IGEM/UP/1B Edition 4: What’s Changed, and How to Calculate Installation Volume
The way you’ve judged a tightness test pass for over a decade is set to change in IGEM/UP/1B Edition 4. Published in March 2026 and mandatory from 1 October 2026, the updated regulation replaces meter-size-based permissible drops with a new driver: Installation Volume (IV).
The test procedure still looks broadly the same, but what decides whether you’ve passed it doesn’t. Here’s what’s new, and how to calculate the IV you’ll now need for every tightness test and purge.
KEY TAKEAWAYS
What’s changed in IGEM/UP/1B Edition 4
The main change:
Permissible pressure drop during a tightness test is now set by Installation Volume (IV), not meter badged capacity.
Additional changes:
- A pipework-only retest is now a mandatory follow-on step when appliances are connected. Any perceptible movement during that retest is a fail.
- “Perceptible movement” is now clearly defined (0.25 mbar on a water gauge, 0.2 mbar on a high-resolution electronic gauge).
- The purge volume formula is standardised: PV = 1.5 × IV, regardless of meter size or pipework diameter.
- Caravans and leisure accommodation vehicles (LAVs) are now in scope.
Note that Edition 3 is withdrawn on 30 September 2026. Edition 4 is mandatory from 1 October 2026.
Why Installation Volume now matters (not meter size)
Under Edition 3, the permissible drop on a tightness test was pegged largely to the meter. If you had a U6, you had a standard set of allowable drops. This is changing because a meter’s badged capacity doesn’t tell you how much gas is sitting in the system you’re testing.
Two systems with the same tiny leak will show different pressure drops over two minutes depending on how much gas is inside them. The bigger the system, the smaller the observed drop — because the leak represents a smaller proportion of the total gas present.
Edition 4 fixes this by scaling the permissible drop to the actual Installation Volume of whatever you’re testing. Every tightness test now starts with an IV calculation.
What is Installation Volume?
Installation Volume is the total internal volume of gas sitting inside the section you’re testing, measured in cubic metres (m³).
For a typical domestic install, it’s somewhere between 0.005 and 0.015 m³ — five to fifteen litres.
IV is made up of three main areas:
- IVp = the pipework — every pipe run between your test point and the end of the section.
- IVm = the meter — if the meter is within the section being tested.
- IVf = the fittings — the volume of the fittings can be taken as 10% of the pipework volume.
How to calculate Installation Volume
- IVt is the total Installation Volume
- IVm is the volume of the meter
- IVp is the volume of the pipework
- IVf is the volume of the fittings
Step 1: Calculate the pipework & fittings volume (IVp + IVf)
Multiply the length of each pipe run by its volume-per-metre, then sum the runs. Use the values in the reference table below, which have already had 10% added to accomodate for the IVf value.
| Pipe | Nominal size | Volume per metre + 10% for fittings (IVp + IVf) |
|---|---|---|
| Copper | 15mm | 0.000154 m³ |
| Copper | 22mm | 0.000352 m³ |
| Copper | 28mm | 0.000594 m³ |
| Copper | 35mm | 0.000924 m³ |
| Steel | ½" | 0.000264 m³ |
| Steel | ¾" | 0.000506 m³ |
| Steel | 1" | 0.000704 m³ |
| Steel | 1¼" | 0.00121 m³/m |
Step 2: Look up the meter volume (IVm)
Use the manufacturer’s data on the meter, or these common values:
| Meter | Internal volume |
|---|---|
| G4 / U6 Diaphragm | 0.008 m³ |
| E6 Ultrasonic Smart | 0.0024 m³ |
| U16 Diaphragm | 0.025 m³ |
Step 3: Add the values together to calculate the total Installation Volume
To speed this process up in the field, use our gas pipe sizing calculator updated for IGEM/UP/1B Ed. 4. Once you add the pipe details, the calculator provides an installation volume automatically.
Worked example
Take a property with the following specifications as an example:
- A U6 domestic meter
- 2m of 22mm copper running from the meter to a tee in the airing cupboard
- 6m of 15mm copper running from the tee to a combi boiler
- 3m of 15mm copper running from the tee to a gas hob
Step 1 — IVp (pipework) + IVf (fittings):
- 2m × 0.000352 = 0.000704 m³
- 6m × 0.000154 = 0.000924 m³
- 3m × 0.000154 = 0.000462 m³
- IVp + IVf = 0.00209 m³
Step 2 — IVm (meter): U6 = 0.008 m³
Step 3 — Total: IVt = 0.00209 + 0.008 = 0.01009 m³ (about 10 litres)
Cross-referencing the Edition 4 natural gas table below, that IV falls in the > 0.010 – ≤ 0.015 m³ band, giving you a permissible pressure drop of 2.5 mbar during the 2-minute tightness test.
