Gas Tightness Testing: The Complete Procedure for Every Scenario

Gas tightness testing is one of those fundamentals that separates a thorough Gas Safe engineer from one who's cutting corners. Get it wrong and you're signing off a potentially dangerous installation. Get it right every time and you've got documented proof the system was sound when you left the job. This guide covers every scenario you're likely to encounter on site — new installs, existing pipework, meter replacements, and the let-by test — along with the pressure drops that are acceptable and the point at which you must condemn.

Equipment and Preparation

You need a calibrated U-gauge or electronic manometer reading in millibars. The U-gauge is the traditional choice and perfectly adequate — just make sure the fluid isn't contaminated and the scale is readable. Electronic manometers are faster and easier to read accurately, especially at low pressures. Whatever you're using, it needs to be in calibration.

Beyond the gauge, you'll need PTFE tape, leak detection fluid (or a proprietary leak detection spray), and a means of isolating individual sections if you're working on a complex installation. Keep a record of the gauge serial number and last calibration date in your paperwork — if a job ever comes back to dispute, you want that documented.

New Installations: Full System Test

For a new installation, the test must be carried out with all appliances isolated (turned off at their individual service valves or disconnected). The objective is to test the integrity of the pipework itself, not the appliances. If you include an appliance with a known weep at the gas valve seal, you'll fail the pipework unfairly — and vice versa.

Tightness Test Procedure (New Pipework)

1. Isolate all appliances at their service valves.
2. Connect your manometer to a convenient test point — typically the test nipple on the meter outlet or a dedicated test point on the installation pipework.
3. Pressurise the system to working pressure (approximately 21 mbar for natural gas domestic installations). Do not over-pressurise.
4. Allow the pressure to stabilise for one minute. This accounts for temperature equalisation and any initial pipe flex.
5. Record the pressure reading.
6. Wait two minutes and record again.
7. Per IGEM/UP/1B, the pass criterion is no perceptible movement on the gauge throughout the test period. For a fluid U-gauge, no perceptible movement is 0.25 mbar or less; for an electronic manometer, 0.2 mbar or less. In practice, a well-made installation with quality fittings will show no measurable drop at all.
8. If the pressure holds, proceed with a visual check of all joints using leak detection fluid — even if the manometer shows no drop.

The no-perceptible-movement criterion exists because any gauge movement beyond the instrument’s resolution indicates a real pressure loss. It does not give you licence to leave small leaks unresolved. If you see any bubbling during the leak detection fluid check, find it and fix it regardless of what the manometer showed.

Purging and Re-testing After Repair

If you do find a leak, purge the system before re-testing. Tighten or remake the fitting, pressurise again, and run the full two-minute test from scratch. Don't just re-pressurise and watch the gauge — you need a fresh timed test on record.

Existing Pipework: Testing Before and After Work

When you're attending an existing property — whether it's a boiler swap, landlord inspection, or fault diagnosis — you should carry out a tightness test at the start of the job, before you touch anything. This establishes a baseline and protects you. If there's already a leak on the system and you don't find it on arrival, you're potentially accepting liability for it once you've signed the paperwork.

Using the Let-By Test

The let-by test checks the integrity of the meter's Emergency Control Valve (ECV). It tells you whether gas is getting past a closed ECV — if it is, the meter or ECV needs reporting to the gas transporter (National Gas or Cadent depending on your region), and the installation cannot be used until it's replaced.

1. Turn off all appliances and service valves.
2. Close the ECV.
3. Connect the manometer to the outlet side of the ECV (the meter test point).
4. Bleed the pressure downstream of the ECV to zero using the appliance connection or a test point.
5. Wait two minutes. If pressure rises from zero, gas is passing the ECV — this is a let-by failure.
6. If pressure remains at zero (or shows no measurable rise), the ECV is sealing correctly. Proceed with the full tightness test.

Existing Pipework Pressure Drop Limits

The same no-perceptible-movement criterion applies to existing pipework, but context matters. Any reading above the gauge resolution warrants closer inspection. Even a tiny perceptible movement should prompt you to check every joint more carefully with detection fluid, because something is leaking.

If you're testing an existing installation and the gauge shows any perceptible movement beyond its resolution threshold, you must treat the installation as having a gas escape. Ventilate the premises, advise the customer not to use any gas appliances, and identify and repair the fault before leaving. Document everything — the initial pressure reading, the drop, the actions taken, and the final re-test result.

Meter Replacements and Recommissioning

After a meter replacement by the transporter, it's your responsibility as the attending gas engineer to test the downstream pipework before recommissioning any appliances. The transporter tests the meter connections — not what comes after it.

The procedure is identical to the new installation test: pressurise to working pressure, stabilise for one minute, record, wait two minutes, record again. On a meter replacement, also carry out the let-by test on the new ECV first. New meters occasionally have manufacturing defects — rare, but it happens.

Once the tightness test passes, relight and commission each appliance in turn, checking working pressure at each burner. This is also the point at which you should document everything with a gas tightness test certificate.

When to Condemn

There's a clear decision tree here. Any perceptible gauge movement beyond the instrument resolution is a fail — full stop. Per IGEM/UP/1B this means 0.25 mbar on a fluid gauge or 0.2 mbar on an electronic manometer. If you can identify and repair the leak, retest and pass the system before leaving. If you cannot locate the leak, or it's in pipework that cannot be accessed without significant work (beneath a concrete floor, for example), you must condemn the installation.

Immediately Dangerous (ID) Situations

If you smell gas before or during testing, or if a leak is confirmed and cannot be immediately made safe, the installation is Immediately Dangerous (ID). Turn off the gas at the ECV, lock the meter if possible, attach a warning notice, and advise the customer that the installation must not be used. Document the ID classification, the reason, and the advice given. You cannot leave an ID installation in a usable state.

At Risk (AR) Situations

If the installation has a defect that doesn't present an immediate danger — a marginally failing tightness test with no smell of gas, or a suspect joint that hasn't started leaking yet — this is classified At Risk. Advise the customer in writing, document the finding, and make clear that the fault must be rectified before the installation can be considered safe. An At Risk classification isn't a reason to walk away without clear written communication.

Documentation

Every gas tightness test should be recorded. Write down the initial pressure, the pressure after two minutes, whether the let-by test was carried out and the result, and the outcome — pass or fail. If repairs were made, note what was done and include the re-test result. This isn't just good practice — it's your evidence that the system was in good order when you left.

For new installations, recommissioning after meter replacement, or any standalone tightness test job, a dedicated gas tightness test certificate gives you and the customer a clear record. For appliance commissioning that follows a tightness test, the gas appliance commissioning certificate picks up from where the tightness test leaves off.

The engineers who keep thorough paperwork are the ones who can defend their work when a job comes back years later with a solicitor's letter attached. Test properly, document everything, and leave every installation better than you found it.

Published February 2026. This article is for general guidance only and does not constitute legal or professional advice. Always refer to the relevant standards and consult qualified professionals for definitive requirements.