Gas tightness testing & purging

What gas tightness testing actually is

A gas tightness test — sometimes called a soundness test — checks the integrity of the gas pipework installation itself. Not the appliances, not the boiler or the warm air heaters. The pipework, fittings, valves and connections that carry gas around your building.

The principle is straightforward: the installation is pressurised, then monitored to see whether that pressure holds. If it drops, gas is escaping somewhere. Where it drops tells you broadly where to look. Leak detection fluid, applied to joints and fittings, shows you exactly where.

For commercial and industrial installations, the procedure follows IGE/UP/1 — the Institute of Gas Engineers and Managers’ standard for soundness testing and purging. This sets out the test pressures, stabilisation periods, and acceptable pressure drop limits depending on the type and size of the installation. It’s a more involved procedure than a domestic soundness test, and the stakes for getting it right are correspondingly higher.

When is a tightness test required?

This is the question that comes up most often, and the honest answer is: more situations than most people realise.

Any new gas installation needs a tightness test before it’s commissioned. That’s obvious enough. But it’s also required after modifications or extensions to existing pipework — if you’re adding a new appliance, rerouting gas to a different part of the building, or extending to a new unit, the affected section needs to be tested before it’s brought into service.

It’s also required when recommissioning a system that’s been isolated or left unused for a period. Vacant premises, buildings that have been refurbished, units that have been empty between tenants — the gas supply can’t simply be turned back on without first verifying the installation is still sound. Pipework that’s been dormant can develop issues: joints that have relaxed, fittings that have corroded, connections disturbed by building work carried out nearby.

Building work is a significant one. If contractors have been working in the building, whether or not directly on the gas, there’s a chance the pipework has been disturbed — a drill through a wall, a floor that’s been dug up, a partition that’s been moved. A tightness test is the only reliable way to verify the installation is still sound.

Finally, tightness testing is always required following a gas escape or emergency isolation. Before the gas goes back on after a reported leak or an emergency shutoff, the installation needs to be proven safe.

How the test is carried out

All appliances are isolated and the installation is pressurised — typically to working pressure for the initial check, then to a test pressure specified by the relevant standard. A manometer measures the pressure precisely. There’s a stabilisation period to allow for temperature equalisation, which can affect readings, particularly on larger installations where the pipework runs through areas at different temperatures.

Once stabilised, the pressure is monitored over a set time period. For commercial installations this is typically longer than the domestic equivalent, and the acceptable pressure drop limits are tighter. A clean result, with no discernible pressure drop within tolerances, means the installation is gas tight and can proceed to purging.

If the pressure drops, the leak needs to be found before anything else happens. Leak detection solution is applied to every joint, fitting and valve in the affected section — gas escaping from a joint will bubble the solution, the location is marked, isolated, and repaired, and the section is retested. This continues until a clean result is achieved. There’s no shortcut and no acceptable level of leakage that gets signed off and left.

What is purging — and why does it matter?

Once a tightness test has passed, the pipework contains air — either because it’s a new installation, or because the system was depressurised for testing or repair work. Before the appliances can be commissioned, that air needs to be displaced by gas. This is purging, and it’s the step that’s least understood by anyone who hasn’t been through the process.

The reason it requires care comes down to the flammability limits of natural gas. Natural gas is flammable when it makes up roughly 5% to 15% of a gas/air mixture by volume. Outside that range it won’t ignite. But as gas is introduced into a pipe containing air, the mixture passes through that flammable range before it becomes predominantly gas. If there’s any ignition source present during that transition — an automatically relighting pilot, an electrical spark, residual heat from an appliance — the consequences are obvious.

Purging procedures control this process. On a simple installation, gas is introduced at a controlled rate and the displaced air/gas mixture is vented to a safe external location, away from any ignition source, until the concentration at the outlet is confirmed as predominantly gas. On larger commercial installations with significant volumes of pipework, the procedure is more involved — purged in sections, with specific flow rates and vent points, following a documented sequence. It’s not a procedure that can be improvised.

Tightness testing vs a gas safety certificate

This is worth being clear about, because the two are often confused, and conflating them can leave a business with a gap in its compliance without realising it.

A gas safety certificate — the annual inspection that results in a formal certificate — assesses the condition and safe operation of gas appliances: the boiler, the warm air heaters, the radiant tube heaters. It covers combustion, fluing, controls, safety devices. It’s an appliance-level check.

Tightness testing is a pipework-level check. It verifies that the gas installation — the infrastructure that delivers gas to those appliances — is structurally sound and leak-free. It doesn’t assess whether the appliances operate safely, and a gas safety certificate doesn’t verify whether the pipework is gas tight. Both matter, and neither substitutes for the other.

Who can carry this out?

Tightness testing and purging on commercial installations must be carried out by a Gas Safe registered engineer who holds the appropriate commercial gas qualifications. Not all Gas Safe engineers are qualified for commercial work — the register distinguishes between domestic and commercial/industrial categories, and the procedures, standards and test requirements for commercial premises are significantly more involved than those for domestic properties.

It’s worth checking. Gas Safe registration is a minimum requirement, but the specific qualification categories on an engineer’s registration card confirm what they’re actually certified to work on.

We carry out commercial tightness testing and purging across Colchester, Essex and Suffolk. Our engineers hold the relevant Gas Safe qualifications for commercial and industrial work, and we follow the IGE/UP/1 procedures throughout. If you’ve had recent installation work, modifications to pipework, or you’re recommissioning a premises after a period of vacancy, get in touch and we’ll talk through what’s involved.