Backflow prevention devices are installed at a building's taps and are not testable. Backflow prevention assemblies are installed at a building's water service connection and is testable and repairable. Examples of backflow prevention assemblies include a reduced pressure zone assembly and a dual check valve assembly. Examples of a backflow prevention device are a hose-bibb vacuum breaker, atmospheric vacuum breaker, or dual check with atmospheric vent.

Guidelines for which equipment to use are written in the District of Columbia's Plumbing Code or in DC Water's Cross-Connection Manual.

Backflow prevention assemblies are required to be tested annually with test reports submitted to the cross-connection control office in Drinking Water Division.

Since backflow prevention devices are not testable, they must be replaced on regular interval usually specified by the manufacturer. DC Water does not require submitting any information on the installation or replacement on these devices.

Thermal expansion is an increase in water volume due to the heating of cold water in a plumbing system. When water is heated in a building's plumbing system with an installed backflow prevention assembly, an increase in pressure may be observed.

Cross-connection control is the elimination or isolation of a physical link between the public water distribution system and any foreign substance. It prevents the contamination of the public drinking water by not allowing the reverse flow of water from a contaminate source back into the drinking water system.

Backflow is the reverse flow of water. Backflow occurs when water from a building or home flows back into the city water mains. Backpressure and backsiphonage are the two types of processes that can cause backflow. Backsiphonage is created when a negative pressure is applied at a building's tap, creating a potential for contaminants to be siphoned into the drinking water system. Backpressure is the condition where a building's plumbing system has a greater pressure than the public distribution water main system forcing water back into the main.

Backpressure and backsiphonage can be prevented by installing backflow prevention devices and/or backflow prevention assemblies on your plumbing system where the water enters into the building.