Backsiphonage
Backflow caused by a drop or loss of pressure in the supply piping that siphons water backward through a cross-connection.
Backsiphonage is a form of backflow that occurs when pressure in the supply line falls below the pressure at a downstream point, creating a partial vacuum that draws liquid backward. It works on the same principle as sipping a drink through a straw. Typical triggers include a water main break, a firefighting operation drawing heavily on nearby hydrants, or an undersized line serving equipment with high demand. Whatever the trigger, the supply side stops pushing and starts pulling.
In the field, backsiphonage is dangerous because it can lift contaminants out of open containers and flooded fixtures. A hose end resting in a laundry sink, a pesticide sprayer attached to a spigot, or an irrigation head standing in a puddle can each become a suction point during a pressure loss. Because the events that cause pressure loss are unplanned and often brief, the reversal can happen and end before anyone in the building is aware of it.
Testers and property owners should understand that some devices protect only against backsiphonage. Vacuum breakers, both atmospheric and pressure types, are designed to admit air and break the siphon, but they are not built to resist pressure pushing from downstream. Matching the device to the mechanism is a fundamental part of cross-connection control, and it is one of the first things a qualified tester evaluates when reviewing an installation.
Related terms
| Backflow | The undesirable reversal of water flow in a piping system that can carry non-potable water or other substances back into the potable supply. |
| Backpressure | Backflow caused when pressure downstream of the potable connection becomes greater than the pressure of the supply itself. |
| 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. |
| 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. |
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