The manifold is the control unit of the sprinkler system. It is composed of a group of control valves that deliver water to different parts of the lawn. Position it in a convenient spot, where you can turn it on and off without getting wet. Try to conceal it with a cover or box, since the plumbing is seldom attractive. One manifold each is needed for a front lawn and another is needed for the back lawn. Together they are referred to as valves and circuits. It should also be placed in a position which is secure and cannot easily be tampered with. You might consider placing the manifold at the side of the house inside the perimeter fence that surrounds your yard for example.
The control valves that make up the manifold are usually globe valves. They are the same type of valves used for faucets in the house. Globe valve shut off the flow of water by pressing a soft disc against a smooth valve seat.
Control valves offten have anti-siphon valves attached to them. Anti-siphon valves, also known as vacuum breakers, prevent the back flow of water into the house supply. As a result they are sometimes required by local ordinances and are always a good idea to include.
Each control valve controls several sprinkler heads. This group of sprinkler heads is called a circuit. Two factors control the number of circus you need. What is the available flow rate? If too many sprinklers are turned on at one time, the water pressure drops, preventing the heads from throwing as far as they are supposed to. For proper coverage, it is best to turn on one circuit at a time.
The other factor to consider is the watering needs of different areas. For example, if a shrub bed is on the same circuit as the lawn, the shrub bed will be watered as often as the lawn, even though it does not need as much water.