How does a gas tightness test work?

A tightness test, sometimes called a soundness test, checks that a gas installation holds pressure and has no leaks. It is a task for a competent, Gas Safe registered engineer working to the current industry procedure, IGEM/UP/1B for typical domestic installations, not something to attempt without the right training and equipment. Here is what the test involves and why each part is there.

When is a tightness test carried out?

More often than people expect. A tightness test is done before working on an installation and again after, so the engineer knows the system was sound when they arrived and sound when they left. It is done whenever new pipework is installed or altered, when a meter is fitted or exchanged, whenever there is a suspicion of a leak, and as part of safety checks on appliances and installations. It is quick, it is decisive, and it is the reason an engineer connects a gauge before touching much else. If gas is ever smelled, testing waits: the smell is dealt with first as an emergency.

What is the let-by test, and why does it come first?

Before the tightness test itself comes a let-by test of the Emergency Control Valve. With the appliances off and the ECV closed, a small pressure is left in the pipework and the manometer is watched for a period. If the pressure rises, gas is passing the closed valve, meaning it is not sealing properly, and that has to be dealt with before anything else. The logic is simple: a tightness test assumes the valve can actually isolate the supply. A valve that lets by would quietly feed gas into the very test that is supposed to prove the system holds, and could mask a real leak.

How is the tightness test performed?

What counts as a pass, and what counts as a fail?

A new or reworked installation must hold with no perceptible pressure drop at all. On an existing installation the procedure allows a small, defined drop depending on the meter and installation volume, but with a hard condition attached: there must be no smell of gas. Any smell of gas means the test is a fail regardless of what the gauge says, because the allowance exists for tired but safe pipework, not for detectable escapes. This is the part of the procedure that most rewards being done by the book rather than by feel.

What happens if the test fails?

The engineer's job becomes finding and fixing the escape, or making the installation safe if it cannot be fixed there and then. That can mean tracing the leak with leak detection fluid or instruments, isolating sections to narrow it down, repairing and retesting, or, where a fault cannot be corrected on the visit, turning off the supply at the ECV with the responsible person's agreement and applying the industry's unsafe situations procedure. What never happens is leaving a failed installation live. And for householders: if you smell gas at any time, do not wait for a test, call the National Gas Emergency line on 0800 111 999.

Is a gas drop test the same as a tightness test?

In everyday use, yes. “Drop test” is trade shorthand for the timed stage of the tightness test, the part where the gauge is watched for a pressure drop, and plenty of engineers and landlords use the two names interchangeably for the whole procedure. The full test is more than the drop, though. The let-by test proves the Emergency Control Valve can actually isolate the supply, stabilisation lets temperature and pressure settle, and only then does the timed reading mean anything: a drop observed without those steps is a number without a context. What the drop is allowed to be comes from the procedure, not folklore. A new or reworked installation must show no perceptible drop at all, and an existing one only ever the small, defined allowance the procedure permits, never with any smell of gas. So when someone books “a drop test on the gas”, this whole sequence is what a competent engineer will actually carry out.

How is an LPG tightness test different?

The shape of the test is the same: let-by, stabilisation, then a timed test with the gauge watched for a drop. What changes is the numbers and the behaviour of the fuel. LPG installations are tested to the current LPG procedures, with their own test pressures and permissible losses rather than the natural gas figures, which is why this guide quotes none for them; the engineer reads them from the procedure current on the day. The fuel behaves differently too. LPG is heavier than air, so any escape pools at low level instead of dispersing, which changes where an engineer checks for trouble when a test fails and why ventilation at low level matters on LPG installations. The qualification boundary is real as well: Gas Safe registration works by category, and an engineer needs the LPG competencies for the installation type, whether that is a house on bulk tank or cylinders, a park home or a holiday let. If your properties run on LPG, our guide to LPG gas safety for landlords covers the wider duties.

What about commercial and medium-pressure tightness testing?

Bigger installations move the job into different procedures entirely. Commercial pipework runs larger and longer than domestic, so tightness testing follows the commercial IGEM procedures rather than the domestic one, with test durations and allowances worked out from the installation's actual volume rather than looked up from a domestic table. New or extended commercial pipework is also strength tested before it is tightness tested, proving the pipework itself can hold pressure before the leak-tightness question is even asked. Medium-pressure supplies, where gas reaches the meter at a higher pressure through a regulator, add steps of their own again: the regulator and the higher-pressure section are dealt with as the procedure requires, not simply gauged like a low-pressure domestic job. And the qualification boundary appears here too, because a first-rate domestic engineer is simply not registered for commercial pipework: commercial and medium-pressure testing belongs to engineers holding the commercial competencies. For what the annual commercial inspection involves, see our CP17 guide.

Why is this an engineer-only job?

Because every part of it depends on competence and calibration: a gauge that reads truly, a procedure that distinguishes stabilisation from a leak, the judgement to classify what is found, and the authority to make an installation safe. Tightness testing is gas work under the Gas Safety (Installation and Use) Regulations 1998, so it must only be done by someone competent, in practice a Gas Safe registered engineer. For engineers, the discipline that pays is recording: readings, times and results logged against the job while still on site. Manifold makes that the path of least resistance, with test results, photos and the certificate captured on your phone at the job, so this visit's numbers sit next to last year's when a question comes later.