Water distribution systems are designed with the intention of the water flowing in a certain direction-from the distribution system to the consumer. However, hydraulic conditions within the system may deviate from the “normal” conditions causing a change of pressure in the system and resulting in water flow in the opposite direction or backwards. Therefore, it is possible (and common) for the water to flow in the opposite direction in unprotected systems. This is called backflow.
Backflow is dangerous because it can allow chemicals, bacteria or pollutants to flow into drinking water in your plumbing system and the drinking water can become contaminated and unusable or even possibly deadly.