Backflow preventer
A backflow preventer is a plumbing device or assembly that stops water from flowing backward out of a building's piping, equipment or containers into the drinking water supply. The plumbing code and water authority where the work is done decide which type a connection needs and how often testable ones are tested.
Also called: Backflow prevention assembly, Backflow device
How backflow happens
Water in a building should move one way: in from the supply. Backflow is the reverse, and it happens in two ways:
- Backsiphonage: supply pressure drops, during a main break or heavy hydrant use for example, and pulls water backward out of the building's piping, the way a straw does.
- Backpressure: pressure on the building's side, from a boiler or a booster pump for example, rises above the supply pressure and pushes water backward.
Either one is a danger at a cross-connection: a point where drinking water piping connects, or could connect, to something that isn't drinking water. Lawn irrigation, boilers, fire sprinklers and a hose lying in a bucket are everyday cases.
Example: A hose is left running into a bucket of cleaning solution. A main breaks down the street, house pressure drops, and the solution is drawn back into the home's pipes. A vacuum breaker on that faucet would have let air in instead. The scenario is made up for illustration.
The main types
| Type | Stops | Where you'll see it |
|---|---|---|
| Air gap | Backsiphonage and backpressure | Open space between an outlet and the rim of the tank or sink it fills |
| Reduced pressure zone assembly (RP or RPZ) | Backsiphonage and backpressure | Connections where a contaminant could harm health |
| Double check valve assembly | Backsiphonage and backpressure | Lower-hazard connections, where the code allows it |
| Pressure vacuum breaker (PVB) | Backsiphonage only | Often on lawn irrigation |
| Hose connection vacuum breaker | Backsiphonage only | Outdoor faucets and utility sinks |
RPs, double checks and PVBs have shutoff valves and test cocks so they can be tested in place, which is why they're called assemblies. An RP's relief valve dumps water when a check leaks or pressure reverses, so it needs drainage and must never sit where it could end up underwater.
Testing is set locally
Testable assemblies wear out quietly: rubber parts age, springs weaken and debris can hold a check open. A tester connects a differential pressure gauge to the test cocks and confirms that each check holds and, on an RP, that the relief valve opens when it should. The result goes on a report for the water supplier or local program.
Who may test, how often and how reports are filed are local decisions, made by the state, the water utility or both. Before you offer testing, ask who certifies testers, whether testing also needs a plumbing license, when tests are required and where reports go. How to start a plumbing business has the full list of questions.
Common mistakes
- Using a plain check valve as protection. A single check can stick open or leak with no way to tell. Install what the code calls for.
- Matching the device to the budget instead of the hazard. A vacuum breaker can't stop backpressure, and the code may not allow a double check on a higher-hazard connection.
- Skipping installation details. Vacuum breakers have to sit higher than the outlets they serve, RPs need drainage and outdoor assemblies need freeze protection. Follow the manufacturer's instructions and your code.
- Losing track of due dates. Record each assembly's location, make, model, size, serial number and last test, and contact the customer before the next test is due.
How it connects to other work
A new assembly may need a building permit, so put the assembly, the permit and the first test on the estimate as separate lines. Recurring tests fit a service agreement, and a trip charge you disclosed upfront covers a visit wasted by a locked gate. Spring irrigation start-ups are the time to check assemblies for freeze damage; see the plumbing seasonal playbook.
Pressure washers and soft washers meet the same risk from the hose side: put a backflow preventer on the customer's spigot connection and never leave a hose end sitting in a tank of cleaning mix.