Backflow preventer information and frequently asked questions - NH and MA

A backflow prevention system refers to a system designed to prevent the flow of contaminated water or other substances (gasoline, oil, etc.) to the drinking water system. The American Backflow Prevention Association defines backflow as:

American Backflow Prevention Association

Backflow preventers and backflow prevention systems can be found anywhere a potable (drinking) water system connects to any source that may allow contaminants into drinking water. This includes (but is not limited to) residential and commercial lawn irrigation systems, water spigots, public drinking fountains, water treatment plants, and in-ground swimming pools. In all cases, backflow preventers protect the drinking water supply by preventing contaminants from crossing into the supply from other systems.

The importance of backflow prevention devices cannot be over-emphasized, as seen in case studies detailed in "Stop Backflow News," a publication of Watts Water Technologies.

Q. What is meant by Degree of Hazard?
A. Degree of Hazard is a commonly used phrase in cross-connection programs, and is simply a determination on whether the substance in the non-potable system is toxic (health hazard) or nontoxic (non-health hazard).

Q. What is toxic?
A. A toxic substance is any liquid, solid or gas which, when introduced into the water supply, creates or may create a danger to health and wellbeing of the consumer. An example is treated boiler water.

Q. What is non-toxic?
A. A non-toxic substance is any substance that may create a non-health hazard, is a nuisance, or is aesthetically objectionable. Examples are food products, such as sugar, soda, etc.

The type of connection and degree of hazard dictate the proper cross connection device. There are five basic products that can be used to correct cross-connections.


  1. Air Gap
  2. Atmospheric Vacuum Breakers, ex. Hose connections
  3. Pressure Vacuum Breakers, ex. Irrigation systems (testable device) - can be used in other non-potable applications if they are not subject to back pressure. Installed above the usage point. Normally tested once a year.
  4. Double Check Valve Assembly (testable device) - used in a low hazard, non-toxic application. Testing is required once a year.
  5. Reduced Pressure Zone Assembly (testable device) - directly connected to a system which may be subject to back pressure or back siphonage, and where there is possibility of contamination which may pose a potential health risk. Toxic or high hazard situation. Required testing, minimum of once a year. Many water districts require two tests a year.