An anti-siphon valve is a control devise that prevents contaminated water from flowing backwards from the irrigation system into the household water supply. 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?
An anti-siphon valve must be installed at least 6 inches above the highest sprinkler head. Fortunately, the anti-siphon valve can be located anywhere in the water line as long as it is between the sprinkler valve and the inlet pipe to the house. This makes it easy to hide it behind a shrub or a bush. Most sprinkler valves are sold with an integral anti-siphon valve. By the way, a sprinkler valve is the faucet that is used to turn the sprinklers on and off. Some sprinkler valves are operated by hand and others are controlled electronically. One valve can control several sprinklers and one electric timer can control several valves.
An anti-siphon valve has no moving parts, therefore maintenance requirements are minimal. As we said, many folks construct their sprinkler systems so that the valves are hidden behind shrubs. This works when it comes to esthetics, but can be a bear if leaves and debris get clogged in the open top of the valve. It's a good idea to occasionally check each anti-siphon valve to ensure that its open top is clean and free of debris.