What Does a Backflow Preventer Do?

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: pressure in the public main drops, for example during a main break or heavy firefighting draw, and water is pulled backward out of the property like a straw.
• Backpressure: pressure on the property side rises above supply pressure, from a pump, boiler or elevated tank, and pushes water backward.

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

• RP (reduced pressure): the most protective type. Two checks with a relief valve in between, used where contamination could make people sick.
• Double check: two checks in a row. Common on fire lines and lower-risk connections.
• PVB and SVB: one check plus an air inlet. You'll see these on sprinkler systems.
• DCDA and RPDA: fire line versions with a small metered bypass that shows if water is being used.

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

Why California requires annual testing

Springs weaken, rubber wears and debris lodges in valves. None of that is visible from outside. An annual field test with a calibrated gauge is the only way to confirm the assembly will actually work when it's needed, which is why water suppliers require it. Schedule yours with our backflow testing service.

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