An atmospheric vacuum breaker is about as simple as backflow protection gets: a body with an air inlet and a float, and no moving parts beyond that.
Under flow, water pressure lifts the float and seals the air inlet shut. The moment the flow stops, the float drops and the inlet opens, letting air into the line. There is no siphon to pull water backward because the line is no longer closed.
What it does not do
Nearly everything. An AVB protects against backsiphonage only — it does nothing about a system that can push water back on its own.
It also cannot sit under constant pressure. The float seals under flow and relaxes when the flow stops, and leaving it pressurized wears that seal out, so codes limit how long an AVB may be pressurized at a stretch. That is why nothing can be installed downstream of one — no valve, no shutoff, no hose end that can be closed. Anything that holds pressure against it defeats it.
It has to sit at least six inches above the highest outlet it protects, because the air gap it creates is what does the work.
Where you find one
Most commonly on irrigation systems, installed downstream of each zone valve rather than once at the point of connection, because the no-valve-downstream rule means one per zone. Also on utility sinks and some equipment connections.
It is not testable
There are no test cocks and no way to attach a gauge, so an AVB cannot be certified and does not go on an annual test cycle. It is checked by inspection — does the air inlet move freely, is the poppet stuck, is it mounted high enough, has anything been added downstream of it since it went in. When one fails it is replaced, not repaired.
Where a provider requires a certified annual test on the connection, an AVB will not satisfy it, and the connection steps up to a pressure vacuum breaker.
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