What is the primary cause of backflow?

The primary cause of backflow in a plumbing system is cross-connection. A cross-connection occurs when there is a direct connection between a potable water supply and a non-potable source, which can allow contaminants to flow back into the drinking water supply under certain conditions. This can happen when there is a change in pressure either from backpressure, which is an increase in pressure causing incoming water to be pushed back, or from a vacuum pressure situation which can create a siphoning effect.

Excessive water pressure, while it can contribute to issues in plumbing, is not itself a direct cause of backflow. Instead, it is the configuration and relationships between supply pipes (the potable source) and other water sources that primarily lead to backflow issues. Therefore, understanding cross-connections is crucial for preventing backflow and ensuring safe drinking water.