This is the smallest and most common backflow device on any property, and most people have one without knowing what it is — the short brass fitting threaded onto an outdoor faucet, between the tap and the hose.
It works on air. While water is flowing, the pressure holds an internal seal shut. When the pressure drops and the flow reverses, the seal opens and lets air into the line, which breaks the suction before anything in the hose can be pulled backward. A hose lying in a flower bed, a garden sprayer with herbicide in it, or an end sitting in a stock tank all stop being a path into the house supply.
What it does not do
It protects against suction only. Anything on the other end that can push water back on its own — a pump, most obviously — defeats it entirely.
It is also not a testable assembly. There are no test cocks, so there is no way to verify one in place and no annual certification to file on it. Where a provider requires certified testing on a connection, a hose bibb breaker does not satisfy that, and the connection steps up to a pressure vacuum breaker or, where the hazard is high or backpressure is possible, a reduced pressure assembly.
Most are not rated for continuous pressure either. A hose left charged against a closed nozzle wears the seal out, which is why the common failure is a breaker that drips from the vent under pressure. That drip is the device telling you it is finished.
Where it belongs
Any outdoor tap, any wall hydrant, any utility sink with a threaded spout, any hose connection in a mechanical room. Plumbing codes require them on hose connections in new construction, and on a great many Western Slope properties built before that, they are simply not there.
On an exterior tap we install them, replace failed ones, and will tell you when the connection has outgrown one. Inside the building we test and repair but do not replace — that line is explained here.
More on the connection itself: hose bibbs and wall hydrants.
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