Cross-Connection Terms and Definitions

Air Gap

An air gap is the un-obstructed vertical distance between a water outlet and the flood rim level of a potential source of contamination; air gap distances are defined in DC Construction Codes.


Backflow is the undesired reverse flow of contaminants or pollutants into the drinking water system caused by either backpressure or backsiphonage.


Backpressure refers to the condition where a system connected to the drinking water system has pressure values greater than the drinking water system.


Backsiphonage refers to the condition where the pressure within a cross-connection falls below the atmospheric pressure.

Backflow Preventer

Backflow preventers are used to protect drinking water supplies from contamination caused by backflow. Backflow preventers only allow water to flow in the desired direction and physically blocks the reverse flow of water.


A cross-connection is a point in the drinking water system where a contaminant or pollutant can flow back into the drinking water system.

Certified Backflow Preventer Inspector.

A professional who has registered with DC Water and is qualified to inspect backflow preventers.

Thermal Expansion

Thermal expansion is an increase in water pressure that occurs when water is heated. This increase in pressure can damage the water heater, cause leaks in household pipes and faucets, or cause other plumbing problems.

Thermal Expansion Device

A thermal expansion device safely manages changes in pressure caused by thermal expansion. It helps to prevent leaks and damage to household plumbing systems.

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