Water Mains Leak Detection in Colnbrook

Symptoms

A property in SL3 had a water meter showing continuous usage, indicating water was being lost somewhere in the supply system even when no taps or appliances were in use. The reading of 1 litre per minute suggested an active leak that, left unresolved, would continue wasting water and could cause ground movement or damage over time.

Closing the internal stopvalve made no difference to the meter reading, which immediately pointed to the leak being on the external mains supply pipework rather than anywhere inside the property. This is a common distinguishing test in water mains leak detection, and in this case it gave a clear steer toward the external supply as the source of the problem.

What was less obvious at the outset was that there were in fact two separate issues contributing to the leak — a consequence of a previous mains replacement that had been carried out without fully decommissioning the old pipework.

Methods Used

With the external mains supply confirmed as the source, tracer gas was introduced into the mains pipework via the cold supply to the kitchen sink. The route of the pipework was then systematically checked above ground.

A strong gas trace was detected at the front of the property, and the area was excavated to expose the pipework beneath.

The excavation revealed open-ended pipework — the old mains had been left connected to the internal cold supply when a mains replacement had been carried out previously, creating a direct path for water to escape. Once the open end was capped and a pressure test was carried out, the result indicated a second leak was still present.

Further inspection inside the kitchen uncovered an obsolete internal stopvalve hidden behind boxing at the previous position of the sink. Pressure testing is a reliable verification tool in water mains leak detection, and in this case it was the step that confirmed both that more work was needed and, ultimately, that the completed repairs were successful.

Investigation Process

1. 1
   Meter reading confirmed active leak

   The water meter was found to be registering continuous usage at 1 litre per minute. Closing the internal stopvalve produced no change in the meter reading, confirming the leak was on the external mains supply pipework.

2. 2
   Tracer gas introduced via kitchen cold supply

   Tracer gas was fed into the mains through the cold supply to the kitchen sink. The route of the pipework was then checked along its length above ground.

3. 3
   Gas trace detected at front of property

   A large tracer gas reading was detected at the front of the property. The area was excavated to expose the pipework at that location.

4. 4
   Open-ended pipework exposed

   Excavation revealed that the old mains pipework had been left connected to the internal cold supply following a previous mains replacement, with the end left open and uncapped.

5. 5
   Open end capped; pressure test indicated second leak

   A 15mm cap was fitted to the open end of the old mains pipework. A pressure test was then carried out, and the result indicated that a second leak was still present.

6. 6
   Obsolete stopvalve found behind boxing

   Further inspection of the kitchen revealed an obsolete internal stopvalve concealed behind boxing at the previous position of the sink. This was the source of the second leak.

7. 7
   Stopvalve removed and pipework capped

   The obsolete stopvalve was removed and the pipework capped with a 15mm cap. This completed the repair to both identified issues.

8. 8
   Pressure test confirmed successful repair

   The mains was pressure tested at 3.5 bar for 30 minutes with no pressure loss, confirming the repair was successful. The pipework was flushed through and the excavated area backfilled.

Result & Outcome

Two separate defects were identified and repaired. The primary leak was at the front of the property, where the old mains pipework had been left connected to the internal cold supply following a previous mains replacement, with the end open and discharging water into the ground. A second leak was then found internally — an obsolete stopvalve hidden behind boxing in the kitchen at the former position of the sink.

Both issues were resolved using 15mm caps. The mains was then pressure tested at 3.5 bar for 30 minutes, confirming no further pressure loss and a complete repair. The excavated area at the front of the property was backfilled once the test was passed and the pipework flushed through.

For a water mains leak detection job of this type, the staged approach — tracer gas to locate the external source, followed by pressure testing to verify the repair — was what identified the second, less visible defect that would otherwise have continued to cause water loss. The meter should now show no usage when no water is being drawn.