How a Backflow Preventer Works
When can backflow occur?
There are two conditions where pressure changes can contribute to the reversal of flow:
Backsiphonage: This happens when there is a negative pressure in the supply piping, allowing downstream substances to be siphoned back into the potable water supply. Under-sized pipes, pipeline breaks, and high withdrawal rates can create vacuums, which contribute to the occurrence of backsiphonage.
Backpressure: This happens when the flow pressure downstream is higher than the supply, allowing downstream substances to be pushed into the potable water supply. This can be created by pumps, thermal expansion and elevation.