Guide · updated 2026-09-18

Gas tightness test: what changes in IGEM/UP/1B Edition 4

Edition 4 is mandatory from 1 October 2026, and Edition 3 is withdrawn on 30 September. The change that matters on site is how you arrive at the permissible drop.

Straight to the questions everyone is asking →

The change in one line

The permissible pressure drop is now set by the installation volume, not by the meter size.

Under Edition 3 you looked at what meter you were stood in front of. Under Edition 4 you work out the volume of the installation you are testing, and the allowed drop follows from that. Two properties with the same U6 meter can now have different permitted drops, because one has 30 metres of 22mm and the other has 8 metres of 15mm.

What that means in practice

You need the installation volume before you know what you are allowed. That is the meter volume plus the volume of the pipework in the section under test, and the standard applies a fittings allowance where you do not know the actual component volumes.

So the job gains a step. Measure or reasonably estimate the pipe runs by diameter, add the meter, total it, and only then do you know what the gauge is allowed to do.

A drop inside the limit is not the end of it

This is the part worth reading twice. Where there is a drop, even one inside the permitted figure, it has to be proved not to be in the pipework. Isolate every appliance and retest. At that point there should be no perceptible movement on the gauge.

A small allowable drop is there to account for appliances, not to give pipework a pass.

Perceptible is not permissible, and that is where people are coming unstuck

These two words are doing different jobs and they are getting mixed up constantly.

Perceptible is about your gauge. It is the smallest movement your equipment can honestly claim to have seen. On a water gauge that is 0.25 mbar. On a digital manometer reading to one decimal place it is 0.2 mbar, and to two decimal places it is 0.25 mbar. At or below that figure, the gauge has not moved. You do not count it, and you do not go any further.

Permissible is the allowance in the standard once the gauge has moved. That is the figure that now comes from the installation volume.

So the order on site is: did the gauge move by more than your equipment’s perceptible figure? If not, you are done. If it did, now you work out the volume and see whether the drop is inside the permitted allowance.

There is a reason so many engineers have this tangled. The first edition of Gas Safe’s Industry Standard Update 133 said that a drop of up to 1 mbar meant you did not need to work out the installation volume at all. The second edition, issued three weeks later, removed that line. If you read the first one and not the second, you are working to guidance that no longer exists.

Do you have to work out the volume on every job?

No, and this is the single most common misunderstanding about Edition 4.

You need the installation volume in two situations. One, the gauge moved perceptibly and you need to know what you are allowed. Two, you are purging, because the purge volume is worked out from it.

A tightness test that shows no perceptible movement needs no volume calculation. Nothing to compare, nothing to work out. Most of the anger about this change assumes you will be doing sums at every property, and you will not be.

Purging changed too, and that one does catch everybody

The purge volume is now one and a half times the installation volume. It used to be a fixed assumption based on the meter and the pipe size, which for most domestic jobs came out at 0.01 m³.

That is the change with the least argument behind it, because the old assumption was measurably wrong. Trainees measured real installations against it and found it was nowhere near enough gas to purge the system safely. That is the honest answer to anyone asking why a working procedure was changed.

The volume also decides how you purge. On natural gas, up to 0.02 m³ can be purged to air. Above that you purge to ignition.

What about a service, or a landlord’s report?

A tightness test is not automatically required on either one.

The standards call for a tightness test on completion of new pipework, after an alteration, replacement or reuse of an existing installation, on a new extension to existing pipework, before work on existing pipework, where a leak is known or suspected, and after any complete loss of pressure.

A landlord’s gas safety record is not on that list. Regulation 36 of the Gas Safety (Installation and Use) Regulations puts a duty on the landlord to keep fittings and flues maintained in a safe condition, but it does not say a tightness test. Gas Safe Technical Bulletin 046 is the document to read on this, and it remains current.

None of which is an argument for skipping one. It is an argument for knowing why you are doing it.

Scope

UP/1B covers domestic-sized installations. Above the installation volume the standard sets as its ceiling you are into IGEM/UP/1A instead, which is a different procedure. Knowing the volume tells you which standard you are working to, which is another reason the volume calculation comes first.

Pipework you cannot see

The standard talks about exposed pipework. It does not ask you to open up walls and floors to measure a gas carcass, and nobody expects you to.

Where a run is hidden, you make a reasonable engineer’s estimate from a visual inspection: where the meter is, where the appliance is, the likely route between them, and the largest size you can see entering and leaving. Look at what comes out from under the boiler to tell copper from corrugated stainless. Genuine steel pipe inside a house is rare enough that most engineers go a career barely meeting it.

An estimate made honestly and recorded is what is being asked for here, not a certainty nobody can have.

What it actually adds to the job

About five minutes for the calculation, and five to ten on top of a normal visit once you are used to it. That is from a trainer who filmed the whole thing end to end on a real property.

It is worth writing the installation volume, the purge volume and the permitted drop on a note at the meter when you have worked them out. The next engineer through the door then has them, and so do you next year.

Where to get the figures

The permitted drops, the volume tables and the test durations sit in the standard itself, and IGEM's standards are copyright. We are not going to reproduce the tables here. Gas Safe Register published the detail in Industry Standard Update 133, and IGEM published its own UP/1B FAQ, and if you are testing to Edition 4 you should be working from your own copy.

If you have not read Edition 4 yet, do that before 1 October rather than after. The procedure has genuinely changed, not just the paperwork.

FAQ

The questions engineers keep asking

When does IGEM/UP/1B Edition 4 become mandatory?

Edition 4 is mandatory from 1 October 2026. Edition 3 is withdrawn on 30 September 2026.

What is the main change in Edition 4?

The permissible pressure drop is now set by the installation volume rather than by the meter size. That means working out the volume of the installation you are testing, from the meter and the pipework, before you know what drop is allowed.

Does a drop within the permitted limit still pass?

Not on its own. Any drop within limits has to be proved not to be in the pipework, by isolating every appliance and retesting, where there should be no perceptible movement.

What is the difference between perceptible and permissible?

Perceptible is about your gauge: 0.25 mbar on a water gauge, 0.2 mbar on a digital manometer reading to one decimal place, 0.25 mbar to two decimal places. At or below that, the gauge has not moved. Permissible is the allowance in the standard once it has, and that figure now comes from the installation volume.

Do I need to work out the installation volume on every tightness test?

No. Only when the gauge has moved perceptibly and you need to know what is allowed, or when you are purging, because the purge volume comes from it. A clean test needs no calculation at all.

Do I need to do a tightness test on a service?

Not automatically. A tightness test is called for on new pipework, after alterations or replacement, on extensions, before working on existing pipework, where a leak is known or suspected, and after a complete loss of pressure.

Do I need to do a tightness test for a landlord’s gas safety record?

Not automatically. Regulation 36 requires the landlord to keep fittings and flues maintained in a safe condition but does not specify a tightness test. Gas Safe Technical Bulletin 046 is the reference.

How do I work out the volume of pipework I cannot see?

By reasonable estimate from a visual inspection. The standard deals in exposed pipework and does not ask you to open up walls. Judge the likely route and use the largest size you cannot see.

What is the purge volume under Edition 4?

One and a half times the installation volume. On natural gas, up to 0.02 m³ may be purged to air; above that you purge to ignition.

Flint is a reference tool for competent, Gas Safe registered engineers. Everything here informs your judgement and never replaces it. Always work to the manufacturer's instructions and the current standards, and check every figure against the appliance data badge and manual.