To prevent contaminated irrigation water from entering the domestic water supply, the law in most places requires you to have an approved backflow prevention device permanently installed on your system. A backflow prevention device is simply a one-way valve that lets water through but doesn't let it turn around and go the other way.
Some backflow preventers are located at the point of connection (which I discuss earlier in this chapter). These devices are called pressure vacuum breakers, reduced pressure devices, or double check valves. Other backflow preventers are installed at each valve; these devices are called atmospheric vacuum breakers. The latter are easier to install than the other kinds, and they can be less costly if you have only a few valves.
Without getting into all the gory details, I can tell you that each type of back-flow device offers a particular level of protection, so you need to talk to your local building department about what kind to use. In most communities, you need a building permit for your backflow device; you may or may not need a permit for the rest of the system. You also need to have the device tested annually by a certified backflow test technician.
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