Office water bill gone up? How to check for a hidden leak

Before you start: find the meter and the stopcock

The water meter for an office usually sits in a small chamber in the pavement, the car park or a landscaped strip by the entrance. The internal stopcock is wherever the main comes into the building, most often in the plant room, a basement cupboard or under the ground-floor kitchen. If nobody knows where either one is, find out now. It is the first thing the engineer will ask, and it is a lot easier to answer on a quiet Tuesday than in the middle of a leak.

In a multi-let building there may be a landlord's main meter with sub-meters for each tenant, or separate supplies per floor. If you only rent part of the building, the managing agent will know how the supply is split, and each meter can be read the same way.

Step 1: read the meter when the building is empty

Pick a time when nothing should be using water: early morning before the cleaners arrive, or a Sunday. Read the meter, including the red dials or the last digits, and read it again thirty minutes later. Many meters have a small spinning disc or star that turns with any flow; if it is turning when the building is empty, water is going somewhere.

If the building has a smart or automatically read meter, ask the water retailer for the half-hourly data. A line that never drops to zero overnight tells you the same thing as a spinning dial.

Step 2: close the stopcock and read it again

Close the internal stopcock fully and watch the meter for ten minutes.

Step 3: the usual culprits inside the building

Small faults cause more rising bills than burst pipes do, and they hide in plain sight because each one looks harmless on its own.

• Toilets and urinals. Lift the lid on the cisterns; water trickling into the overflow or down the back of the pan is a fault. Older cistern-fed urinals flush all night and all weekend unless the control is working.
• Taps and showers. One drip a second is roughly 15 litres a day, and every floor has its own washrooms.
• Tea points and kitchens. Boiling-water taps and water coolers plumbed into the supply, dishwashers and ice machines with a feed that weeps.
• Plant. A pressurisation unit topping up the heating every day, a safety valve dripping into its tundish (the small open funnel below it), a booster pump or cold water tank that keeps refilling.
• Air conditioning and humidifiers. Some systems have a water feed of their own; a valve that no longer shuts off can run for months.

Isolate each suspect in turn and watch the meter. When it slows, you have found one of them; when it stops, you have found the last.

Step 4: if the meter turns with the stopcock closed

The supply pipe is leaking somewhere between the meter and the building. Walk its likely route and look for water coming up through joints in the paving, a damp patch in the car park that never dries or grass that stays green in a dry spell. Do not expect to see much, though. Under tarmac and paving, a leak can run for months without showing on the surface. Finding it is a job for acoustic listening, leak-noise correlation and tracer gas, followed by a targeted excavation once the spot is known. How ADI investigates water mains and car park supplies.

Turning the meter reading into litres

Water meters read in cubic metres, and one cubic metre is 1,000 litres. A leak of 20 litres an hour sounds small. Left running, it is 480 litres a day and more than 14 cubic metres a month, in a building where everyone has gone home.