Damp Patch, a Moving Meter and a Wall Full of Electrical Conduit
Six months after a ground-floor kitchen extension, a damp patch appears on the bedroom wall at skirting level, 120 mm above the floor, on the opposite side of the wall from a concealed 15 mm CPVC line feeding the kitchen sink. The line runs 5 m inside the wall at 300 mm depth and was pressure-tested at 10 bar for one hour before plastering. All taps and the washing machine valve are closed, the overhead tank is isolated, and the water meter's 0.1 L dial makes one complete turn in 12 minutes. The owner wants to break the whole 5 m wall, replace the pipe and re-tile; the plumber suggests injecting sealant from the damp side. The wall also carries the main electrical conduit to the bedroom sockets, running about 100 mm from the pipe.
1. Confirm whether the loss is on the supply line and estimate the daily water loss.
Read the worked answer
With every fixture closed and the tank isolated, the meter should be stationary. One full turn of the 0.1 L dial in 12 minutes means:
0.1 L ÷ 12 min = 0.0083 L/min ≈ 0.5 L/hour ≈ 12 L/day, about 360 L/month.
Because the meter is downstream of the tank and upstream of the concealed line, movement confirms a loss on the supply side, not a drain leak. A drain leak would not move the meter. The quantity is small - consistent with a pinhole, a cracked socket or a damaged joint - but it is continuous and pressurised, so it will enlarge and travel through the wall.
Before accepting this, check that no cistern float valve or appliance is passing, and that the meter is not creeping due to a faulty non-return valve. If the meter stops when the kitchen branch is isolated, the leak is confirmed on that branch.
2. Describe a step-by-step method to locate the leak with minimum damage, including the electrical safety precautions.
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Safety first Isolate the bedroom socket circuit at the consumer unit, lock off or tag it, and use an RCD-protected supply for any test equipment. Do not drill or cut within 100 mm of the conduit; water tracking along the conduit is a shock and fire risk.
Locating
- Isolate the kitchen branch at the nearest stop valve and confirm the meter stops; this proves the branch is the leaking section.
- Map moisture with a moisture meter along the pipe route. The highest reading usually indicates the leak, not the lowest damp patch.
- At night, use an acoustic listening stick or leak detector on the wall; a pinhole under pressure gives a hiss.
- Use an infrared camera if available to see the cool wet track.
- Cut a single 300 × 300 mm inspection opening at the highest reading, not the whole 5 m run.
- Repair with a compression fitting or replace the damaged section; pressure-test at 1.5 times working pressure, flush, then patch and re-tile.
If the pipe is found touching the electrical conduit, re-route that section in surface conduit rather than re-burying it.
3. Compare the owner's proposal with the plumber's sealant proposal and recommend one, with reasons.
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Breaking the whole 5 m wall This guarantees access and a new pipe, but it is expensive, disruptive and unnecessary if the leak is local. It also creates a large making-good and tiling job and may disturb the electrical conduit.
Sealant injection from the damp side This is not a repair for a pressurised pipe. Sealant cannot reliably bond to a wet, pressurised surface, may travel along the pipe and block it, and can divert water into the electrical conduit. It hides the symptom and leaves a worsening leak inside the wall. It should be rejected.
Recommendation Use targeted diagnosis and a localised opening at the leak. This is the lowest-risk, lowest-cost option that actually repairs the cause. If the pipe route is too close to the conduit or has already been damaged in several places, re-route the branch on the surface in a conduit and abandon the buried section. Record the new route on a sketch for future maintenance.