A backflow preventer is a device that stops water in your sprinkler system from traveling back into the public water supply. Yes you need one. This device is for public safety and you must have one to meet the building code. There are two most commonly used devices for residential sprinkler systems. The first is a Double Check Valve Assembly which is a single device installed directly after your main stop valve below ground, in a valve box. This device requires annual testing by a certified tester, usually a $35 to $45 charge. The other is an Atmospheric Vacuum Breaker (AVB). The AVB is a part of your zone valve. The zone valve with the AVB will need to be installed a minimum of 6" above the ground. This device does not need to be tested. The down side is, they're more susceptible to freeze damage and they're ugly.
If you have any more questions please call or stop by, we would be happy to answer them.