Pressure vacuum breaker (PVB)
A testable backflow prevention assembly with a spring-loaded check valve and an air inlet valve that protects against backsiphonage only.
A pressure vacuum breaker is a testable assembly containing a spring-loaded check valve and an independently operating, spring-loaded air inlet valve, along with shutoff valves and test cocks. When supply pressure drops, the air inlet opens and admits atmospheric air into the body, breaking any siphon before it can draw water backward. Unlike an atmospheric vacuum breaker, the PVB is designed to operate under continuous supply pressure, so it can be installed upstream of downstream control valves.
The PVB is a workhorse of irrigation protection because sprinkler systems combine several risk factors: heads that sit in soil and standing water, zone valves that keep the line pressurized, and outdoor installation. Its critical limitation is that it protects against backsiphonage only. It has no defense against backpressure, so it must never be used where a pump, elevated piping, or any other source of downstream pressure could push water back toward the supply.
Testers and property owners should pay attention to installation details. A PVB must be mounted above the highest downstream outlet so gravity cannot defeat the air inlet, and because the air inlet can spit water during normal operation, it belongs where minor discharge causes no damage. Outdoor units need freeze protection or seasonal winterization. As a testable assembly it requires periodic field testing, typically annual where required, set by the water authority.
Related terms
| Spill-resistant vacuum breaker (SVB) | A testable vacuum breaker that functions like a pressure vacuum breaker but is designed to avoid spilling water during pressurization, making it suitable for indoor use. |
| Atmospheric vacuum breaker (AVB) | A simple, non-testable backsiphonage prevention device that uses a float or disc to seal against flow and admit air when pressure is lost. |
| Backsiphonage | Backflow caused by a drop or loss of pressure in the supply piping that siphons water backward through a cross-connection. |
| Annual testing | The periodic field testing of backflow prevention assemblies, typically annual where required, set by the water authority, that verifies each assembly still works. |
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