Backflow Preventer

A backflow preventer is a mechanical or combined mechanical-atmospheric device installed in a water line to block reverse flow. Code requires them at the service entrance and at specific fixtures or systems—irrigation, boilers, pool fills, chemical injection points—anywhere non-potable or contaminated water could siphon or push back into the clean supply during pressure drop or surges.

There are several types: dual-check valves (two checks in series, cheap and passive), atmospheric vacuum breakers (vented, work only under gravity), pressure vacuum breakers (vented but handle some pressure), and the gold standard for high-hazard applications—reduced pressure backflow preventers (RPBs). An RPB has two checks and a relief valve that bleeds water if it senses backpressure, ensuring water never reverses even under worst-case conditions. It's bulkier and needs occasional inspection, but it's the only device that stops both backpressure and backsiphonage.

In the field, size the preventer to match line velocity and pressure, install it on a drip leg with proper clearance for the relief drain, and remember that an atmospheric vent needs air, so don't cap it. Many jurisdictions require annual testing of RPBs by a licensed cross-connection specialist. A failed backflow preventer is a health code violation and a liability problem—don't ignore it.