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Irrigation systems are generally subject to local and state regulation as to how they're protected. The systems come into contact with irrigation runoff water, which can be potentially fully of numerous different toxic substances. These substances include animal feces, fertilizer, trash and debris, petroleum from vehicle leaks and, really, just about anything that ends up on the ground. For that reason, it's important that the flow of water not only through the system, but back into the system, is regulated by precision devices.
Backflow is a counterintuitive proposition. An irrigation backflow preventer seems superfluous when one considers that the water that flows through the pipe only flows one way. There are conditions, however, that can lead to there being negative pressure in a plumbing system. When this occurs, whatever water is in the pipe will flow backward, potentially contaminating all of the fresh water that hasn't yet made its way through the system. If this contamination is pushed backwards past household faucets, pollutants from the irrigation system may infiltrate the fresh water. Of course, this carries with it those same hazards that are carried in the pipe. In the case of irrigation runoff water, this contamination can be nasty.
There are sometimes local regulations that will determine what types of irrigation backflow preventers you can use. There is a range of devices available, which increase in price along with their complexity. They are, however, an absolute necessity, even if you live in an area where they're not required, such as on a rural plat of land. Backflow prevention is a feature of all modern plumbing and it is a large part of the reason that tap water is potable in most modern cities. You'll find several different choices available to you.
One of the least expensive options for an irrigation backflow preventer is a double check backflow preventer. These devices are very simple, having no vents for excess water or air. They use two valves to control backflow. There are designs available that are similar and that are called "dual check" preventers. These are not the same devices as double check preventers. Double check preventers can be differentiated by the presence of two cocks on the device that allow it to be tested. These devices can be mounted below ground, and they attach to the main water supply for your sprinkler system.
If you have a larger system or, in particular, if you have a commercial system, you'll need what's called a reduced pressure backflow preventer. These are generally the most expensive of irrigation backflow preventer devices. They are also among the devices that must be installed above ground. They have the advantage, however, of not needing to be installed above the level of all the sprinklers, as is the case with devices such as a vacuum-driven backflow prevention device. Reduced pressure preventers can be very expensive relative to other options. When installed correctly, they offer a very high level of protection.
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