valve tightness are verified, using the assembly's dedicated test cocks and vent.

AVB

Atmospheric Vacuum Breaker

Not testable — inspection only

Where it's used: Simple hose-bibb style and low-hazard applications. No shutoff valve or continuous pressure may be downstream.
How it works: A float or poppet drops to admit air when supply pressure drops. Backsiphonage protection only.
How it's tested: There are no test cocks, so it cannot be field tested. It is visually inspected for correct installation, height and operation.

RPDA / DCDA

Detector Assemblies (Fire Lines)

Testable — annual

Where it's used: Fire sprinkler and standpipe service lines where low-flow leakage or unauthorized use must be detected.
How it works: A full-size RP or DC assembly with a parallel metered bypass containing a smaller assembly, so any small flow registers on the meter.
How it's tested: Both the main assembly and the bypass assembly are tested independently and both must pass.

Air Gap

Not a mechanical assembly

Where it's used: Drain lines, sinks, ice machines, and equipment discharge.
How it works: A physical vertical separation — at least twice the pipe diameter — between the supply outlet and the flood rim. Nothing can be siphoned back across an air gap.
How it's tested: Verified by inspection and measurement; it can be defeated by a hose or extension, so it is checked during a cross-connection survey.

Backpressure vs. Backsiphonage

Backsiphonage happens when supply pressure drops — a water main break or heavy fire-hydrant draw — creating a vacuum that sucks water backward. Backpressure happens when the downstream system is at higher pressure than the supply, such as a pump, boiler or elevated tank pushing water back toward the main. Vacuum breakers stop only backsiphonage; RP and double check assemblies stop both.

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