We often find in residential homes that dirty water from the hillside pool and/or the lawn irrigation sprinkler system will back-siphon or backflow down the hosepipe and into the clean water tank. To prevent this from happening, the house's external tap should be fitted with a backflow prevention device.

A backflow preventer is a means or mechanism to prevent backflow. The basic system for preventing backflow is a mechanical backflow preventer device, which provides a physical barrier to the backflow of water to prevent cross-contamination issues. The main types of mechanical backflow preventers are the reduced-pressure principle assembly, the pressure vacuum breaker assembly, and the double-check valve assembly. A secondary type of mechanical backflow preventer is the residential dual check valve.

An example is the piping between a public water system or consumer's potable water system and an auxiliary water system, cooling system, or lawn sprinkler irrigation system. A cross-connection is any temporary or permanent connection between a public water system or consumer's potable (i.e., drinking) water system and any source or system containing nonpotable water or other substances.