Cross Connection and Backflow Prevention

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What is cross-connection?


Please note:  this list is provided to you as a convenience only and is not comprehensive.  The District does not recommend any specific contractor.  View the full list of certified backflow testers by county.



A cross-connection is any actual or potential connection between the public water supply and a source of contamination or pollution.  In homes, the most common type of cross-connections include:

 hose connected to hose bib with vacuum breaker      hoses connected at hose bibs
 in-ground irrigation system     in-ground irrigation systems
 fire sprinkler system     fire sprinkler systems (not "flow-through" type)
 boiler unit     boiler systems
 pool auto-fill system     pool or pond auto-fill systems, or boat launches

Commercial facilities, including industrial, restaurant, and medical facilities must protect against other cross-connection hazards.

What is backflow?

Clean, safe water is designed to flow from the main into homes.  In unusual instances, this normal direction of flow is reversed, resulting in "backflow."  In a backflow situation, water that has contacted chemicals or other hazards can enter the drinking water system through cross-connections in homes or commercial buildings.

Why do I need a backflow prevention device (backflow assembly)?

If you have one of these cross connections that poses a potential hazard to the drinking water supply, state and federal law require you to install a backflow prevention device on your connected system.  This ensures backflow – or potential water contamination - cannot affect the public drinking water.  The law's annual test requirement makes sure the device keeps working properly.

What type of backflow device does my system need?

irrigation system in box with installed backflow device    double check valve assembly backflow device     For irrigation and fire sprinkler systems:  most residences need a double-check valve assembly (DCVA)
 boiler system installed with backflow device    Reduced Pressure Backflow Assembly (RPBA) device    

For boiler systems and other higher hazards:  a reduced-pressure backflow assembly (RPBA) is required.

 outdoor hose bib installed with vacuum breaker device    vacuum breaker device     All homes should install vacuum breakers on hose connections, easy to find at hardware stores.

If you have questions about what type of backflow device to install on your property, please contact us at (425) 398-4419 or


Backflow can result from two types of situations:  back-siphonage or back-pressure.

Back-siphonage can happen when a rapid drop in water pressure (such as a main break, or a fire incident that uses large quantities of water very quickly) creates a vacuum, which can cause a reverse flow or suction in the water lines of your home. 

If a water line (such as a garden hose) is submerged in a polluted or contaminated substance when the pressure drops, that substance can be drawn into the water supply.  When the pressure returns to normal, the substance will flow in the opposite direction.  Without a proper backflow device preventing the opposite flow, those contaminants can be pulled into your home water lines, or even possibly your neighbors'.

Backflow explanation diagram

  • Irrigation Safety and Cross-Contamination flier

To prevent cross-connections from occurring, follow these precautions:

  • Install an approved backflow prevention device on any cross-connected systems, test upon installation and annually by a state certified backflow tester.
  • Never submerge a garden hose in a bucket, sink, or anything else. 
  • Do not use a spray attachment on your garden hose without first installing a backflow prevention assembly. 
  • Keep the end of the garden hose clean and free of contaminants.    




If you have questions regarding cross-connection or backflow assemblies, contact (425) 398-4419 or email