Short definition
An atmospheric vacuum breaker (AVB) is a backflow device that opens to atmosphere when supply pressure drops below atmospheric, admitting air to break a siphon and prevent contaminated downstream water from being drawn back into the supply. Listed under ASSE 1001. It’s the simplest mechanical backflow protection — used where the install can satisfy a strict set of geometry rules.
What it is
The AVB is a brass body containing a check disc held closed by water pressure during normal flow, plus an air-inlet port that opens when supply pressure falls below atmospheric (a “vacuum” condition). When the disc lifts off its seat under negative pressure, atmospheric air rushes in through the inlet, breaking any siphon that might otherwise pull contaminated water backward. No springs, no diaphragm — almost no moving parts beyond the disc.
The cost of that simplicity is strict installation rules:
- At least 6 inches above the highest downstream outlet. Below that elevation, the device doesn’t function reliably.
- No shutoff valve downstream. A closed valve downstream creates back-pressure conditions the AVB isn’t designed for.
- Back-siphonage protection only, not back-pressure. If pressurized water can push back through the device (a boiler, a chemical-feed pump, a fire-pump test connection), an AVB is the wrong protection — that’s an RPZ application.
Why it matters to a homeowner
You’ll most often hear “AVB” from an inspector or a sprinkler installer. On hose bibs the device is usually called a HBVB (hose-bib vacuum breaker, ASSE 1011) — a small AVB integrated into or screwed onto the threaded outlet. On irrigation systems, AVBs were once common but are mostly displaced by PVBs (pressure vacuum breakers, ASSE 1020), which are rated for the continuous-pressure conditions of irrigation.
If an inspector flags an AVB, the typical issues are:
- Installed too low — must be 6 inches above the highest downstream outlet.
- Installed downstream of a shutoff valve — defeats the device’s design conditions.
- The disc has deteriorated from chlorinated water or mineral deposits — leaks at the air port during normal flow.
The fix is usually to replace with the right device for the application: HBVB on hose bibs, PVB on irrigation, RPZ on high-hazard cross-connections.
When you’ll encounter this term
- Inspector flags a missing or improperly installed AVB on a hose bib or sprinkler riser.
- Sprinkler installer chooses between AVB (older, cheaper, basic) and PVB (continuous-pressure, modern preference).
- Plumber recommends replacement of an old AVB whose air port is dripping during normal flow.
Common variants
- AVB vs. PVB (pressure vacuum breaker). AVB is for non-continuous use (rated for back-siphonage only, no shutoff downstream). PVB is for continuous-pressure systems like irrigation, with shutoffs and test cocks for annual testing.
- AVB vs. HBVB (hose-bib vacuum breaker). HBVB is the small AVB designed to thread directly onto a hose-bib outlet (ASSE 1011 standard).
- AVB vs. RPZ. RPZ is for high-hazard applications and protects against both back-siphonage and back-pressure. AVB is back-siphonage only and lower-hazard.