Published in March 2026 by the Institution of Gas Engineers and Managers (IGEM), Edition 4 became effective on 1 October 2026, replacing Edition 3.

For Gas Safe engineers, understanding these changes is essential when carrying out applicable gas installation testing. In particular, the revised requirements place greater emphasis on installation volume and the correct assessment of pressure movement.

At Certificates4UK Ltd, we work with gas boilers and installations across Surrey. Keeping our technical knowledge up to date is an important part of carrying out gas work professionally.

In this guide, we explain the main changes in IGEM/UP/1B Edition 4, including what engineers need to know about tightness testing, permissible pressure drops, and purging.

Technical note: This article is intended for appropriately qualified Gas Safe engineers. It is an educational overview, not a replacement for the complete IGEM standard, applicable safety procedures, or manufacturer instructions.

What Is IGEM/UP/1B Edition 4?

IGEM/UP/1B is a technical standard covering tightness testing and direct purging of small natural gas (NG), liquefied petroleum gas (LPG), and LPG/air installations.

The standard provides requirements for testing installation pipework, including meters, within its defined scope.

For engineers, gas tightness testing is more than watching a pressure gauge. It involves understanding the installation, identifying the appropriate test conditions and correctly interpreting the results.

The latest edition makes the relationship between installation volume and permissible pressure loss particularly important.

When Did IGEM/UP/1B Edition 4 Become Effective?

IGEM published Edition 4 in March 2026.

The new edition became effective on 1 October 2026, while Edition 3 was withdrawn on 30 September 2026.

Therefore, engineers should now be working from the updated requirements where IGEM/UP/1B applies.

This is particularly relevant when reviewing older training material, test sheets or working procedures that still refer to Edition 3.

Which Gas Installations Does IGEM/UP/1B Edition 4 Cover?

The standard applies to specified small NG, LPG and LPG/air installations.

Its scope includes installations meeting the relevant limitations, including:

• Installation volume not exceeding 0.035 m³
• Meter badged capacity not exceeding 16 m³/h
• Nominal pipework bore not exceeding 35 mm
• Applicable operating-pressure limitations
• Specified installation and testing conditions

For example, the standard identifies nominal operating pressures not exceeding 21 mbar for the relevant NG and LPG/air installations and 37 mbar for the relevant LPG installations.

The full scope contains additional conditions that engineers must check before applying the procedures.

Engineer perspective: Never assume a particular tightness-testing procedure applies simply because the installation is in a domestic property. Confirm the installation falls within the standard’s scope first.

What Has Changed From Edition 3 to Edition 4?

The most important changes highlighted in the Edition 4 quick-reference material concern installation volume, permissible pressure loss, and purging calculations.

1. Permissible Pressure Loss Depends on Installation Volume

The updated approach uses installation volume and fuel gas when determining the applicable permissible pressure-loss limits.

This means meter size alone is not sufficient to establish the relevant limit.

An engineer may need to calculate the total installation volume to assess a pressure loss correctly.

2. Pipework-Only Retesting Requires No Perceptible Pressure Loss

Where a pressure loss is identified on an installation with existing appliances, the engineer must establish whether the loss originates from the installation pipework or the appliances.

A perceptible pressure loss on a pipework-only retest constitutes a failure.

There is no permissible leakage allowance for installation pipework itself.

3. Purge Volume Is Calculated Using Installation Volume

The updated standard requires installation volume to be calculated whenever purging is undertaken.

The purge volume is determined using a multiplier of 1.5 times the installation volume.

Importantly, IGEM explains that the purging procedure itself remains unchanged, while the method of determining purge volume has been revised for some installations.

Gas Tightness Testing: Understanding Pressure Loss

A tightness test is intended to establish whether an installation meets the applicable gas-tightness requirements.

However, engineers must distinguish between testing installation pipework and assessing an installation with existing appliances connected.

These are not interchangeable situations.

For example, a permissible pressure-loss limit may apply when assessing an installation containing existing appliances, provided the pipework has been confirmed gas-tight, and all relevant conditions are met.

That does not mean a pressure loss can automatically be accepted on new installation pipework or a pipework-only retest.

Why the Difference Matters

Imagine attending a property where an existing gas installation shows perceptible pressure movement during testing.

It would be incorrect simply to compare that movement against a pressure-loss table and assume the installation passes.

The engineer must first establish whether the installation pipework is gas-tight.

If the pipework-only test shows perceptible pressure loss, the installation fails the relevant requirement and appropriate action is necessary.

If the pipework is confirmed gas-tight, the loss associated with existing appliances can then be assessed under the applicable criteria.

This distinction is one of the most important points for engineers to understand.

What Is Installation Volume in Gas Tightness Testing?

Installation volume (IV) represents the total internal volume of the relevant gas installation.

It includes the volume of the pipework, meter, and fittings.

The calculation can be expressed as:

IV = Pipework Volume + Meter Volume + Fittings Volume

For example, an installation may contain several lengths of different-sized pipework, together with a gas meter and fittings.

The engineer must account for the relevant components when calculating the total installation volume.

The correct volume matters because it can determine which permissible pressure-loss category applies.

Do Engineers Need to Calculate Installation Volume Every Time?

Not necessarily for every tightness-test assessment.

IGEM’s official FAQ explains that, in certain circumstances, where the observed pressure loss is below the smallest permissible limit for the fuel gas, calculating installation volume is not required solely to compare that loss against the permissible limits.

