A zone that will not shut off is either a valve that cannot seal or a controller that is still energising it. Unplug the controller: if the zone stops, the fault was electrical; if it keeps running, the valve is mechanically stuck open and the water has to be stopped at the irrigation shutoff valve. This wastes water continuously, so stopping it comes before diagnosing it.
Stop the water first
- Switch the controller off, or unplug it. This removes power from every station. If the zone stops within a minute or two, the problem was on the electrical side and the valve is fine.
- If it keeps running, close the irrigation shutoff valve. A valve stuck open mechanically does not care what the controller is doing. The isolation valve is the only thing that stops it — commonly near the water meter, at the backflow assembly, or in a box near the house.
- Check the valve’s own controls before assuming failure. Irrigation valves have a manual bleed screw and usually a flow control stem. Either one left cracked open by whoever last worked on the system produces exactly this symptom and takes a quarter turn to correct.
- Note which zone and whether it ever stops. Continuous flow versus a zone that eventually shuts off much later are different faults and the distinction matters.
Why a valve fails to close
Understanding the mechanism makes the causes obvious. An irrigation valve is held closed by water pressure, not by the solenoid. Mainline pressure sits above a rubber diaphragm and pushes it down onto its seat. When the controller energises the solenoid, the solenoid opens a small bleed port that releases that pressure, the diaphragm lifts, and water flows to the zone. De-energise the solenoid, the port closes, pressure rebuilds above the diaphragm, and it seals again.
So anything that prevents pressure rebuilding above the diaphragm, or prevents the diaphragm seating, leaves the valve open:
- Debris on the diaphragm seat — a grain of grit, a fragment of pipe shaving, or sediment. Extremely common, and often fixed by cleaning rather than replacing anything.
- A stiff, distorted or torn diaphragm. Age hardens the rubber until it no longer conforms to the seat.
- A solenoid whose plunger has stuck open, so the bleed port never closes.
- A solenoid still receiving voltage, which is a controller fault rather than a valve fault.
- The manual bleed screw or flow control left open.
- Very high supply pressure, which can hold a marginal diaphragm off its seat and shortens the life of every component on the system.
The electrical version
If unplugging the controller stops the zone, the valve is mechanically sound and the controller is at fault. The usual cause is a failed triac — the solid-state switch on a station output that has failed in the conducting state, so it keeps passing 24 volts whether the program calls for it or not.
The tell is that the controller’s display looks entirely normal while the zone runs continuously, sometimes even with the unit showing off. A controller that cannot stop a zone is a controller that has lost control of that output, and it is usually a replacement rather than a repair on that station.
The much simpler electrical cause is worth ruling out first: a program with overlapping start times, or two programs both running, can chain zones together so the system appears never to finish. That is a scheduling error, not a fault, and it is corrected in the program.
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What it is costing while it runs
| Aspect | Consequence |
|---|---|
| Water use | A full zone running continuously is among the highest-volume faults possible on the system |
| Landscape | Continuously saturated soil kills turf and shallow-rooted plantings as effectively as drought |
| Hardscape | Saturated base beneath driveways, walkways and patios leads to settling and cracking |
| Compliance | Water running into a gutter, street or storm drain may put you in breach of a local water waste ordinance |
| The valve itself | A diaphragm held off its seat by debris wears against it, so the longer it runs the more likely a rebuild becomes necessary |
The repair
Most of these are valve rebuilds: the bonnet comes off, the diaphragm and seat are cleaned or the diaphragm is replaced, a new solenoid goes on if that is the culprit, and the valve body stays where it is. No excavation beyond opening the box, and the parts are inexpensive. As a general North Georgia market figure a rebuild with a new diaphragm and solenoid commonly runs roughly $90 to $225 per valve — a market estimate, not a quote.
Where several valves are the same age, it is worth discussing them together. Valves installed at the same time in the same water tend to fail within a year or two of each other, and dealing with the manifold in one visit costs far less than three separate calls. See irrigation valve repair.
Related answers
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Services that fix this
Irrigation Valve Repair
The component that decides whether a zone runs at all — diagnosed, rebuilt or replaced.
Read more →Sprinkler Zone Repair
One zone dead while the rest run fine — the most common irrigation call there is, and the most diagnosable.
Read more →Controller Repair
The brain of the system: no power, dead stations, lost schedules and surge damage diagnosed and repaired.
Read more →Where we work
Irrigation Medics repairs irrigation systems across Hall County and the Lake Lanier communities, the Gwinnett corridor through Duluth, Suwanee and Buford, and North Fulton and Forsyth — Johns Creek, Alpharetta, Milton, Roswell and Cumming. Same rates throughout. See the service area.