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.

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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