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
- 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
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.
Related questions
Is backflow testing really every year?
In California, backflow prevention assemblies must be tested at least once a year. They also need a test right after installation, after any repair, and after being moved. Your water district sets the exact due date and usually mails a notice before it.
Do houses have backflow devices too?
Plenty of homes do, usually because of a sprinkler system, a fire sprinkler line, a pool fill or a well. If you got a notice, your district already has a device on file for your address.