Water Mains Leak Detection in Enfield Town

Symptoms

At a property in N14, the water meter was registering a continuous flow of approximately 0.5 litres per hour with all known outlets closed. There were no visible signs of a leak — no wet patches, no surface water, nothing obvious to indicate where water was being lost.

The reading was low but persistent, consistent with a slow underground loss on the mains supply.

Adding to the difficulty, the internal stopvalve had seized and could not be closed. This meant the internal and external mains pipework could not be isolated from each other and had to be assessed together.

This is the kind of scenario where water mains leak detection requires a methodical approach — working around access constraints while still producing a reliable result.

With no isolatable sections and no visible symptoms, the investigation needed to rely entirely on specialist equipment to confirm the presence of a leak and then pinpoint its location.

Methods Used

Because the internal stopvalve had seized, the internal and external mains could not be tested separately — the full mains run had to be assessed as a single system. Tracer gas was introduced into the pipework via the garden tap, and a pressure test at 3 bar was held for 30 minutes.

All pressure was lost during that period, confirming an active leak on the mains.

With the leak confirmed, the mains route was systematically surveyed for escaping tracer gas. A strong trace was detected in the back garden close to where the mains enters the property, approximately 1.5 metres from the meter.

This gave a precise excavation target. In water mains leak detection, combining a pressure test with tracer gas surveying in this way is particularly effective when there are no surface signs and no ability to isolate individual sections.

Investigation Process

1. 1
   Meter reading noted

   The water meter was observed registering approximately 0.5 litres per hour with all known outlets closed and no visible signs of a leak on site.

2. 2
   Internal stopvalve found seized

   Attempting to isolate the internal mains, the internal stopvalve was found to have seized completely and could not be closed, requiring the full mains run to be tested as one system.

3. 3
   Tracer gas introduced and pressure test conducted

   Tracer gas was introduced into the pipework through the garden tap, and a pressure test at 3 bar was run for 30 minutes. All pressure was lost during the test period, confirming an active leak.

4. 4
   Mains route surveyed for tracer gas

   The route of the mains was checked for escaping tracer gas. A strong trace was detected in the back garden where the mains enters the property, approximately 1.5 metres from the meter.

5. 5
   Excavation carried out

   The back garden was excavated at the identified location, exposing the mains pipework. A leak was found at the joint between the lead mains and an obsolete stopvalve.

6. 6
   Leak section cut out and replaced

   The leaking section — the junction between the lead mains and the obsolete stopvalve — was cut out and replaced using 1 metre of 25mm MDPE pipe and two universal transition couplings.

7. 7
   Post-repair pressure test passed

   A second 3 bar pressure test was held for 30 minutes with no pressure loss recorded, confirming the repair was successful.

8. 8
   Excavation backfilled and mains flushed

   The excavation was backfilled and the mains was flushed through to clear the system before reinstatement.

Result & Outcome

The leak was located in the back garden of the property in N14, approximately 1.5 metres from the meter, at the point where the lead mains connected to an obsolete stopvalve. This junction had failed and was the source of the continuous low-level loss the meter had been registering.

The leaking section was cut out and replaced with 25mm MDPE pipe using two universal transition couplings, removing both the failed joint and the obsolete stopvalve from the system. A post-repair pressure test at 3 bar held for 30 minutes with no pressure loss confirmed a clean result.

For water mains leak detection jobs where no surface signs are present and no internal isolation is possible, the combination of tracer gas and pressure testing proved decisive — delivering a confirmed leak location and a verified repair without unnecessary excavation elsewhere. The meter reading should now return to zero when all outlets are closed.