What is a backflow preventer?

A backflow preventer is a one-way valve installed on water lines to stop contaminated water, wastewater, or non-potable fluids from siphoning backward into the clean drinking water supply. Backflow occurs when water pressure inside a building exceeds street supply pressure, or when a system depressurizes suddenly, creating a vacuum that pulls contaminated water upstream.

In Ontario, backflow preventers are required by plumbing code on water services serving certain fixtures and systems, including irrigation systems, booster pump systems, lawn treatments, industrial processes, and other cross-connection hazards. They protect public health by preventing bacteria, chemicals, pesticides, and other contaminants from entering the municipal water supply or shared building systems.

Common types include:

• Reduced pressure zone (RPZ) devices - the most protective option, with two check valves and a relief valve that actively discharge backflow. Installed outdoors or in an accessible location with proper clearance.
• Double check valve (DCV) assemblies - two check valves in series, suitable for moderate hazards where space is tight or outdoor installation is not practical.
• Atmospheric vacuum breakers (AVB) - simpler, cheaper devices for low-hazard indoor uses like single hose bibs or laboratory sinks.

Installation location depends on what you are protecting. Water services to the entire building may need a meter-mounted preventer. Individual fixtures like garden taps or irrigation lines need their own protection. Licensed plumbers in Ontario conduct cross-connection surveys to identify hazards and size, install, and test preventers to meet code. Most devices require annual or five-yearly certification testing.