Backflow Prevention

The spot you are guarding is called a cross-connection: any place a potable water line can touch water that is not safe to drink. Backflow is that water traveling the wrong way. It happens two ways. Backsiphonage is a pressure drop, say a main breaks down the street, pulling water backward like a straw. Backpressure is something downstream, usually a pump, pushing harder than the city main.

The hardware that breaks the path is ranked by how much it protects. An air gap is the strongest, an open vertical space at least twice the width of the outlet opening, because air cannot siphon. Below it sit a reduced pressure assembly, a pressure vacuum breaker set at least 12 inches above the highest pipe downstream, and an atmospheric vacuum breaker set at least 6 inches above.

A desert lot piles these up fast. The plumbing code puts a vacuum breaker on every sillcock and hose bibb, and a vacuum breaker or reduced pressure assembly on yard irrigation. Inject fertilizer through that irrigation and the code steps you up to the reduced pressure assembly. Add a pool auto-fill valve, an evaporative cooler makeup line, and on unserved land a cistern topped off by hauled water, and one house carries half a dozen.

Here is the part owners get backward. Arizona's cross-connection rule, A.A.C. R18-4-215, is what makes a water utility require and yearly test these assemblies. It exempts single-family homes used only as residences, unless a cross-connection problem turns up. So on a normal custom home the plumbing code drives the devices, and your plumbing inspection is where they get checked.

For example, a Scottsdale build with a zoned drip system gets a pressure vacuum breaker about a foot above the highest emitter. Under the state rule that device only holds up on irrigation with no chemicals and no backpressure. Start running fertilizer through it and it has to become a reduced pressure assembly.