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The irrigation system that came with our house (looks like a DIY job) has a backflow protector on the main line and all the valves are anti-siphon. The valves are old and some of them leak so we're planning to replace them. I'm wondering if there's a reason why the valves are anti-siphon instead of inline. I've seen a few instances of this configuration on the Internet, but with no reasonable explanations.

As far as I know, our municipality doesn't have any rules requiring valves to be anti-siphon. In fact, we live in an area that can freeze overnight as late as June and as early as September, so moving to inline to get the valves underground would help make the system more resilient to freezing.

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I agree it seems redundant to use anti-siphon valves in combination with a backflow prevention device. But people don't always choose these valves because they needed the anti-siphon feature. Maybe the person who chose them just doesn't like operating or maintaining valves that are underground (harder to bend and reach) in a box full of spider webs. They could have preferred the up-and-down arrangement of the above ground anti-siphon type rather than the basic inline type.

Your "authority having jurisdiction" gets to have the final word on the matter, however. Call your water supplier and ask them whether you have an accepted type of backflow prevention device and whether they require anti-siphon valves too.

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