Frequently Asked Questions

  1. What is cross-connection control and how does it protect the public drinking water system?
    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.
  2. What are backflow, backpressure, and backsiphonage?
    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.
  3. How do I prevent backsiphonage and back pressure?
    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.
  4. What is the difference between a backflow prevention device and a backflow prevention assembly?
    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.
  5. How do I know if I need to install a backflow prevention assembly or a backflow prevention device?
    Guidelines for which equipment to use are written in the District of Columbia's Plumbing Code or in DC Water's Cross-Connection Manual Section 4 (PDF 181 kb) .
  6. Are backflow prevention assemblies required to be tested? If so, how often?
    Backflow prevention assemblies are required to be tested annually (PDF 361 kb) with test reports (PDF) submitted to the cross-connection control office in Drinking Water Division.
  7. What is DC Water's policy regarding backflow prevention devices?
    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.
  8. What is thermal expansion and how does it relate to the installation of a backflow assembly?
    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.

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