Plumbers narrow a hidden leak by elimination. Confirm movement at the meter, close the main to separate the service line from the building, then close zone or riser valves one at a time while watching the meter or a submeter. A pressure-hold test on the suspect section then confirms it before pinpointing.
What is the starting point?
The meter. EPA's guidance for commercial facilities says to read the meter when all water-using equipment is off and nobody is using the restrooms, then read it again an hour later. A significant change means a leak somewhere in the distribution system or the facility. Many meters also show a leak indicator, such as a spinning dial or flashing symbol.
If the meter isn't moving, the building may not be leaking at all, and the high bill may be a use or billing question. Comparing bills helps with that.
How is the building divided?
- 1Main valve closed. With the building's main shut, if the meter still moves, the leak is between the meter and that valve, on the buried service line. That case has its own page.
- 2Zone and riser valves closed one at a time. With the main open, close one zone and watch the meter. When the flow drops, the leak is in the zone just closed.
- 3Submeters. EPA says submeters can help identify leaks and show when equipment is malfunctioning, and that a monitored submeter reading that is unexpectedly high may point to a leak or a malfunction. Submeters versus the utility meter explains the comparison.
- 4Pressure hold. Once a section is isolated, a plumber can watch whether it holds pressure on a gauge. A section that loses pressure with its valves closed has a leak in it. This is the same logic as the meter test, applied to a smaller piece of pipe.
This only works if the valves work. A valve that doesn't fully close muddies every reading. Phoenix Water Services recommends knowing where your shutoff is and testing it regularly, and warns against using the city's meter valve yourself, since that takes a special tool. Testing the main before a leak hunt saves time.
What happens once the section is found?
Isolation tells you which run of pipe, not the exact spot. A plumber then pinpoints it with listening and other tools; how those work is covered separately. Each shutdown also affects tenants, so plan the shutoffs and notify them first.
EPA adds that a facility that cannot account for more than 10 percent of its water use during a water assessment may have leaks in distribution lines or equipment and warrants further investigation. What a detection quote should cover is worth asking before work begins.