Backflow is the undesirable reversal of flow in a potable water distribution system. Water that is always under pressure can only flow in one direction. Then how can water flow in reverse? Water will always flow towards the point of lowest pressure. If a water main were to break or if the fire department opened several fire hydrants to help fight a fire, the pressure in the water main could drop. The demand upstream could cause a reversal in flow.
Cross connections and the possibility of backflow need to be recognized so they do not occur. A garden hose submerged in a hot tub, swimming pool, car radiator, or attached to an insect/fertilizer sprayer could siphon the liquid back into the water main.
Backflow Prevention Devices
Backflow prevention devices are designed to protect the public water system from these types of concerns.