Gas Tightness Testing UK — The Let-By and Soundness Test for Gas Safe Engineers in 2026

What a Tightness Test Is and Why It Is Mandatory

A tightness test (sometimes called a soundness test) proves that a gas installation — the pipework, fittings, valves and connected appliances — holds pressure without leaking. The standard procedure for testing and purging is set out in IGEM/UP/1B for domestic installations and IGE/UP/1A for the broader range of low-pressure domestic and light commercial work. These industry standards define how the test is carried out, the stabilisation and test periods, and the maximum permitted pressure drop for a given installation volume and operating pressure.

The test matters because even a small, slow leak on a natural gas installation is a serious hazard. Gas escapes you cannot smell at low concentration can accumulate, and any leak represents a risk of fire, explosion or — where combustion is affected — exposure to CO. Carrying out a correct tightness test is how you demonstrate, with a measured result rather than a guess, that the installation is safe to leave in service. It is also a documented requirement of being a competent person on the Gas Safe Register.

When You Must Carry Out a Tightness Test

A tightness test is not just a new-install task. You must carry one out in a range of situations, and the trigger is broadly "whenever the integrity of the installation could have changed or is in doubt". The common cases are:

• On a new installation — before any gas is admitted to appliances and before commissioning, to prove the new pipework is sound.
• After working on the installation — any time you break into the pipework, add or remove a section, replace a meter, or alter fittings.
• Before commissioning — as part of bringing an appliance or installation into service.
• On a periodic check — for example during a landlord gas safety check or a routine service, to confirm continued integrity.
• When investigating a smell of gas — a reported escape, or your own suspicion of a leak, must be confirmed or cleared with a proper test rather than relying on leak-detection fluid alone.

If you are ever unsure whether a test is required, the safe default is to do one. It takes minutes and it is the only objective way to prove the installation holds pressure.

The Let-By Test vs the Tightness Test

These two checks are run back to back but they test different things, and conflating them is a common mistake.

The let-by test checks the isolation valve or meter control valve itself. With the valve closed and pressure applied to the test side, you watch for a rise in pressure. A rise means gas is passing the closed valve — the valve is "letting by" — which would corrupt any tightness reading because gas would be feeding into the installation during the test. The let-by test must show no rise before you proceed.

The tightness test then checks the installation downstream. With the installation charged to the test pressure and the control valve closed, you watch for a drop in pressure over the test period. A drop beyond the permitted figure for that installation volume indicates a leak somewhere in the pipework, fittings or appliances. In short: the let-by test proves the valve holds, and the tightness test proves the pipework holds.

The Test Procedure, Step by Step

The following is a high-level overview of the procedure. Always follow the full requirements of the relevant standard (IGEM/UP/1B or IGE/UP/1A) and the specific figures it sets for the installation you are testing.

1. Preparation

Establish the installation volume, identify the operating pressure, and make sure the installation is in the correct state for testing — appliances connected as appropriate, pilot lights off, and any sections that should be isolated correctly valved off. Warn the occupier that gas will be off during the test.

2. Fit the pressure gauge

Connect a suitable pressure gauge — a manometer (water gauge) or a calibrated electronic gauge — at an appropriate test point, typically the meter test point or a designated test nipple. The gauge must be sensitive enough to read the small pressure changes involved, working in mbar.

3. The let-by test

Raise the pressure, close the control valve, and watch the gauge for the specified short period. If the reading rises, the valve is letting by and must be dealt with before you can carry out a valid tightness test.

4. Stabilisation period

Bring the installation up to the test pressure, then allow a stabilisation period before timing the test. Pressure and temperature settle during this time — starting the test before the installation has stabilised is a frequent cause of false fails, because thermal effects show up as an apparent pressure change.

5. The test period and permitted drop

Once stable, time the test period set by the standard and record the pressure at the start and end. Compare the measured drop against the maximum permitted pressure drop for that installation volume and operating pressure. If the drop is within the permitted figure, the installation passes. If it exceeds the permitted drop, the installation fails and a leak must be traced.

