Landlord Gas Safety Certificate
CP12 gas safety certificates carried out by a Gas Safe registered engineer, with proper tightness testing under real operating conditions.
View service →Gas Leak Found During Pressure Testing: Boiler Safely Capped, N7 London
By Ralf - Gas Safe Registered Engineer | 28 October 2026 | Landlord Compliance
A gas safety check in N7, North London on 20 September 2026 is a good example of why the testing sequence matters, not just the end result. The property's boiler initially looked fine at rest - it was only once it was actually running that the real fault showed itself.
The testing sequence
Ralf carried out a full gas tightness and pressure test on the supply and boiler: an initial let-by test at 10 mbar for one minute, a stabilisation period at 20 mbar for one minute, then a tightness test at 20 mbar for two minutes with the boiler switched off. With the boiler off, there was no pressure drop at all on the supply to the boiler - on the face of it, a clean result.
The test was then repeated with the boiler switched on and operating normally. Under those conditions, the pressure dropped by 4.6 mbar - a result that would have been missed entirely if the test had stopped after the boiler-off stage.
Tracing the fault
Because the supply pipework itself had already tested clean with the boiler off, the drop under operating conditions pointed to the boiler itself rather than the pipework feeding it. Ralf opened the boiler and checked the gas valve and fan/combustion assembly directly, using a gas leak sniffer detector to pin down the source. The leak was traced to the connection where the gas pipe joins the fan/combustion assembly - not visible or detectable without opening the casing and testing under real operating conditions.
The safety call
With a confirmed leak at the boiler itself, the gas meter was capped and the boiler left isolated - not safe to operate until the fault is properly repaired. It's a straightforward decision in principle but still the right one to make without hesitation: a boiler that's been identified as unsafe doesn't get left running while parts are sourced or a repair is scheduled.
Why this matters for landlords specifically
This is exactly the kind of fault a proper annual gas safety check (CP12) is designed to catch - one that wouldn't show up from a visual inspection or a boiler-off test alone. See our Landlord Gas Safety Certificate page for details, or our Islington N1 area page for coverage of North London more broadly. Call 07472 251465 to book a check.
Common Questions
A let-by test checks whether the gas control valve itself is holding pressure correctly when closed. A tightness test checks the whole system - pipework and appliance - for any pressure drop over a set time, first with the boiler off and then under actual operating conditions. Testing both off and on matters: a system can hold pressure perfectly at rest and still have a leak that only shows up once the boiler is actually running.
If a boiler isn't safe to operate, capping the supply is the correct immediate action regardless of whether parts or further work are needed to complete the repair - leaving a known gas leak live while sourcing a part is not an acceptable trade-off. Safety takes priority over convenience every time, even if it means a property is without heating for longer than a same-visit fix would take.
Related Services
Emergency Boiler Repair in London
If a safety check has found a fault that needs resolving before the boiler can run again.
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Call Pristine Gas Ltd on 07472 251465. Gas Safe Registered, proper tightness testing under real operating conditions, covering all London postcodes.
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