A 1.5 mbar fall on a tightness test with a long-standing smell at the hob
You have replaced a corroded 15 mm copper tee and about 1.2 m of pipework in a kitchen, on the outlet side of the meter. The installation supplies a 24 kW open-flued boiler in the utility room, a four-burner hob on a rigid connection and a cooker bayonet. After reassembly you carry out a tightness test at the meter outlet test point using a calibrated gauge, following the domestic procedure in IS 813 and IGEM/UP/1B. After the stabilisation period the gauge falls 1.5 mbar, then holds steady at that reading for the remainder of the test period. A let-by test on the emergency control valve shows no rise on the gauge. The customer mentions that "there's a bit of a smell of gas near the hob sometimes, but it's been like that for ages". The kitchen was only finished last month and the pipework you replaced is now behind a sealed service void with an access panel at one end.
1. How do you interpret the 1.5 mbar fall and the stable reading afterwards?
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Neither reading is a pass on its own, and the stable tail does not convert a fail into a pass.
- The pass/fail criterion is volume-dependent. The permissible pressure drop is set by the test method for the installation volume, test pressure and duration you are using. You must measure/calculate the installation volume and compare 1.5 mbar against the allowance for that volume - you cannot judge the figure from memory or by comparison with another job. If 1.5 mbar exceeds the allowance, it is a fail, full stop.
- The stable tail is diagnostic, not reassuring. Typical innocent causes of a fall that then settles are: insufficient stabilisation (pipework and any liquid in the gauge still equalising after handling), regulator creep from the meter governor, and temperature change. Genuine leaks normally continue to fall, so a settled reading points towards those effects - but it does not exclude a leak that has become temporarily sealed by dirt or by a tap settling.
- The reported smell changes the burden of proof. A long-standing smell near the hob means you must treat the installation as leaking until proven otherwise, regardless of arithmetic. Do not purge or relight yet.
- Also verify the gauge - a liquid gauge misread by a meniscus, or a gauge with a leaking connection, produces false results. Record every reading, the volume used and the ambient conditions.
2. The let-by test is satisfactory. Set out the sequence you use to localise the problem, and state how your actions would differ if the let-by test had shown a rise.
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Satisfactory let-by tells you the ECV is seating: gas is not passing the closed valve, so the drop is downstream of it.
Localisation sequence
- Isolate all appliances at their own inlet: turn off the boiler service valve, turn off the hob taps and disconnect/cap the cooker bayonet at its plug. This removes appliance internals, tap grease and bayonet seals from the test.
- Re-test the pipework only. If it now holds, the fault lies in an appliance or its connection - test each appliance connection individually and examine the hob taps, which commonly leak at the tap spindle and are a credible source of a smell that "has been there for ages".
- If the pipework still falls, sectionalise using existing isolation points, working back from the meter, re-testing each section. Pay particular attention to the new tee and the sealed service void, which must be opened for examination at the access panel - a joint you cannot see is a joint you cannot clear.
- Never use a flame to search. Use leak-detection fluid on joints and a suitable gas detector, and ventilate as you work.
If let-by had shown a rise the ECV is passing and cannot isolate the installation. You must not rely on it: stop, ventilate, inform the occupant, and refer the ECV/meter installation to the gas network operator for attention; only then can a meaningful downstream test be completed.
3. The leak turns out to be a hob tap seal. Describe the work and checks needed before you hand the installation back, and what you record.
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The repair is only half the job; proving the installation is sound and documented is the rest.
- Repair properly - replace the offending tap seal/valve with the manufacturer's part (or isolate and cap the appliance if parts are unavailable and the customer agrees to a documented isolation). Do not "nip up" a tap that is worn.
- Re-test - reassemble, then repeat the tightness test on the whole installation at the meter outlet test point, allowing full stabilisation and comparing against the permissible drop for the measured installation volume. A satisfactory let-by check should be repeated too, since the ECV was operated.
- Purge correctly - after any work that admits air, purge the affected section to a safe point, never into a room, cupboard or void, and confirm with a gas detector that air has been displaced before attempting ignition.
- Recommission - relight, check gas rate against the data badge (for a 24 kW appliance roughly 2.2 m³/h of natural gas), working pressure, flame picture, and combustion CO/CO₂ ratio; confirm the hob burners light and hold on all settings and that the cooker bayonet is correctly engaged and leak-free.
- Records - the original failing readings, the let-by result, the fault found, parts fitted, all test readings, purge details and the final combustion readings on the job sheet; explain the cause of the long-standing smell to the customer and advise them to report smells to the network emergency line in future rather than living with them.