How Installation Volume Affects the Permitted Drop

The permitted pressure drop is not a single fixed number — it depends on the volume of the installation. A larger installation contains more gas, so a given physical leak produces a smaller measured pressure drop than the same leak would on a small installation. To account for this, the standard sets the maximum permitted drop relative to the installation volume: smaller installations are allowed a larger permitted drop, and larger installations a tighter one.

This is why establishing the installation volume in preparation is not a formality. Apply the wrong volume and you apply the wrong permitted drop — potentially passing an installation that should fail, or failing one that is actually sound. There is also a maximum permitted drop figure that applies as an upper limit regardless of volume, so the result is always read against both the volume-derived figure and any stated maximum. The table below illustrates how the principle works (always use the exact figures from the current standard for the installation in front of you).

Quick Reference: Tightness Test Stages and Pass Criteria

What to Do If the Test Fails

A failed tightness test means the installation is leaking and you must not leave it in service in that state. The priority is to make the situation safe, then trace and locate the leak.

• Trace and locate the leak systematically — isolate sections to narrow down where the drop is coming from, then work along joints, fittings and appliance connections.
• Use leak-detection fluid on suspect joints and fittings. Bubbling shows the escape point. Leak-detection fluid is a locating tool — it does not replace the measured tightness test for proving the whole installation.
• Rectify and re-test. Once you have remade the joint or replaced the failed component, repeat the full tightness test to confirm the installation now passes.
• Never leave a leaking installation in use. If you cannot make it sound, the installation must be made safe — isolated and the gas turned off — and the situation handled under the appropriate procedure for a gas escape or unsafe situation, with the responsible person informed.

Documentation, Warning and Advisory Notices

Recording the result is part of the job, not an afterthought. Note the test pressure, the stabilisation and test periods, the measured pressure drop and the permitted drop you compared it against. Where the installation passes, that record demonstrates you carried out a proper test. Where you find an unsafe situation, the correct warning or advisory notice must be issued under the recognised unsafe situations procedure, the appliance or installation labelled appropriately, and the responsible person made aware in writing.

Good records protect you. If a question arises later about the state of an installation when you left it, a clear, dated test record showing the figures and the action you took is your evidence that you worked competently and safely.

Your Duties Under the Gas Safety (Installation and Use) Regulations

The Gas Safety (Installation and Use) Regulations place legal duties on anyone working on gas. You must be competent and registered with the Gas Safe Register to carry out gas work, and you must not leave an installation or appliance in a condition that is dangerous to any person. Confirming tightness is a core part of meeting that duty: it is how you prove there is no escape of gas before you hand the installation back.

The regulations also require that where you know an appliance or installation is dangerous, it is not used until the defect is corrected, and that the relevant people are informed. A failed tightness test that you cannot rectify on the spot falls squarely into this — making it safe and notifying the responsible person is not optional. Treating the tightness test seriously, every time, is how you stay both compliant and safe.

Frequently Asked Questions

Is a tightness test the same as a soundness test?

Yes — "tightness test" and "soundness test" are used interchangeably to describe the procedure that proves the installation holds pressure without leaking. The standard procedure is set out in IGEM/UP/1B and IGE/UP/1A.

Why do I need a stabilisation period before timing the test?

Pressure and temperature change as the installation settles after you charge it. If you start timing too soon, those thermal effects show up as an apparent pressure change and can cause a false pass or fail. Allowing the stabilisation period gives you a reading you can trust.

Can I just use leak-detection fluid instead of a tightness test?

No. Leak-detection fluid is a tool for locating where a leak is once a test has shown the installation fails. It cannot prove the whole installation is sound the way a measured tightness test does. Both have their place, but the tightness test is the mandatory check.

Why does installation volume matter for the result?

A larger volume of gas masks a leak by producing a smaller pressure drop for the same escape. The permitted drop is therefore set relative to volume — larger installations get a tighter permitted figure — so you must establish the volume to apply the correct pass criterion.

What if the let-by test shows a rise in pressure?

A rise means the control or isolation valve is passing gas. You cannot carry out a valid tightness test while the valve lets by, because gas feeding in would mask a leak. The let-by must be resolved first.