How does a gas tightness test work under IGEM/UP/1B Edition 4?
A tightness test proves a gas installation holds pressure, and it is a job for a Gas Safe registered engineer. From 1 October 2026 it follows IGEM/UP/1B Edition 4: a let-by test at the emergency control valve, natural gas raised to 20 mbar, 1 minute to stabilise, then a 2 minute test. The permissible drop is read from the installation volume and the fuel gas, not the meter size, and any perceptible movement must be shown not to come from the pipework.
Reviewed by Jordan Valentine-Dunn, Gas Safe registered engineer · Portsmouth Gas Heating · Last reviewed September 2026
A tightness test, sometimes called a soundness test or a drop test, checks that a gas installation holds pressure and has no leaks. It is gas work under the Gas Safety (Installation and Use) Regulations, so it is a task for a competent, Gas Safe registered engineer working to the current procedure. For small natural gas, LPG and LPG/Air installations that procedure is IGEM/UP/1B. Edition 4 was published in March 2026, Edition 3 is withdrawn on 30 September 2026, and from 1 October 2026 every test must follow Edition 4.
What changed in Edition 4?
| Under Edition 3 | Under Edition 4 |
|---|---|
| The permissible drop depended on the meter size. | The permissible drop depends on the installation volume (IV) and the fuel gas: tables 2, 3 and 5 below. |
| A small drop within limits was accepted with existing appliances connected. | Any perceptible movement, even within limits, must first be proved not to come from the pipework: isolate the appliances and retest with no movement allowed. |
| Purge volume could be assumed as 0.01 m³ for pipe up to 28 mm on a meter up to 6 m³/h. | Purge volume is 1.5 × IV on every installation, so the IV is worked out on every job. |
| No stated rule for a low test pressure. | If the test pressure cannot be reached, isolate and investigate first. Natural gas may be tested at 18 mbar where 20 cannot be achieved, with IGEM/G/13 operating-pressure checks to follow. |
| Which edition was used was not recorded. | Gas Safe Register recommends recording which edition the test followed on the work record. |
When is a tightness test carried out?
Before working on an installation and again after, so the engineer knows the system was sound on arrival and sound on leaving. On a new installation and whenever pipework is altered, extended or reused. When a meter is fitted or exchanged. Whenever an escape is known or suspected, after a complete loss of supply or installation pressure, and immediately before purging. Routine testing is part of servicing and safety checks too.
How is the tightness test performed?
Visual inspection and set-up
Inspect the installation. Cooker lids up, permanent pilots out, appliances set as the procedure requires.
Let-by test at the emergency control valve
With the ECV closed, hold 7 to 10 mbar in the pipework for 1 minute. A rise means gas is passing the closed valve, which has to be sorted before the tightness test means anything.
Raise to the test pressure
Natural gas is tested at 20 mbar. If 20 mbar cannot be reached, isolate the installation and investigate the low pressure; a test at a minimum of 18 mbar is allowed, with the IGEM/G/13 operating-pressure checks carried out after a pass. LPG and LPG/Air are tested at their own pressures.
Stabilise for 1 minute
Temperature stabilisation, so the gauge settles before timing begins. Skipping it is how a sound system gets condemned and an unsound one passed.
Test for 2 minutes
Watch the gauge and stay alert for any smell of gas. What counts as no movement depends on the gauge: 0.25 mbar or less on a water gauge or an electronic gauge reading to two places, 0.2 mbar or less on an electronic gauge reading to one place.
Read the result
Pipework only, or only new appliances connected: no perceptible movement is allowed. Existing appliances connected and no smell of gas: compare the drop with the permissible limit for the installation volume in the tables below.
Prove the pipework
New in Edition 4. Any perceptible movement, even within limits, must be shown not to come from the pipework. Isolate every appliance and test again: no movement allowed. An appliance that cannot be isolated stays connected and the no-movement rule applies.
Reinstate and check
Refit the test nipple, reconnect the appliances, turn the gas on, and leak-detection-fluid test every disturbed joint and the pipework between each isolation valve and its appliance. There must be no smell of gas.
How much pressure drop is allowed?
The limits below apply only with existing appliances connected and no smell of gas reported. IGEM's stated basis is a leakage rate of 19 ml per minute for the installation volume, so a larger installation gets a smaller permissible drop.
| Installation volume (IV) | Maximum permissible drop |
|---|---|
| Up to 0.005 m³ | 8 mbar |
| Over 0.005 to 0.010 m³ | 4 mbar |
| Over 0.010 to 0.015 m³ | 2.5 mbar |
| Over 0.015 to 0.035 m³ | 1 mbar |
| Installation volume (IV) | Maximum permissible drop |
|---|---|
| Up to 0.025 m³ | 1.5 mbar |
| Over 0.025 to 0.035 m³ | 0.5 mbar |
| Installation volume (IV) | Maximum permissible drop |
|---|---|
| Up to 0.0025 m³ | 2 mbar |
| Over 0.0025 to 0.005 m³ | 1 mbar |
| Over 0.005 to 0.01 m³ | 0.5 mbar |
| Over 0.01 to 0.035 m³ | No perceptible movement |
A shortcut that saves the arithmetic: if the drop is below the smallest limit in the table for that fuel gas (1 mbar for natural gas, 0.5 mbar for LPG/Air), it is within limits and the installation volume does not need working out for the comparison. It still needs working out for the purge. LPG has no shortcut, because its largest band allows no movement at all.
