What Is Backflow? A Plain-English Guide

Backflow, in One Paragraph

Water normally flows one way: from the city main into your home. But pressure drops — a water-main break, a fire hydrant in use, a pump failure — can reverse the flow. When that happens, water sitting in your irrigation pipes (mixed with fertilizer, pesticides, and soil bacteria) can get sucked back into the clean drinking-water supply. That reverse flow is called backflow, and the valve that prevents it is a backflow preventer.

Why Phoenix cares so much

Phoenix is an irrigation-heavy city in the desert. Nearly every home has a sprinkler system, which means nearly every home has a cross-connection between potable water and lawn chemicals. That's why the annual test is law, not a suggestion.

Signs your device may need attention

• Water constantly dripping or spraying from the device
• Visible corrosion, cracks, or freeze damage on the brass body
• A failed test report from a previous year
• Sprinkler heads with low pressure (sometimes a stuck check valve)

Backflow vs. back-siphonage

You'll hear two terms. Backpressure is when downstream pressure exceeds supply pressure (e.g., a boiler pushing back). Back-siphonage is when supply pressure drops and creates suction. Your preventer guards against both — which is why the annual gauge test checks each valve independently.

Now that you know what it is, here's what the test costs and what Phoenix requires.