Gas Tightness Testing: Procedure, Readings & Documentation

Why Tightness Testing Matters

Every gas installation must be confirmed gastight before being put into use. A leaking gas installation can cause explosions, carbon monoxide poisoning, and death. Tightness testing isn’t optional — it’s the single most important safety check you perform.

Tightness testing is required by the Gas Safety (Installation and Use) Regulations 1998, and the procedure is defined in IGE/UP/1B (now IGEM/UP/1B Edition 3). Gas Safe auditors specifically check that tightness test results are documented correctly.

No tightness test result on a CP12 or commissioning record is one of the most common audit failures. Getting the procedure right and documenting it properly protects you, your customer, and your Gas Safe registration.

Equipment You Need

• Manometer (U-gauge or digital) capable of reading to 0.1mbar accuracy
• Suitable pressure test point (usually at the meter or test point tee)
• Stopwatch or timer
• Method of isolating each appliance (isolation valves or appliance gas cocks)
• Leak detection fluid (LDF) for confirming suspected leak locations
• Warning notice labels (for if the test fails)

Step 1: The Let-By Test

The let-by test must be performed before the tightness test. Its purpose is to confirm the meter isn’t passing gas when all appliances are off. If the meter is letting gas by, your tightness test reading will be meaningless.

Step 2: The Tightness Test

Once the let-by test has passed, proceed with the full tightness test. This tests the entire installation from the meter outlet to every appliance.

Reading Your Results

Documenting the Test

Proper documentation is what separates a professional gas engineer from one who gets flagged at audit. Record the test on the relevant certificate (CP12 Section 3, or commissioning record) and include:

• Starting pressure and final pressure
• Pressure drop (calculated)
• Pass or fail result
• Test duration
• Gauge type (U-gauge or digital)
• Date and time of the test
• If you photograph your gauge reading, include the photo with the job record
• For a failed test, record what action was taken (see next section)

What to Do If It Fails

A failed tightness test means there is a gas leak on the installation. Follow this procedure:

1. Do NOT put the installation into use.
2. Systematically check all joints and connections with leak detection fluid (LDF).
3. Check meter connections, appliance connections, and any recently disturbed pipework first.
4. Once the leak is found and repaired, repeat the full tightness test.
5. If you cannot find the leak, classify the installation as At Risk (AR) or Immediately Dangerous (ID) as appropriate.
6. Attach a warning label and notify the responsible person.
7. If the installation was previously in use and has developed a leak, turn off the gas at the ECV and ventilate the property.

Testing Existing Pipework Before You Start

On any existing property — a boiler swap, a landlord inspection, a fault call — test before you touch anything. It takes a few minutes and it establishes what the installation was doing when you arrived.

The reason is liability as much as safety. If the system already has a leak and you do not find it on arrival, you have no way of showing it was not something you caused, and your paperwork at the end of the job says the installation was sound when you left. A visual inspection alone is never enough on existing pipework: it may have been disturbed, corroded, or altered by someone else since it was last tested.

A result that passes but sits near the top of the permissible drop for that installation is worth treating as a warning rather than a pass. Go over the joints with LDF before you sign anything — something close to leaking now will be leaking later.

Purging and Re-testing After a Repair

When you find and fix a leak, the repair does not count until it is proven. Remake or tighten the fitting, purge the system, pressurise again, and run a complete fresh timed test from the beginning — stabilisation period included.

Re-pressurising and watching the gauge for a few seconds is not a test and gives you nothing to record. The re-test result is the evidence that the installation was sound when you left it, so it needs the same procedure and the same documentation as the original.

Meter Replacements and Recommissioning

After the transporter replaces a meter, the pipework downstream is yours to prove. The transporter is responsible for the meter and its own connections — not for whatever the installation pipework beyond it has been doing for the last twenty years.

1. Carry out the let-by test on the new ECV first. New does not guarantee sound.
2. Run the full tightness test on the downstream installation exactly as you would for new pipework.
3. Only once it passes, relight and commission each appliance in turn.
4. Check the working pressure at each appliance as you go.
5. Record the meter change, the test readings, and the commissioning results together.

Immediately Dangerous and At Risk

If you cannot find the leak, or it sits in pipework you cannot reach without significant work — buried under a solid floor, for example — the installation has to be classified under the Gas Industry Unsafe Situations Procedure rather than left as it is.

Immediately Dangerous (ID)

A confirmed leak that cannot be made safe, or gas smelled before or during testing, is Immediately Dangerous. Turn the gas off at the ECV, cap or lock the meter where you can, attach a warning notice, and make clear to the responsible person that the installation must not be used. Record the classification, the reason for it, and the advice you gave. An ID installation cannot be left in a usable state.

At Risk (AR)

A defect that is not an immediate danger — a marginal result with no smell of gas, or a suspect joint that has not started leaking — is At Risk. Turn off the affected appliance or section with permission, advise the responsible person in writing, and state plainly that it must be put right before the installation can be considered safe. At Risk is not permission to walk away quietly; the written record is the whole point of the classification.

The Do’s and Don’ts

Do

• Always perform the let-by test first — if the meter is letting by, your tightness test is meaningless.
• Wait for the stabilisation period (1–2 minutes) before starting your timed test.
• Record both pressures (start and end) — not just ‘pass’ or ‘fail.’
• Test the entire installation with all appliance isolation valves open.
• Use leak detection fluid on every joint if the test fails — don’t guess.
• Photograph your gauge reading as evidence for your records.
• Zero your gauge before every test — a drift of 0.5mbar will skew your reading.

Don’t

• Skip the let-by test because you’re in a hurry. It takes 1 minute and validates your whole test.
• Start timing before the pressure stabilises — you’ll get a false fail.
• Record a pass without actually testing. Gas Safe auditors check for this and it’s a disciplinary offence.
• Use washing-up liquid as leak detection fluid — it can corrode fittings. Use proper LDF.
• Ignore a borderline result (3–4mbar drop). Investigate with LDF even though it technically passes.
• Put the installation into use after a failed test. No gas, no exceptions.

LPG Tightness Testing

LPG installations (propane or butane) follow the same basic procedure but with important differences. LPG is heavier than air and pools at low level, making leak detection even more critical.

Medium Pressure Installations

Medium pressure installations (above 75mbar) are found in commercial and industrial settings. Testing requires specialist equipment rated for the higher pressure, longer test durations based on pipe volume per IGEM/UP/1B, and a competent person holding specific medium pressure qualifications. The principles are the same but the stakes are higher.

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