What does "anti-siphon" mean?
To many people, "anti-siphon" is a technical term best understood by plumbers and engineers. In reality, it's a very easy concept to grasp.
Anti-siphon describes the design of a plumbing product (in our case the 400A Toilet Fill Valve) that prevents "backflow" or "back-siphonage". Either is a circumstance in which water (possibly used or impure) unintentionally flows back into the drinkable water supply via negative pressure. Negative pressure in a municipal water supply is not common, but can occur (as could happen when a nearby fire hydrant is opened).
Why is backflow hazardous?
Common cleansers, insecticides and untreated organic matter all pose a threat to household health and safety if accidentally back-siphoned from the toilet tank into the drinking water supply. Backflow/back-siphonage must be prevented to ward off such contamination.
Plumbing codes and standards both require anti-siphon design.
According to current codes and standards, anti-siphon operation is required for all toilet tank fill valves. The difference between standard vs. code is much like national vs. local.
Standards are product design and performance requirements set by ASSE (American Society of Sanitary Engineering). ASSE standard 1002-86 requires anti-siphon back flow preventors on all toilet fill valves..
Codes are based upon such standards and adopted/enforced on a local (city, county or state) level.
Is your fill valve OK? Three ways to identify a code-approved fill valve.
 Look for the UPC shield or the words "anti-siphon" somewhere on the valve.
- Note the presence of an air entrance at the top the valve. This could be a series of holes surrounding the top or a large air entrance around the valve operating lever. When installed, this air entrance is above the water line where it can draw air in.
- The valve height adjusts to allow a 1" gap between the overflow pipe and the "Critical Level" as marked on the valve. This 1" difference directs water to the overflow pipe instead of the fill valve's air entrance (where it could pollute drinkable water).
Other backflow danger zones.
Toilets aren't the only potential backflow culprits. Kitchen faucets-on-a-hose, sprinkler systems and beverage dispensers also need some type of anti-siphon protection. Even an ordinary garden hose can pose a threat when submerged in a bucket or connected to a sprayer containing chemicals.
To ensure peace-of-mind, employ anti-siphon practices throughout the house and yard. If in doubt about a fixture's backflow prevention, buy and install inexpensive backflow prevention devices for all threaded faucets.
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