What Is a Backflow Preventer and Why Do I Need One?

The Basics

A backflow preventer is a mechanical device installed in a plumbing system to ensure that water flows in only one direction — from the public water supply into your building — and cannot reverse flow back into the public water main.

Backflow occurs when the pressure in your plumbing system reverses, which can happen under two conditions:

Back-pressure: Your system pressure exceeds public water main pressure, pushing water backward. This can happen with 

boilers, pumps, or pressure-boosting equipment.

Back-siphonage: A sudden loss of pressure in the public main (caused by a water main break, fire hydrant use, or heavy demand) creates a vacuum that sucks water backward from your system into the public supply.

Why Does Backflow Matter?

If backflow occurs, whatever is in your plumbing — including chemicals, fertilizers, irrigation water, industrial fluids, or even toilet water — can be pulled back into the public drinking water supply, contaminating it for everyone downstream.

High-hazard backflow events have included fertilizer from an irrigation system contaminating drinking water, industrial chemicals entering residential water supplies, and hospital medications entering city water mains. These are public health emergencies.

Who Needs a Backflow Preventer?

Any property that has a connection between the public water supply and a potential source of contamination is required to have a backflow preventer. This includes virtually every commercial, industrial, and institutional property, as well as any residential property with an irrigation system, pool, or other auxiliary water connection.

Who Installs It?

Backflow preventers must be installed by a licensed plumber or fire protection contractor, and must be installed in accordance with local plumbing codes and water authority requirements. After installation, the device must be tested by a certified backflow tester before the water service is activated.