Backflow is the backward movement of normal water flow. This can result from a drop in the customer’s water pressure or from suction in the water supply caused by a break in the water distribution piping. This can cause contaminants to drain back into the drinking water supply.

A cross-connection is a physical link between a drinking water supply and any connection to a non-drinking water source or apparatus. An example of a cross-connection is a drinking water pipe which is connected to a contaminated source, such as chemical substances or well water without backflow protection.

A Double Check Valve backflow assembly is installed for low hazard (non-toxic) conditions. A Reduced Pressure Zone backflow assembly is installed for high hazard (toxic) conditions.

Typical backflow assembly installations are located at your water meter connection or property line. This includes water service to domestic, irrigation, or dedicated fire systems.

Mechanical backflow preventers or assemblies have internal check valves, seals, springs and moving parts that are subject to wear or weakness over time. These assemblies need to be tested annually to ensure that they are working properly in protecting the water supply.

Backsiphonage is backflow caused by a negative pressure (i.e. a vacuum or partial vacuum) in a public water system or consumer’s potable water system. The effect is similar to drinking water through a straw and can occur when there is a stoppage of water supply due to nearby fire fighting, a break in a water main, etc.

Backflow into a public water system can pollute or contaminate the water in that system (i.e. backflow into a public water system can make it unusable or unsafe to drink), and each water supplier has a responsibility to provide water that is usable and safe to drink under all foreseeable circumstances and must take reasonable precautions to protect its public water system against backflow. Furthermore, consumers generally have absolute faith that water delivered to them through a public water system is always safe to drink.

A backflow preventer is a means or mechanism to prevent backflow. The basic means is an air gap, which either eliminates a cross-connection or provides a barrier to backflow. The basic mechanism is a mechanical backflow preventer, which provides a physical barrier to backflow. The principal 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 air gap is a vertical, physical separation between the end of a water supply outlet and the flood-level rim of a receiving vessel. This separation must be at least twice the diameter of the water supply outlet and never less than one inch. An air gap is considered the maximum protection available against backpressure backflow or backsiphonage but is not always practical and can easily be bypassed.

A RDC is similar to a DC in that it is a mechanical backflow preventer consisting of two independently acting, spring-loaded check valves. However, it usually does not include shutoff valves, may or may not be equipped with test cocks or ports, and is generally less reliable than a DC. A RDC is effective against backpressure backflow and backsiphonage but should be used to isolate only nonhealth hazards and is intended for use only in water service connections to single-family homes.