What Is a Backflow Preventer?

A backflow preventer is a mechanical assembly installed on a water line that stops water from flowing backward from a property into the public water supply. It keeps contaminated or non-potable water, like irrigation water with fertilizer or water from a boiler or fire system, from entering the drinking water everyone shares.

How backflow happens

• Backsiphonage: when street pressure drops, say during a water main break or when firefighters open hydrants, water can get sucked backward out of buildings.
• Backpressure: when something on the property, like a pump or boiler, pushes water harder than the street supply, it can force water the wrong way.

How the assembly stops it

Inside are spring-loaded check valves that close if water tries to reverse. The higher-protection types also have a relief valve or air inlet, so if a check ever leaks, the water dumps out or air gets pulled in instead of flowing backward.

The main types

• Reduced pressure principle (RP): two checks plus a relief valve between them. Highest protection, for high hazards.
• Double check valve (DC): two independent checks. For lower hazards and many fire lines.
• Pressure vacuum breaker (PVB) and spill-resistant vacuum breaker (SVB): a check and an air inlet, for backsiphonage only, typically irrigation.
• Detector assemblies (DCDA, RPDA): DC or RP with a metered bypass, used on fire sprinkler services.

Compare them side by side in RP vs. double check vs. PVB.

Why California requires annual testing

Over time springs get weak, rubber parts wear out and grit gets stuck in the valves, and you can't see any of it from outside. Testing with a calibrated gauge once a year is how anyone knows it still works. Book yours through our testing page.

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