Backflow is an unwanted reversed flow due to back pressure or backsiphonage.
Back pressure occurs when the pressure of one system in an interconnected fluid system fails, resulting in the pressure of one system exceeding the pressure of another.
Backsiphonage is when the pressure of the line supplying the faucet is less than the pressure in the supply line to the bathtub. Resulting in flow from the bathtub into the open faucet.
When this happens contaminated fluid can be drawn up into the water system; which poses as a serious health concern in the safety of your drinking water.
It is for this reason that building codes mandate strict backflow prevention measures.
Examples of instances that can cause backflow are:
Backflow prevention is an automatic process that requires a backflow preventer, or an assembly of differential valves, to prevent backflow. By separating drinking water and polluted fluid with valved connections, contamination is greatly reduced.
Backflow prevention is mandatory, very complex and can result in severe penalties if not regulated.