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How to choose a backflow device for an irrigation system

In most homes the landscape irrigation system and the potable water system are one and the same -- there is no separation. Here's where the danger comes in. Sprinkler lines lay in gardens and lawns that are often sprinkled with weed killer, pesticides, and other poisons. Irrigation systems occasionally draw water inward (backwards). This phenomenon is known as a siphon or backflow. Backflow can result in contaminated water at a tap inside the home. Is there any wonder why the building code requires that all outside irrigation lines have some sort of anti-siphon protection?

Backflow can be defined as the unwanted reverse flow of any liquid, solid or gas in a piping system.  In an irrigation application, this means that water within the irrigation system may find its way back into the potable (drinkable) water system during a backflow incident.

"A backflow preventer is a device designed to prevent potable water from being contaminated with non-potable water during a condition of backflow."

There are two main types of backflow: Back Pressure and Back Siphonage.

    • Back pressure is when pressure downstream of the device is greater than the pressure upstream. Water hammer is an example of backpressure.
    • Back siphonage occurs when a vacuum is created upstream and water gets literally sucked backwards. An open fire hydrant, or water main break can cause back siphonage. Not all backflow devices are created equal. 

Choosing a Backflow Device
These are a must in all sprinkler systems if you want to protect yourself from physical harm as well as the law. The backflow preventer prohibits the contamination of your personal water supply. And while the anti-siphon valve already has such a stabilizing mechanism built in, the Globe and Angle Valve are incomplete in such regards. Thus, if you have selected to install a Globe or Angle Valve, you must also choose one of the following backflow preventers.

 APAS Anti-Siphon Irrigation Valve
Atmospheric Vacuum Breaker recommends that you avoid this type of backflow device.  It is extremely unreliable!

If you choose this type, purchase one unit for each control valve located on the landscape.  These are the cheapest of backflow preventers; however, most cities don't allow them in irrigation systems.  When installed, each must be at least 6" above the highest sprinkler head in the zone it protects. 

If you have several control valves, specifically six or more, AVBs will cost you more in the long run.  A single pressure vacuum breaker becomes the more practical choice.  The images to the right depict a typical atmospheric vacuum breaker and one integrated into an anti-syphon zone control valve.

Pressure Vacuum Breaker
Comparable in function to the AVB but only requires the installation of one breaker for the entire irrigation system. Accordingly, the number of control valves on your property is irrelevant. It should be mounted on the mainline leading to all control valves, 6" above the highest sprinkler head.
This is one of our Best Selling types of backflow devices.
Reduced Pressure Backflow Preventer
The best that money can buy, this preventer is almost always found in a commercial sprinkler system and can be a great compliment to a residential system as well. The device allows for the application of fertilizers or other chemicals into the irrigation system, a function that no other preventer performs. Used mostly in dangerous situations, it should be set up 12" above ground.

Double Check Backflow Preventer
This preventer allows for the flow of pure water that is not susceptible to contaminants in the air. Though this device does not allow for the application of chemicals into the irrigation system, it is cheaper in comparison to the RPBP. Typically, it is installed beneath the ground in a vault and sometimes in basements, a tactic that protects them from freezing.
This is one of our Best Selling types of backflow devices. It works great when installing the backflow device in a basement or below grade.

The Problem With Double Check
Though the preventer is built with two check valves, in the instance that dirt or other objects gets stuck in one valve, it is likely that the same will happen to the other valve at the same time. Thus, the two valves could fail at the same time, in turn causing the water supply to be contaminated by noxious substances.

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