Changes to gas tightness testing mean engineers need to be confident not only in carrying out the physical test, but also in calculating installation volume, assessing pressure loss and applying the correct requirements for the fuel gas and installation being tested.
IGEM/UP/1B Edition 4 came into effect on 1 October 2026, replacing Edition 3. The updated standard covers tightness testing and direct purging for small natural gas, LPG and LPG/Air installations within its defined scope.
To support engineers working with the updated requirements, Staffordshire Training Services has developed STS GasAssist, a free gas tightness testing, pressure drop and purge calculator.
STS GasAssist guides engineers through installation volume calculations, test setup, readings and the final review. It can also be used offline once initially set up, making it practical for properties, meter cupboards and other areas where mobile reception may be limited.
GasAssist is an engineering aid rather than a replacement for technical competence. It does not certify a test, declare an installation safe or replace the requirement to follow the current controlled standard.
IGEM/UP/1B Edition 4
IGEM/UP/1B Edition 4 applies to defined small gas installations using natural gas, LPG or LPG/Air. One of the principal scope limits is a maximum installation volume of 0.035 m³ for an individual dwelling or non-domestic premises.
Other scope limits also apply, including those relating to operating pressure, pipe size and meter capacity. Engineers must therefore establish that an installation falls within the scope of the procedure rather than relying on installation volume alone.
One of the significant considerations under Edition 4 is the relationship between installation volume and permissible pressure loss in circumstances where an allowance can be applied. This makes accurate calculation of the installation volume particularly important.
Our guide to changes to tightness testing standards covers the wider transition to the updated procedure. In contrast, our article on gas pressure, flow, tightness testing and purging looks more broadly at the principles involved in this area of gas work.
Installation Volume and Pressure Loss
Installation volume is an important part of the updated tightness testing procedure because it can determine the maximum permissible pressure loss for qualifying existing installations with existing appliances connected.
Calculating installation volume involves more than measuring the total length of gas pipework. Different pipe sizes contain different volumes of gas per metre, while the meter and relevant valves and fittings also contribute to the total volume of the section being tested.
GasAssist allows the engineer to select each pipe material and size before entering the length of the run. Further runs can then be added where an installation contains several pipe sizes.
The gas meter can also be included in the calculation. Listed meter options include G4/U6 diaphragm, E6 ultrasonic and G10/U16 diaphragm meters. Where another meter is installed, the engineer can enter a verified manufacturer volume.
Valves and fittings can then be accounted for using the appropriate method before GasAssist calculates the total installation volume.
Natural Gas Pressure Loss Allowances
For an existing natural gas installation with existing appliances connected, the installation volume is used to establish the applicable maximum pressure loss where all of the conditions for applying an allowance are satisfied.
The applicable figures incorporated into GasAssist are:
| Installation Volume | Maximum Pressure Loss |
|---|---|
| Up to and including 0.005 m³ | 8 mbar |
| Over 0.005 m³ up to and including 0.010 m³ | 4 mbar |
| Over 0.010 m³ up to and including 0.015 m³ | 2.5 mbar |
| Over 0.015 m³ up to and including 0.035 m³ | 1 mbar |
These figures must not be interpreted as an acceptable leakage allowance from the installation pipework.
Where there is perceptible pressure loss within the relevant allowance, the engineer needs to establish whether the loss is associated with the pipework or the connected appliances. Appliances should be isolated and the pipework and meter retested with no perceptible movement.
If an appliance cannot practicably be isolated, it can remain connected for the retest, but the requirement is again no perceptible movement.
A pressure loss above the applicable allowance, or perceptible movement during the required retest, means the source of the escape must be identified and repaired or the installation made safe in accordance with the current procedure.
No Perceptible Movement
The pressure loss allowances for existing installations should not be treated as a general tolerance that applies to every tightness test.
New installations, installations containing only new appliances and pipework-only tests require no perceptible movement, assessed using the applicable gauge criterion.
This distinction is particularly important when using a calculation tool. A result that falls within a stated pressure loss allowance does not automatically mean that an installation has passed its tightness test.
The engineer still needs to apply the complete procedure, consider the circumstances of the installation and complete the necessary isolation, retesting and final checks.
Before completing the outcome, equipment should be reinstated as appropriate and disturbed joints checked. Pipework between isolation valves and appliances should also receive the applicable checks, with the engineer confirming that there is no smell of gas.
Any accepted appliance-related pressure loss also requires an appropriate site-specific risk assessment.
LPG And LPG/Air Installations
Edition 4 also covers LPG and LPG/Air installations that fall within its specified scope. The natural gas pressure loss figures must not be applied to these installations because different limits are used according to the fuel gas.
GasAssist allows the engineer to select the actual fuel gas so that the appropriate values can be applied to the calculation.
For LPG/Air, the figures are:
| Installation Volume | Maximum Pressure Loss |
|---|---|
| Up to and including 0.025 m³ | 1.5 mbar |
| Over 0.025 m³ up to and including 0.035 m³ | 0.5 mbar |
For LPG, the figures are:
| Installation Volume | Maximum Pressure Loss |
|---|---|
| Up to and including 0.0025 m³ | 2 mbar |
| Over 0.0025 m³ up to and including 0.005 m³ | 1 mbar |
| Over 0.005 m³ up to and including 0.010 m³ | 0.5 mbar |
| Over 0.010 m³ up to and including 0.035 m³ | No perceptible movement |
As with natural gas, a pressure loss that falls within an allowance does not remove the requirement to establish that the pipework itself is gas tight.
Gas engineers looking to extend their scope of work into LPG can complete our ACS LPG Changeover Initial Assessment at our Stafford training centre.
Calculating Installation Volume
Manually calculating installation volume can become more involved where an installation contains several pipe sizes.