How is the installation volume worked out?
Installation volume is the internal volume of the pipework, the valves and fittings, and the meter, in cubic metres. Multiply each run of pipe by the volume per metre for its size, add the fittings (10% of the pipework volume unless the actual volume is calculated), and add the meter. For hidden pipework, assume the largest size you can see runs throughout, so the volume is overstated and the permissible drop is never larger than it should be. The standard's own figures sit in its Appendix 7; the ones below are the figures published by two independently built calculators that agree with each other, one of them audited against the standard.
| Pipe | Litres per metre |
|---|---|
| Copper 15 mm | 0.14 |
| Copper 22 mm | 0.32 |
| Copper 28 mm | 0.54 |
| Copper 35 mm | 0.84 |
| Steel ½" (15 mm) | 0.24 |
| Steel ¾" (20 mm) | 0.46 |
| Steel 1" (25 mm) | 0.64 |
| Steel 1¼" (32 mm) | 1.10 |
| PE SDR11 20 / 25 / 32 mm | 0.19 / 0.33 / 0.53 |
| U6 or G4 diaphragm meter | 8 litres (0.008 m³) |
| E6 ultrasonic (smart) meter | 2.4 litres (0.0024 m³) |
| U16 or G10 diaphragm meter | 25 litres (0.025 m³) |
Worked example: a U6 meter, 2 m of 22 mm copper to a tee, 6 m of 15 mm to the boiler and 3 m of 15 mm to the hob. Pipework 2 × 0.32 + 9 × 0.14 = 1.9 litres, fittings 0.19 litres, meter 8 litres: 10.09 litres, or 0.01009 m³. That falls in the over 0.010 to 0.015 m³ band, so with existing appliances connected the permissible drop is 2.5 mbar, and the purge volume is 1.5 × 0.01009 = 0.0151 m³, purged to air.
Manifold's free tightness test calculator does this arithmetic: enter the runs and the meter, read the permissible drop for the band, and see what Edition 4 makes of your reading. On the check page the same card saves the test against the job and prints it on the Landlord Gas Safety Record.
What counts as a pass, and what counts as a fail?
A smell of gas fails the test whatever the gauge shows. Pipework only or new appliances: any perceptible movement fails. Existing appliances connected: a drop above the limit for the band fails; a drop within it is acceptable only once the pipework-only retest shows no movement and no smell of gas, and a site-specific risk assessment says the appliance can stay connected. A test that fails under Edition 4 cannot be re-run under Edition 3 to pass it.
What happens if the test fails?
The engineer's job becomes finding and fixing the escape, or making the installation safe if it cannot be fixed there and then: tracing with leak detection fluid or instruments, isolating sections to narrow it down, repairing and retesting, or, where a fault cannot be corrected on the visit, following the unsafe situations procedure in IGEM/G/11.
What about purging?
Purge volume is 1.5 times the installation volume on every job. Natural gas purges to air up to a purge volume of 0.02 m³ and to ignition above that. LPG and LPG/Air always purge to ignition. Because the purge needs the installation volume, Edition 4 means working it out even when the tightness test itself did not need it.
Which installations does IGEM/UP/1B cover?
Installation pipework, including the meter, where the maximum operating pressure at the emergency control valve outlet is 2 bar or less, the pipe is 35 mm or smaller, the meter's badged capacity is 16 m³/h or less, and the installation volume is 0.035 m³ or less, in domestic and non-domestic premises. Natural gas includes blends with up to 20% hydrogen. Larger installations go to IGEM/UP/1, and meter installations tested without downstream pipework to IGEM/UP/1C.
Is a gas drop test the same as a tightness test?
In everyday use, yes. Drop test is trade shorthand for the timed stage of the tightness test. The full test is more than the drop: the let-by test, the stabilisation, the reading against the permissible limit, and now the pipework-only retest.
Always use a calibrated gauge and work to the current procedure. This guide is a high-level overview of the public summaries of IGEM/UP/1B Edition 4, not a substitute for the standard or proper training. Tightness testing is a job for a competent, Gas Safe registered engineer.
Frequently asked
Sources
- Gas Safe Register, Industry Standard Update 133 (Edition 2): IGEM/UP/1B Edition 4 tightness testing
- Registered Gas Engineer, Updated tightness testing standard: what you need to know (May 2026)
- IGEM, IGEM/UP/1B Edition 4: tightness testing and direct purging of small LPG/Air, NG and LPG installations
- IGEM, frequently asked questions on IGEM/UP/1B
- HSE, gas health and safety
Related guides
Last reviewed September 2026. This guide is general information, not legal or safety advice, gas safety work must be carried out by an appropriately Gas Safe registered engineer. Rules can change, so check the linked official sources for the current position.