Gas Tightness Test Procedure: Let-By & Pressure Drop Guide

Summary

Gas tightness testing is a mandatory check after any work on a gas installation — whether a new boiler installation, a pipe repair, or the reconnection of an appliance. The test verifies that no gas is escaping from the pipework or fittings. Unlike flue gas analysis (which checks combustion products), tightness testing checks the integrity of the physical pipe system itself — every joint, valve, fitting, and connection from the meter outlet to the appliance isolation valves.

The procedure uses the working gas pressure already in the system (typically around 20–21 mbar for natural gas on a standard domestic installation) as the test pressure. The engineer closes off the meter and notes the pressure, then waits. A pressure drop beyond the permissible limit for that installation's volume indicates a leak. This is different from higher-pressure purging tests used on new pipework, which use air or nitrogen at higher pressures.

Standard in transition (as of this review, August 2026): IGEM/UP/1B Edition 4 was published in March 2026 and becomes mandatory from 1 October 2026. Edition 3 remains valid until it is withdrawn on 30 September 2026 — both editions are technically live during this transition window, and Gas Safe expects engineers to move to Edition 4 as soon as practicable. The single biggest change: permissible pressure drop is no longer a flat figure tied to meter size — it is now calculated from the Installation Volume (IV) of the pipework under test, banded across several ranges (smaller IV = larger permissible drop, since a small leak represents a bigger proportional loss in a small volume of gas). Edition 4 also formally defines "perceptible movement" on the gauge (0.25 mbar on a water gauge, 0.2 mbar on a high-resolution electronic gauge) and adds a mandatory pipework-only retest step when appliances are connected. Do not rely on a single "1 mbar" or "4 mbar" rule of thumb under either edition — calculate the installation volume and check the current permissible-drop table.

The "let-by test" is a related but distinct check. It tests whether the meter governor (the regulator controlling pressure downstream of the meter) is seating correctly and not letting gas past when it should be closed. A let-by fault means gas is leaking through a closed governor — serious, and requiring a DNO call, not a DIY fix. Engineers must be familiar with both tests and when each is appropriate.

Key Facts

• Standard domestic working pressure — approximately 20–21 mbar at the meter outlet (natural gas); measured with a manometer
• Test pressure — working pressure; no additional pressurisation is required for a standard domestic tightness test
• Pressure drop limit — not a single flat figure; permissible drop over the 2-minute test is set by the Installation Volume (IV) of the pipework under IGEM/UP/1B Edition 4 (mandatory from 1 October 2026) — larger permissible drops for smaller IV, down to roughly 1 mbar at the top of the domestic IV band (>0.010–≤0.035 m³); calculate IV and check the current table rather than assuming a flat number
• Let-by test — separate test for meter governor integrity; carried out before the main tightness test; if let-by is confirmed, call the DNO — do not proceed with the main test
• Purging — after a tightness test is passed, pipework must be purged of air before appliances are commissioned; purge per IGEM/UP/1B guidance
• Gas detector — use a calibrated gas detector (not just soap solution) to locate leaks confirmed by a pressure drop; check all joints and fittings
• Soap solution — still useful for confirming individual joints once a leak is suspected by the pressure test; apply sparingly and look for bubbles
• Emergency controls — know the location of the emergency control valve (ECV) — typically adjacent to the gas meter; turn off if a significant leak is detected
• Appliance isolation valves — during tightness test, all appliance isolation valves should be in the closed position; this tests the installation pipework only, not the appliances
• New installations — may require a strength test (higher pressure with air or nitrogen) before the working pressure tightness test; IGEM/UP/1B specifies when this is required
• Reporting — always record tightness test results in writing; include date, pressure at start, pressure at end, duration, and outcome (pass/fail/leak found)
• RIDDOR — dangerous gas escapes may trigger RIDDOR reporting obligations; consult HSE guidance if in doubt
• DNO involvement — any fault upstream of the meter (including let-by) must be reported to the gas transporter/emergency service; do not attempt to repair the meter or governor

Quick Reference Table

Detailed Guidance

Pre-Test Checks

Before carrying out a tightness test:

1. Confirm the property has a gas supply (meter present and connected)
2. Identify the Emergency Control Valve (ECV) — typically a lever valve at the meter, turns 90° to close (lever across = off)
3. Note any smell of gas — if strong smell is detected, do not operate switches, ventilate the property, and call the national gas emergency service on 0800 111 999 before proceeding
4. Identify all appliances and their isolation valves; close all isolation valves
5. Allow the system pressure to stabilise at working pressure before beginning

Let-By Test Procedure

The let-by test is carried out first. It checks whether the meter governor is seating (closing) correctly when the downstream pressure builds up to the governor set pressure. A governor that is "letting by" is allowing gas through even when it should be closed.

LET-BY TEST PROCEDURE

1. Ensure all appliances and isolation valves are CLOSED
2. Connect manometer to test point (test nipple after meter)
3. Note working pressure (should be ~20 mbar)
4. Close the Emergency Control Valve (ECV) at the meter
5. Watch the manometer for 2 minutes

RESULT:
├── Pressure RISES → Let-by detected
│   ├── Gas is leaking past the meter governor
│   ├── DO NOT proceed with installation work
│   ├── Call the gas emergency service: 0800 111 999
│   └── Advise customer; vacate if significant
│
└── Pressure STABLE (no rise) → Let-by test PASSED
    └── Proceed to working pressure tightness test

Working Pressure Tightness Test Procedure

TIGHTNESS TEST PROCEDURE

1. Re-open ECV; confirm pressure at working level (~20 mbar)
2. All appliance isolation valves remain CLOSED
3. Close ECV again to isolate the system
4. Note the starting pressure on the manometer
5. Wait 1–2 minutes (observe clock — be precise)
6. Note the final pressure

