What It Is
An irrigation backflow preventer is a device installed where a lawn sprinkler system connects to the potable water supply. It prevents fertilizers, pesticides, and soil contaminants from being siphoned backward into drinking water lines if supply pressure drops. Most municipalities require a testable backflow preventer on all in-ground irrigation systems, with annual certification by a licensed tester.
What It Does
When a sprinkler system runs, sprinkler heads are typically at or below ground level, submerged in or near soil that may contain fertilizers, pesticides, animal waste, or other contaminants. Under normal conditions, water flows from the supply line to the heads. But if supply pressure drops during the irrigation cycle - a common event during peak demand or a main break - a siphon can form that draws whatever is at the sprinkler heads backward through the pipes and into the house supply.
The irrigation backflow preventer stops that from happening. It sits on the supply side of the irrigation system, between the main water line and the sprinkler controller or valve manifold, and physically blocks reverse flow regardless of what happens to system pressure.
Where It Is Located
The device is typically installed above ground, near the point where the irrigation supply branches off the main water line - often close to the foundation or in a utility area near the water meter. It may be in a protective box or enclosure to keep it accessible for testing. You will recognize it as a brass assembly with two shutoff handles, a test cock, and sometimes a small pressure relief port.
How It Works
Most irrigation systems use a reduced pressure zone (RPZ) assembly or a pressure vacuum breaker (PVB) as the backflow preventer. An RPZ device uses two spring-loaded check valves separated by a relief valve that vents to atmosphere if pressure between the checks drops - providing a visible signal of any failure. A PVB uses a check valve combined with an air inlet valve that opens on loss of pressure, admitting air to break any siphon. Both types are testable, meaning a certified tester can confirm the internal components are functioning properly without disassembling the device.
Common Problems or Signs of Failure
- Continuous dripping from the relief port on the RPZ assembly between irrigation cycles
- Device fails the annual test - check valves not seating, relief valve stuck
- Visible corrosion, mineral scale, or physical damage to the brass body
- The assembly was never installed on a system that has been operating for years - common in older systems installed before code requirements were enforced
- Shutoff handles are corroded in the open position and cannot be closed for testing
Can a Homeowner Check It?
- Locate the backflow preventer assembly near the irrigation supply connection and confirm it exists
- Check whether it is dripping from the relief port when the irrigation system is not running
- Look for a test report sticker or tag - most municipalities require annual testing with a record kept on file
- Confirm the device is above ground and accessible, not buried under landscaping
When to Call a Plumber
If your irrigation system does not have a backflow preventer, or if the existing device is leaking or overdue for its annual test, a licensed plumber who is also a certified backflow tester can handle both the installation and the certification. Most municipalities send annual testing notices to property owners, but the homeowner is responsible for compliance. Failing to have a device tested can result in a notice to disconnect the irrigation supply.
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