Two Types of Backflow
Backsiphonage may occur due to a loss of pressure in the main water supply – a water main break, a fire fighting emergency, or system repair. This creates a siphon in the plumbing system which can draw water out of a sink, bucket or tank, and back into the potable water system.
Backpressure may be created when a source of pressure (such as a boiler) creates a pressure greater than the pressure supplied from the main water system. This can cause contaminated water to be pushed into the plumbing system through an unprotected cross-connection.