Water moving while the system is off means something on the pressurised side is failing, because the mainline holds pressure around the clock even when no zone is running. The three causes are a mainline leak, a zone valve that is no longer sealing shut, and low-head drainage, which is not a fault at all. Close the irrigation shutoff valve to stop the loss, then work out which of the three you have — only one is harmless.
Understanding what stays pressurised
This confuses people reasonably: if the controller is off, why is water moving? Because an irrigation system has two pressure states, not one.
The mainline runs from your supply to the zone valves and is pressurised continuously, twenty-four hours a day, all season. It has to be, so that any zone can start instantly when the controller calls for it. The lateral lines beyond each valve are only pressurised while that particular zone is energised.
So a fault anywhere on the mainline, or in a valve that should be holding the mainline back, leaks constantly. That is why this category of fault produces the largest water bills — it is not limited to run times.
The three causes, and how to tell them apart
Low-head drainage deserves explanation because it gets diagnosed as a leak constantly. When a zone shuts off, the water sitting in the lateral line has to go somewhere, and gravity drains it out of whichever heads are lowest on the slope. On a sloped North Georgia lot that can be a noticeable amount of water pooling around two or three heads after every cycle. It stops on its own within minutes and the meter does not move, and the diagnostic question is simply: does it stop? A leak does not stop.
| Cause | What you see | Serious? |
|---|---|---|
| Mainline leak | Soggy ground or water surfacing with the system off, meter turning constantly, often reduced pressure system-wide | Yes — highest waste, and can undermine hardscape |
| Valve not sealing | Water trickling continuously from heads on one specific zone, meter turning, that zone always damp | Yes — continuous loss, and usually a rebuildable valve |
| Low-head drainage | Water weeps briefly from the lowest heads on a zone after it finishes, then stops. Meter still. | No — normal behaviour |
Confirming it with the meter
- Shut off every indoor water use. No dishwasher, washing machine, ice maker or shower, and make sure the controller is not running a cycle.
- Watch the meter for fifteen minutes. Record the reading, wait, read again. Any movement means water is flowing.
- Close the irrigation shutoff valve and repeat. If the meter now holds steady, the loss is in the irrigation system. If it still moves, call a plumber — the fault is on the house side.
- Walk the property while it is pressurised. Look for water at heads on a single zone, which points to a valve, versus a soggy area unrelated to any head, which points to the mainline.
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Diagnostic-first visits across North Georgia. Call 770.335.8595 or request service — you get an estimate before any work begins.
What to shut off, and when
For a constant leak, switching off the controller does nothing, because the controller does not control mainline pressure. The valve you need is the irrigation shutoff — a dedicated valve that isolates the sprinkler system from the house supply, commonly found near the water meter, at a backflow assembly, or in a box close to where the irrigation line leaves the house.
Closing it stops the loss entirely while you arrange a repair, and costs you nothing but the ability to water. If you cannot find it, the guide on locating the irrigation shutoff valve covers the usual places. It is genuinely worth knowing where it is before you need it.
Why a valve stops sealing
Valves close by using mainline pressure against a rubber diaphragm. Three things defeat that: the diaphragm stiffens or tears with age, grit or debris lodges on the seat so the diaphragm cannot seat fully, or the solenoid fails in a way that leaves its bleed port open.
A fourth cause is worth checking first because it costs nothing: the manual bleed screw on the valve bonnet, or the flow control stem, left cracked open by whoever last worked on the system. That produces exactly the same symptom and takes a quarter turn to fix.
Where the valve itself has failed, it is usually rebuilt rather than replaced — a new diaphragm and solenoid in the existing body. See irrigation valve repair.
Related answers
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Services that fix this
Irrigation Leak Repair
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The component that decides whether a zone runs at all — diagnosed, rebuilt or replaced.
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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.