THE SHORTCUT
Rather than doing this by hand every time, you can use our updated pipe sizing calculator to quickly get the installation volume, purge volume, and permissible drop for any section of pipework. If you’re sizing a new set of pipework, you’ll be able to get all these results from the same set of inputs.
The new permissible drop tables
Once you have your IV, compare it to the relevant table for the fuel gas type. If your observed drop on the 2-minute test is within the permissible limit for your IV band, you can proceed to the pipework-only retest. If it’s outside the limit, trace and repair the escape, or make the system safe.
Natural gas
| Installation Volume (IV) | Maximum permissible pressure drop |
|---|---|
| ≤ 0.005 m³ | 8 mbar |
| > 0.005 – ≤ 0.010 m³ | 4 mbar |
| > 0.010 – ≤ 0.015 m³ | 2.5 mbar |
| > 0.015 – ≤ 0.035 m³ | 1 mbar |
LPG
| Installation Volume (IV) | Maximum permissible pressure drop |
|---|---|
| ≤ 0.0025 m³ | 2 mbar |
| > 0.0025 – ≤ 0.005 m³ | 1 mbar |
| > 0.005 – ≤ 0.01 m³ | 0.5 mbar |
| > 0.01 – ≤ 0.035 m³ | No perceptible movement |
LPG/Air
| Installation Volume (IV) | Maximum permissible pressure drop |
|---|---|
| ≤ 0.025 m³ | 1.5 mbar |
| > 0.025 – ≤ 0.035 m³ | 0.5 mbar |
For the full step-by-step tightness test procedure under Edition 4 — let-by test, stabilisation, the 2-minute test, pipework-only retest, and everything in between — see our updated gas tightness test procedure guide.
What counts as “perceptible movement” now
Edition 4 tightens up a phrase that used to be judged by eye. “Perceptible movement” on a gauge is now defined numerically:
- Fluid (water) gauge — a movement of 0.25 mbar or less is considered not perceptible.
- Electronic gauge (standard resolution) — “no perceptible movement” means a maximum of 0.25 mbar.
- Electronic gauge reading to one decimal place — “no perceptible movement” is a maximum of 0.2 mbar.
On a pipework-only retest (after isolating appliances), any perceptible movement is an automatic fail.
The new purging formula
The purging procedure itself hasn’t changed, but the way you calculate purge volume has.
Under Edition 3, purge volume depended on meter size and pipework diameter. Under Edition 4, there’s one formula for every installation:
PV = 1.5 × IV
So if you’ve already done an IV calculation for your tightness test, your purge volume is easily calculated.
Timeline — what to do before 1 October 2026
Edition 4 came into effect immediately on publication in March 2026, but runs alongside Edition 3 for a six-month transition.
Now — 30 September 2026: both Edition 3 and Edition 4 are acceptable. Gas Safe Register has agreed not to defect businesses that aren’t yet fully conversant with Edition 4 during inspections, and will provide guidance where needed.
30 September 2026: Edition 3 is formally withdrawn.
1 October 2026: Edition 4 is mandatory. All tightness testing and purging must follow the new procedure.
Three practical things to do now:
- Familiarise yourself with the IV calculation. Run it for a couple of jobs you’ve recently done so the arithmetic becomes automatic.
- Update your record-keeping. Edition 4 makes pipe schedules (diameters, lengths) operationally valuable forever — not just a design-time artefact. Capture them once, use them at every test.
- Check your refresher training schedule. If your ACS refresher is due in the back half of 2026, make sure the course you’re booking covers Edition 4 explicitly. Not all providers have updated yet.
If you have an inspection coming up, you won’t get caught out by these changes until 1 October. However, it’s worth knowing what inspectors will be looking for.
Putting it all together
The change that really matters is quieter: your pipe schedule is now a long-term asset. The numbers you capture during installation — diameter, length, material — feed pipe sizing at design-time, IV at testing-time, and purge volume forever after.
Use our free Gas Pipe Sizing Calculator to get both the sizing pressure drop and the Installation Volume you’ll need for your next tightness test. And if you want that data stored alongside your jobs, certificates, and gas safety records, that’s what Gas Engineer Software is for.