Written 31 August 2026
IGEM/UP/1B Edition 4: what actually changes on 1 October
Edition 3 gets withdrawn on 30 September 2026. From 1 October you work to Edition 4. This page is the short version — what is different, what you do differently on site, and what stays exactly as it was.
The test itself is broadly the same. What has changed is how you judge the result. Permissible pressure drop now comes from the Installation Volume of the system rather than the meter size, and if the gauge moves at all, you can no longer treat that movement as coming from the appliances — you have to prove the pipework on its own is tight. Purge volume is also now calculated from Installation Volume.
The dates, straight
- IGEM published Edition 4 (Communication 1903) in March 2026. It came into effect immediately.
- Edition 3 (with Amendments October 2012, Communication 1759) ran alongside it for six months.
- Edition 3 is withdrawn on 30 September 2026.
- Gas Safe Register issued Industry Standard Update 133. The version that stands is Edition 2 of that ISU, issued 15 April 2026, which supersedes the original of 25 March 2026 after industry reported confusion with the first one.
- Through the transition, Gas Safe Register agreed not to defect a business that was not yet fully conversant with the new procedure, and to give guidance at inspection instead. That ends when the transition ends.
Change 1 — Installation Volume replaces meter size
Under Edition 3 you looked up the permissible drop against the meter. Under Edition 4 the permissible limits are defined by gas type and Installation Volume (IV). So where a drop needs assessing, you calculate the IV of the installation — or of the part of it being tested — and compare the observed drop against the permissible limits table for that fuel gas.
The practical effect: a job that used to be a table lookup now starts with a volume calculation. Pipe runs, diameters and the meter all feed into it.
Change 2 — any perceptible movement is not the appliances' fault
This is the one that changes what you do with your hands. If there is no perceptible movement, the installation is gas tight and you are done.
If there is perceptible movement, the observed drop has to be assessed against the permissible limits for the gas type and IV. And even where the drop is within permissible limits, that is not the end of it: you then have to establish that the drop is not coming from the installation pipework. To do that you isolate all appliances from the gas supply and repeat the test on pipework only. On that pipework-only test there can be no perceptible movement at all for it to pass.
Where it is not reasonably practicable to isolate the appliances, the test is carried out with them connected — and the no-perceptible-movement criterion applies. If movement is recorded, that is a fail: trace and repair the escape, or disconnect the installation from the gas supply and make it safe.
Same outcome if the drop falls outside permissible limits in the first place — trace and repair, or make safe.
On a fluid (water) gauge: a movement of 0.25 mbar or less is considered not perceptible. So if you can see the gauge move, it is fair to infer the pressure has altered by more than 0.25 mbar.
On an electronic gauge: no perceptible movement means a maximum of 0.25 mbar — except for gauges that read to one decimal place, where it means a maximum of 0.2 mbar.
The test sequence, in order
- Visual inspection. Make sure appliances are set up properly — cooker lids upright, permanent pilots extinguished.
- Turn off the Emergency Control Valve (or the appropriate valve).
- Connect the gauge and carry out the let-by test at 7–10 mbar for one minute.
- If let-by passes, raise the pressure to the relevant tightness test pressure for the fuel gas and allow one minute for temperature stabilisation.
- Adjust the pressure where necessary, then test for a further two minutes.
- Pipework only, or only new appliances connected? There can be no perceptible movement.
- Existing appliances on the system? If there is any perceptible movement, calculate the Installation Volume and compare the drop against the permissible limits table for that fuel gas.
- Within limits — isolate all appliances and repeat on pipework only. No perceptible movement, or it is a fail.
- Can't isolate an appliance — test with it connected and apply the no-perceptible-movement criterion.
- Outside limits — trace and repair the escape, or make the system safe.
- Finish up: remove the gauge, refit the test nipple, reintroduce the gas, and test all disturbed joints with leak detection fluid — along with all installation pipework between any isolation valve and the appliance it serves. There must be no smell of gas.
Change 3 — purge volume
The purging procedure is unchanged. How you determine the purge volume is not. Meter size and pipework diameter used to decide it. Now you calculate the Installation Volume and apply the same formula on every installation:
Where the numbers are
The permissible limits tables are not reproduced here on purpose. They belong to IGEM and Gas Safe Register, and a table copied onto a website is exactly how somebody ends up working from a stale one. Get them from the Standard itself, or from the controlled copy of ISU 133 in the Technical Information area of the Gas Safe Register engineer portal.
- IGEM/UP/1B Edition 4 — the Standard itself, from IGEM
- Gas Safe Register engineer portal — Technical Information area, for the controlled copy of ISU 133
An Industry Standard Update is an overview. Gas Safe registered engineers and businesses remain responsible for being fully apprised of the whole published Standard and how it applies in practice.
Questions engineers keep asking
When is IGEM/UP/1B Edition 3 withdrawn?
30 September 2026. Edition 4 was published in March 2026 and came into effect immediately, but the two ran alongside each other for six months so businesses had time to train. From 1 October 2026 Edition 4 is the one to work to.
What is the main change in Edition 4?
Two things. First, the permissible pressure drop on a tightness test is now worked out from the Installation Volume of the system, not from the meter size alone. Second, where there is any perceptible movement of the gauge, that movement is not attributable to the installation pipework — so you have to prove the pipework itself is tight.
What counts as 'no perceptible movement'?
On a fluid (water) gauge, a movement of 0.25 mbar or less is treated as not perceptible — so if you can see it move, you can infer the pressure has changed by more than 0.25 mbar. On an electronic gauge the pass criterion is a maximum of 0.25 mbar, except gauges that read to one decimal place, where it is a maximum of 0.2 mbar.
Do I still do a let-by test?
Yes. The let-by test is unchanged: 7–10 mbar for one minute, with the Emergency Control Valve (or the appropriate valve) turned off.
How long is the tightness test itself?
After the let-by passes, raise the pressure to the relevant tightness test pressure for the fuel gas, allow one minute for temperature stabilisation, adjust the pressure if you need to, then test for a further two minutes.
Has purging changed as well?
The purging procedure itself is unchanged, but the way you work out the purge volume has. Where meter size and pipework diameter used to determine the purge volume, you now calculate the Installation Volume and apply PV = 1.5 × IV on all installations.
Will I be defected for not knowing Edition 4 yet?
During the six-month transition, Gas Safe Register agreed not to defect a business that is not fully conversant with the new procedure at inspection, and to give guidance instead. That transition ends on 30 September 2026.
Doing the sums on a phone, on site
The Gas Hub App works out the Installation Volume from the pipe runs you enter, times the test for you, and puts the verdict and your evidence photos straight onto the record. It was built to Edition 4 from the start, so there is nothing for you to change on 1 October.