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g/plumbing • Posted by u/backflow_ben •

Backflow on an irrigation system — what type is required and why?

Getting push-back from a homeowner about the backflow preventer requirement on their new irrigation system. They say "my neighbor's system doesn't have one."

AHJ requires a double-check valve assembly (DCVA) for residential irrigation here, but the homeowner is insisting their neighbor gets away with just a hose bib backflow.

I know the hose bib backflow (AVB) is code-compliant in some jurisdictions for garden hose use, but not for permanently installed irrigation systems with sub-soil heads. Can someone help me explain the technical reason this matters, so I can explain it to the homeowner and not just say "because code says so"?

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u/water_heater_wally •

This is why I like backflow — there's real public health science behind every requirement. It's not bureaucracy, it's just applied chemistry and fluid mechanics.

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u/drain_dave •

In short: the neighbor's setup is probably code-violating too, or it's an older install that predates the current requirement. "My neighbor has it" is not a code argument. Explain to the homeowner that backflow contamination events happen and cities have been sued over them. A DCVA protects everyone including them.

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u/pipefitter_pat •

The technical reason: irrigation heads are sub-surface, meaning they can be submerged in contaminated soil/water. If water pressure drops (water main break, firefighting draw-down), there's negative pressure that can suck contaminated water from around the submerged heads back into the potable supply. A DCVA prevents back-siphonage AND backpressure. An AVB (hose bib backflow) only prevents back-siphonage and must not be installed where it can be submerged — which sub-surface irrigation heads regularly are.

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