A vacuum breaker is a backflow-prevention device that lets air into a water line the moment pressure drops, breaking the suction that would otherwise siphon dirty water back into the clean supply. It is the small threaded device on outdoor faucets and the anti-siphon part built into many irrigation and utility valves.
A vacuum breaker is a small backflow-prevention device that works by letting air into a pipe at exactly the right moment. Its job is to stop back-siphonage, the kind of backflow that happens when pressure in the supply line drops and creates suction, pulling water backward. The moment that suction appears, the vacuum breaker opens a tiny air inlet. Air rushes in, the suction is broken, and dirty water cannot be drawn back toward the drinking supply.
The mechanism is simple. Inside is a lightweight check element held in place by normal water flow. When water flows forward or the line is pressurized, the element stays seated and the air inlet is closed. When forward pressure disappears and reverse suction begins, the element drops or shifts, opening the vent to the atmosphere. That breaks the vacuum the same way lifting your finger off a straw lets the liquid fall back out. No suction, no backflow.
The most common place you will see one is on an outdoor faucet. That little finned cap threaded onto a hose bibb is a hose-connection vacuum breaker. It matters because a garden hose is a classic hazard: leave the end sitting in a pool of fertilizer, a bucket of soapy water, or a swimming pool, and a drop in street pressure could siphon that liquid back into the house supply. For example, a hose left in a wading pool during a water-main break could pull pool water toward the kitchen tap without a vacuum breaker in the way. The device guards against exactly that.
Plumbing code requires vacuum breakers in many spots beyond hose bibbs, including irrigation systems, utility and mop sinks, and other outlets where a hose or submerged inlet could create a cross-connection. A vacuum breaker is closely related to an air gap: both prevent backflow, but an air gap uses open space while a vacuum breaker uses a mechanical air inlet, which lets it protect places where an open gap is not practical. Vacuum breakers only stop back-siphonage from suction; a different device is needed where backflow could be driven by back-pressure instead.