A domestic installation could, for example, have a larger pipe leaving the meter before reducing through several sizes as it supplies different appliances. Each section has its own internal volume according to its pipe size and length.
GasAssist breaks this calculation down by individual pipe runs. The engineer selects the pipe size, enters the length and then adds another run where required.
The calculator includes reference volumes per metre for the supported pipe materials and sizes. An option is also provided for other pipework where the engineer has a verified volume per metre.
Once the pipework volume has been calculated, the applicable meter volume and valves and fittings are added to produce the overall installation volume.
This makes the arithmetic quicker, but it also provides a clearer working record. Rather than simply having a final installation volume written in a notebook, the engineer can see the individual figures used to produce the result.
Purge Volume Calculations
Installation volume also forms the basis of the purge calculation.
For installations being purged under the applicable procedure, purge volume is calculated using:
Purge Volume (PV) = 1.5 × Installation Volume (IV)
For example, an installation with a calculated installation volume of 0.020 m³ would require the following calculation:
PV = 1.5 × 0.020
PV = 0.030 m³
GasAssist brings the installation volume, tightness testing information and purge calculation into the same guided process. This reduces the need for engineers to carry out separate calculations or move between different notes while completing the work.
The calculated purge volume does not replace the remaining requirements of the purging procedure. Engineers must continue to follow the complete current procedure applicable to the installation.
Using the Free GasAssist Calculator
The free STS GasAssist calculator takes the engineer through four main stages.
1. Installation
Select the relevant pipework, enter individual pipe lengths and add the meter, valves and fittings. GasAssist then calculates the installation volume.
2. Test Setup
Enter the information relating to the installation and the tightness test so that the correct criteria can be applied.
3. Readings
Record the applicable test readings as the tightness testing procedure is completed.
4. Review
Check the information entered, calculations and resulting outcome before completing the working note.
GasAssist is designed to reduce the amount of manual arithmetic required on site while prompting the engineer to consider the relevant information at each stage. The engineer remains responsible for entering accurate information, selecting the appropriate procedure and interpreting the result correctly.
Offline Access On Site
A calculation tool is of limited use if it stops working as soon as an engineer enters a meter cupboard or property with poor mobile reception.
GasAssist has therefore been designed to work offline once the initial setup has been completed.
Open GasAssist while connected to the internet and submit your contact details if prompted. Keep Remember me on this device selected and allow the calculator to load. The calculator and locally stored drafts can then continue to work without an internet connection.
On an iPhone or iPad, GasAssist can be opened in Safari and added to the Home Screen. Android users can use Chrome to add or install the calculator on their device.
This provides quick access in much the same way as a conventional app, without having to find the website each time a calculation is required.
Contact submission and administration functions require an internet connection, but the core calculator and saved drafts can continue to be used offline.
Saving a Test Working Note
GasAssist does more than provide a final installation volume or pressure loss figure. Engineers can retain a record of the information used during the calculation.
The Print working note / save PDF function allows the working note to be printed or saved as a PDF. Calculation data can also be exported separately where a digital copy is required.
Draft jobs are stored locally within the browser on the device being used. Engineers should therefore save or export any information they need to retain before clearing a completed job or changing devices.
The working note provides a useful record of the calculation and information entered into GasAssist. It does not replace any documentation required by the applicable technical standard, Gas Safe Register, an employer or other relevant procedures.
GasAssist Scope
The current version of GasAssist is intended for domestic installations with an installation volume up to 0.035 m³.
If the calculated installation volume exceeds this limit, GasAssist stops the pressure loss assessment rather than attempting to apply criteria outside its intended scope.
Engineers must remember that installation volume is only one consideration. The applicable standard contains other scope requirements, so a volume below 0.035 m³ does not by itself establish that the procedure is suitable for an installation.
This is an important part of the way GasAssist has been developed. The calculator is there to support an engineer’s assessment, not make engineering decisions on their behalf.
It cannot identify an incorrectly measured pipe run, detect a gas escape, confirm that an appliance has been correctly isolated or determine whether an installation should remain in service. Those decisions depend on the competence of the engineer carrying out the work.
Gas Training In Staffordshire
Changes to tightness testing procedures demonstrate the importance of gas engineers maintaining their technical knowledge throughout their careers. Pressure, flow, tightness testing and purging are fundamental areas of gas work, but engineers also need to be able to adapt as standards and industry procedures change.
At Staffordshire Training Services, our gas training in Stafford combines technical knowledge with practical training at our training centre.
For people entering the gas industry through an alternative route to an apprenticeship, our Gas Managed Learning Programme provides structured training alongside supervised workplace experience. Tightness testing, pressure, flow and purging form part of the wider technical knowledge required as candidates work towards becoming competent gas engineers.
We also provide ACS assessment and specialist gas training for existing engineers looking to maintain or extend their scope of work.
STS GasAssist has been developed with the same practical approach. It can take care of some of the arithmetic, guide engineers through the information required and provide a useful working record, but it does not replace the knowledge behind the test.
Engineers still need to understand the procedure, recognise the significance of the readings they obtain and make the correct safety decisions on site.
The STS GasAssist Tightness & Purge Calculator is free to use and, once initially set up, can be kept on a phone for offline access whenever it is needed on site.
Technical note: GasAssist and this article provide supporting information for competent gas engineers. They do not reproduce or replace the complete requirements of IGEM/UP/1B Edition 4 or other applicable technical guidance. Engineers should always work to the current controlled standard and procedures relevant to the installation.
Related Articles
- Update To Tightness Testing Standards
- Gas Tightness Testing Fault-Finding Techniques
- Gas Pressure, Flow, Tightness Testing and Purging Explained
- Gas Pressure Testing and Appliance Performance For Engineers
- Gas Purging Procedures For Gas Engineers
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