Absolutely frightening that your locale would allow an anti-siphon valve. They have been outlawed for more than 30 years in most states. They do not protect against backpressure hence are a hazard to our drinking water supply. From a cost standpoint, if you use this awful device, you must have one for every zone. Smarter choice is to install one brass pressure vacuum breaker that protects the entire system. You only need one.