Diagnosing Low Pressure in a Single Irrigation Zone

Why One Zone Has No Pressure

When a single irrigation zone shows little or no pressure while others work fine, the problem is usually at the zone valve or in the lateral lines downstream of that valve. A hissing sound at the valve when it's energized often indicates a restriction—debris in the valve seat, a partially closed flow control, or a failing solenoid plunger.

Step-by-Step Troubleshooting

1. Check the manual bleed: Open the valve's manual bleed screw (thumbscrew) slightly. If pressure improves, the solenoid plunger may be swollen or the valve diaphragm is damaged.
2. Inspect for debris: Shut off the main water, disassemble the valve, and clean the seat, ports, and diaphragm. Small rocks or grit often lodge where the pipe reduces to ½″ or ¾″ at the sprinkler heads.
3. Verify full water supply: Ensure the main shut-off valve and any isolation valves are fully open. A partially closed gate valve can choke flow.
4. Listen for hissing: With the zone running, listen at the valve. A loud hiss suggests a restriction inside the valve body.

When to Rebuild or Replace

If cleaning doesn't help and the valve is a current model, install a rebuild kit (diaphragm, spring, solenoid plunger). For obsolete valves, replacement is the only reliable option. Remember: manually opening an old valve via the bleed screw can sometimes cause it to stick in the open position.

Quick Test for Multiple Valves Running

If pressure drops across all zones, check the water meter while a single zone runs. A spinning meter with only one zone active may indicate a stuck valve or crossed wires causing multiple valves to open simultaneously.