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The City of Portland, Oregon

Portland Water Bureau

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Air Gap Backflow Prevention

Unlike other backflow assemblies, air gaps are a non-mechanical means of backflow prevention. 

Air gaps are very effective whether backsiphonage or backpressure is the backflow condition being isolated. Air gaps may be fabricated from commercially available plumbing components. An acceptable air gap must be twice the supply pipe diameter, but never less than one inch, measured vertically above the receiving vessel. Air gaps must be rigidly mounted and the discharge end of the supply pipe must not be threaded or provided with a means for making connections. Use of an air gap means that there will be clear vertical separation between the pressurized supply pipe and a non-pressurized receiving vessel.

In addition to the above, the following should be considered when deciding upon whether or not to use an air gap in your particular installation:

  • In a continuous piping system, each air gap would require the added expense of reservoirs or tanks and perhaps the installation of a secondary pumping system.
  • The air gap may be easily defeated if the proper gap isn't maintained, thus negating backflow protection.
  • At an air gap, the water is exposed to the surrounding air with its inherent airborne pollutants or contaminants and thus may no longer be potable.  
  • For these and other reasons, air gaps must be inspected frequently to assure that they are providing adequate backflow protection.

The general configuration of an approved air gap is illustrated below: