What is Backflow?

Backflow is the flow of water in the wrong direction.  Backflow valves keep contaminants out of drinking water when working correctly. 

There are two types of backflow.  They are backpressure and backsiphonage. 

Backpressure is the backflow caused by a downstream pressure that is greater than the upstream or supply pressure in a water system.  Backpressure can result from an increase in downstream pressure, a reduction in the potable water supply pressure, or a combination of both.  Increases in downstream pressure can be created by pumps, temperature increases in boilers, etc.  Reductions in potable water supply pressure occur whenever the amount of water being used exceeds the amount of water being supplied, such as during water line flushing, fire fighting, or breaks in water mains.  

Backsiphonage is backflow caused by a negative pressure in a water system.

Helpful links:

American Backflow Prevention Association

University of Southern California Foundation for Cross-Connection Control and Hydraulic Research