Install or Repair Backflow Valves
Be sure and check with local officials about what kinds of backflow valves can be used in your area. There are many different kinds of backflow valves, including an atmospheric vacuum breaker, a cheaper valve installed on the pipe right after the control valve. It needs to be installed higher than the highest sprinkler head operated by that valve-check with local officials to find out if there's a code. You probably need to use this kind if you have six or more valves.
Anti-siphon valves, the most common type, have a built-in atmospheric vacuum breaker and also must be installed higher than the highest sprinkler head.
Pressure vacuum breakers are similar to the ones above except that you only need to install one of them along the mainline leading to the control valves.
Reduced pressure backflow preventers are more expensive, used mostly in commercial irrigation installations. If you want a top-quality valve, this is the one. Double check backflow preventers; they're legal in many places, illegal in others.
-- Tips courtesy of HomeAdvisor.com