There are two main causes of backflow: backsiphonage and backpressure.
- Backsiphonage can occur when there is a loss of water pressure in the system due to a break in the main line or a heavy demand such as a firefighting effort. Backsiphonage is the reversal of normal flow caused by negative pressure, vacuum or partial vacuum, in the supply piping. This condition allows liquids to be siphoned back into the water distribution system, the same effect as using a straw to drink from a glass.
- Backpressure can occur when the downstream pressure becomes greater than the supply pressure. This is normally not an issue with residential services unless a booster pump is in use for an irrigation system.