RESULT:
├── Pressure DROP exceeds the permissible limit for this
│   installation's calculated volume (IGEM/UP/1B table) → FAIL — LEAK PRESENT
│   ├── Open ECV to restore pressure
│   ├── Use calibrated gas detector on all joints
│   ├── Apply soap solution to suspected joints
│   ├── Locate, repair, and retest
│   └── Do NOT commission appliance until pass is achieved
│
└── Pressure DROP within the permissible limit → PASS
    ├── Under IGEM/UP/1B Edition 4, also carry out the mandatory
    │   pipework-only retest — any perceptible movement fails
    ├── Record test result with start pressure, end pressure,
    │   duration, calculated installation volume, and outcome
    └── Proceed to purging before commissioning appliance

Do not use a flat "1 mbar" or "4 mbar" pass/fail figure. The permissible drop depends on the calculated Installation Volume (IV) of the pipework under test — see the transition note above.

Locating Leaks After a Failed Test

When a tightness test fails, the leak must be located before proceeding. Methods in order of preference:

1. Calibrated gas detector — electronic combustible gas detector (CGI); sweep all accessible pipework joints, valves, and fittings; start at the meter and work downstream. A detector gives a percentage of lower explosive limit (%LEL) reading; any reading above background indicates gas presence.

2. Soap solution — apply a soap-based detection fluid (purpose-made leak detection spray, or washing-up liquid diluted with water) to each joint. Bubbles form where gas is escaping. Use this to confirm individual joints after the CGI has indicated the area.

3. Isolation method — for large installations or where leaks are not quickly found, isolate sections of pipework by closing intermediate valves and retesting each section. This narrows the leak to a specific section.

Do not use a naked flame to detect gas leaks.

Purging After Tightness Test

After a successful tightness test, the installation pipework must be purged to remove any air introduced during the test or repairs. Purging is carried out per IGEM/UP/1B:

• Open the ECV and allow gas to flow through the pipework
• For short runs of small pipe, purging at the appliance connection point for the required period is sufficient
• For longer runs, follow IGEM/UP/1B Table 1 for purge times based on pipe diameter and length
• Purge into a safe external atmosphere or use a purge bag to capture gas safely in confined areas

Recording and Reporting

A written record of the tightness test must be made at every visit and must include:

• Date and time
• Address
• Engineer name and Gas Safe registration number
• Starting pressure (mbar)
• End pressure (mbar)
• Duration of test (minutes)
• Result: PASS or FAIL
• If fail: action taken, repair made, retest result
• Any unsafe situations identified and action taken

This record should be retained by the engineer and a copy provided to the customer (or landlord/agent as appropriate).

Frequently Asked Questions

How is a tightness test different from a gas safety check?

A gas safety check (CP12 for landlords) includes a tightness test as one of its components, but also covers appliance inspection, flue check, and combustion testing. A standalone tightness test is simply the pipework pressure check — it's required after any work on gas pipework before an appliance is reconnected.

Can a small pressure drop (e.g. 0.5 mbar) be accepted?

Whether it's acceptable depends on the permissible drop for the calculated Installation Volume of the pipework being tested (IGEM/UP/1B) — there is no single flat figure across all installations. Under Edition 4, "perceptible movement" itself is now formally defined (0.25 mbar on a water gauge, 0.2 mbar on a high-resolution electronic gauge), so a genuinely tiny reading may not even count as movement. Separately, temperature change in the test environment can affect pressure readings — a warm room cooling slightly can cause an apparent pressure drop. Allow for this by testing in stable conditions and repeating if results are borderline. Do not accept a borderline result without investigating.

What do I do if I find a leak I cannot reach (pipework under concrete)?

If the leak is in embedded pipework that cannot be accessed, the options are: rerouting the pipework to an accessible route, or (with appropriate specialist advice) using a corrosion-resistant approved gas pipe sleeve system. The appliance must not be recommissioned until the leak is rectified.

When is a strength test required instead of a working pressure test?

A strength test (at higher pressure, typically with air or nitrogen) is required for new pipework before it is connected to the gas supply. Once installed and pressure-tested at higher pressure, a working pressure tightness test is carried out before the gas is turned on. The working pressure test is the routine check after any reconnection or repair work.

Regulations & Standards

• Gas Safety (Installation and Use) Regulations 1998 — all gas installation work must be carried out by Gas Safe registered engineers; unsafe situations must be reported

• IGEM/UP/1B — Tightness testing and direct purging of small LPG/Air, NG and LPG installations: Edition 4 published March 2026, mandatory from 1 October 2026 (Installation Volume-based permissible pressure drop, formally defined "perceptible movement", mandatory pipework-only retest); outgoing Edition 3 remains valid until withdrawal on 30 September 2026

• Gas Safe Technical Bulletin TB008 — Guidance on tightness testing procedures for Gas Safe engineers (referenced in current industry training material; specific current wording not independently confirmed in this review — verify against Gas Safe Register's own technical bulletin library before relying on it for a specific detail)

• BS 6891:2015+A1:2019 — Installation and maintenance of low-pressure gas pipework in domestic premises: includes tightness testing requirements

• Gas Safe Register — Technical Guidance — Official Gas Safe tightness testing guidance

• IGEM/UP/1B Publication — Full procedure documentation (subscription required)

• HSE Gas Safety Guidance — HSE guidance on gas safety requirements

• BS 6891 Standard Summary — Domestic gas pipework installation requirements

• gas safe requirements — Gas Safe registration and legal obligations

• unsafe situations — ID, AR, NCS classification when leaks are found

• boiler installation — Installation checklist including tightness test stage

• boiler servicing — Annual service checklist