Plumbing Tips: What Does Anti-Siphon Or Backflow Prevention Mean?

Anti-siphon and back-flow prevention devices are a crucial part of outdoor plumbing.

Whether you own a home or business with outdoor plumbing, chances are good that you've heard the terms "anti-siphon" or "back-flow prevention." These terms, though technically different, generally both refer to the blocking of a fluid's backward motion through a pipe, particularly one with an open outlet, such as a spigot or lawn irrigation system.

To better understand how these devices work, consider this example. When you open a spigot, water flows outward because pressure inside the pipe is positive, meaning that the water pressure exceeds atmospheric pressure. However, when the spigot is closed, the pressure inside the pipe becomes negative, or less than atmospheric pressure. The result is a force known as "siphon," which causes the backward flow of water through the pipe, a movement known as back-flow. So, anti-siphon refers to a mechanism that blocks the action of siphoning, and back-flow prevention refers to a mechanism that prevents backward flow. The difference between these terms anti-siphon and back-flow prevention is subtle, because they both refer to mechanisms that keep water flowing in only one direction, but you will often hear them being used interchangeably.

You may already know that anti-siphon and/or back-flow prevention devices are widely required, but you may want to know why. The answer is, "contamination." Anytime a hose is placed in a yard or an underground sprinkler is operated, there's a chance for contaminants -- such as pet excrement, fertilizer, lawnmower gas, etc.-- to enter the line. Without anti-siphon or back-flow prevention, these contaminants could be sucked into the source water, where they may be ingested in drinking water. To prevent this scenario from playing out, most states mandate that anti-siphon and/or back-flow prevention devices be installed in outdoor plumbing. Furthermore, of the states that mandate these devices, most also require that they be installed by a certified plumber. Some states, however, allow homeowners to do the installation upon completion of a brief safety course. Remember, it is a good idea to check your local plumbing codes before installing anti-siphon or back-flow prevention devices yourself.



If you're considering performing such an installation, many anti-siphon and back-flow prevention devices are available to you. A brief list includes the anti-siphon valve, which is often built into a standard house spigot; the AVB, or atmospheric vacuum breaker; the PVB, or pressure vacuum breaker; and the DCV, or dual-check valve. Each of these devices works in a slightly different way, so the guidelines for their installation vary. For instance, for an AVB to be installed, it is often required that it be placed in-line after a sprinkler valve, and that it be mounted six inches above grade. A PVB, on the other hand, is often required in-line before any sprinkler valves, at a minimum of twelve inches above the highest sprinkler in the yard. As for the DCV, most states restrict its use altogether, because it is hard to tell by visual inspection if the device is working properly. Again, these requirements vary widely throughout the country, so it is advisable to learn what these requirements are prior to installation.

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