However, the pipework must still be confirmed gas-tight, and all applicable acceptance conditions must be met.

If the observed loss exceeds the smallest permissible limit, installation volume may need to be calculated to establish the relevant maximum.

An important exception: If purging is undertaken, installation volume must be calculated even if it was not required for the earlier tightness-test assessment.

Natural Gas Permissible Pressure Drops

The supplied Edition 4 quick-reference sheet provides the following natural gas installation-volume categories.

Natural gas installation volume	Maximum pressure 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

These figures are provided for technical reference and must be interpreted alongside the full standard.

They do not establish a general allowance for leaking installation pipework.

The applicable gas type, test configuration, appliance condition, and other acceptance criteria remain essential.

Why Can Different Installation Volumes Have Different Limits?

A pressure drop does not represent the same quantity of gas loss in every installation.

Installation volume affects the relationship between pressure movement and gas leakage.

This is why the updated standard assesses permissible pressure loss with reference to installation volume rather than relying solely on meter size.

For engineers, the practical lesson is straightforward: understand the installation before interpreting the test result.

What Does No Perceptible Movement Mean?

The Edition 4 reference sheet identifies thresholds associated with perceptible movement for different measuring instruments.

These include fluid gauges and electronic pressure-measuring instruments.

However, the displayed reading is only one part of the assessment.

Engineers must also consider instrument suitability, accuracy, resolution, and the applicable requirements of the full standard.

A digital gauge displaying a small pressure change does not automatically mean that the change is acceptable.

Likewise, an engineer should not assume that an instrument is suitable simply because it provides a numerical pressure reading.

IGEM/UP/1B Edition 4: Gas Purging Changes

Purging is another important area covered by Edition 4.

The standard requires installation volume to be calculated whenever purging is undertaken.

The purge-volume calculation is:

Purge Volume (PV) = 1.5 × Installation Volume (IV)

For example, if an installation volume is 0.020 m³:

PV = 1.5 × 0.020 = 0.030 m³

This is an illustrative calculation, not a complete purging procedure.

The engineer must follow the applicable requirements of the full standard, including the necessary safety precautions.

What If Installation Volume Wasn’t Needed for the Tightness Test?

This is an important clarification from IGEM.

An engineer may not have needed to calculate installation volume during a particular tightness-test assessment.

However, if purging is subsequently required, installation volume must still be calculated to determine the correct purge volume.

The two requirements should not be confused.

Why Engineers Should Review Their Testing Procedures

When a technical standard changes, the challenge is not simply learning a new set of figures.

It is understanding when each requirement applies.

At Certificates4UK Ltd, our approach to technical work is based on checking the relevant conditions rather than making assumptions.

The same principle applies to gas tightness testing.

Before interpreting a result, engineers should be confident that they are using the correct standard, testing the appropriate section of the installation and applying the correct acceptance criteria.

Older working habits should never take priority over current technical requirements.

What Should Engineers Check?

Engineers should review their existing tightness-testing documentation, technical references and installation-volume calculation methods.

They should also ensure that their pressure-measuring instruments are suitable and that they understand the distinction between testing installation pipework and assessing existing appliances.

Where uncertainty exists, the complete IGEM publication should be consulted.

Gas Safe Engineers and Gas Safety Inspections in Surrey

Certificates4UK Ltd has provided gas safety and property maintenance services since 2011.

Our work includes gas safety inspections, boiler servicing and fault diagnosis for homeowners, landlords and property professionals across Surrey.

We cover areas including West Molesey, Walton-on-Thames, Esher, Weybridge, Hersham, Thames Ditton, Cobham and surrounding locations.

Keeping up with current gas industry standards is an important part of professional engineering practice.

For landlords, the annual Gas Safe with CP12 inspection remains a separate responsibility from the technical procedures for tightness testing and purging.

Frequently Asked Questions About IGEM/UP/1B Edition 4

Is IGEM/UP/1B Edition 4 mandatory from October 2026?

Edition 4 became effective on 1 October 2026, replacing Edition 3. Engineers should use the current applicable technical standard for work within its scope.

What is the biggest change in IGEM/UP/1B Edition 4?

One of the most significant changes is the relationship between installation volume, fuel gas, and permissible pressure-loss limits.

Does Edition 4 allow gas pressure loss on installation pipework?

No. Perceptible pressure loss on a pipework-only retest constitutes a failure. Permissible pressure-loss criteria for installations with existing appliances must not be interpreted as an allowance for leaking pipework.

Do I need to calculate installation volume for every tightness test?

Not in every circumstance. IGEM provides specific situations where installation volume does not need to be calculated solely for assessing a pressure loss. However, the relevant conditions must be satisfied.

Do I need installation volume for purging?

Yes. Where purging is undertaken, installation volume must be calculated to determine the purge volume, even where that calculation was unnecessary during the preceding tightness test.

What is the purge-volume calculation under Edition 4?

The purge volume is calculated as 1.5 times the installation volume, subject to the full standard’s requirements.

Can I use an Edition 4 quick-reference sheet instead of the complete standard?

No. A quick-reference sheet is useful for revision, but it cannot cover every requirement, exception and safety condition in the complete technical publication.

Does IGEM/UP/1B apply to every gas installation?

No. The standard has defined limitations covering factors such as installation volume, meter capacity, pipework size, operating pressure and gas type.

Where can engineers find the official Edition 4 guidance?

The complete publication and technical FAQs are available from the Institution of Gas Engineers and Managers (IGEM).