Gas

Gas Tightness (Let-By and Drop) Test Procedure

The tightness test proves an installation holds pressure with no gas escaping — a let-by check on the meter valve, a stabilisation period, then a timed test where the pressure must not fall by more than the permitted amount.

Updated 20 Aug 20268 min read

A gas tightness test (strength and tightness testing) is carried out on a new installation, after any work that breaks into the pipework, and whenever soundness is in doubt. On a domestic natural gas installation the method follows IGEM/UP/1B (or IGEM/UP/1A for the strength-tested new installation), and the permitted pressure drop depends on the meter, the pipework volume and the test pressure.

This guide walks through the standard domestic procedure for a U6/G4 diaphragm meter: the let-by test, the stabilisation period and the timed tightness test with its permitted drop. It describes the method and where the pass/fail figures come from rather than reproducing the full IGEM tables — always work to the current IGEM procedure and use a correctly ranged gauge. Gas work must be carried out by a Gas Safe registered engineer.

Key takeaways

The test has three stages: a let-by test on the emergency control valve, a stabilisation (temperature-settling) period, then the timed tightness test.
For a domestic U6/G4 meter the tightness test is run at a nominal test pressure of around 20 mbar and timed (commonly two minutes after stabilisation).
A let-by must show no rise on the gauge — any rise means the valve is passing gas and must be dealt with before continuing.
The permitted pressure drop is not a single fixed number: it depends on the installation volume and gauge, and is read from the IGEM/UP/1B procedure.
Purge and re-test if you disturb the pipework; a failed test means find and fix the leak, never sign it off marginal.

When and why you test

A tightness test proves the installation is sound — that with the gas on and appliances isolated, no gas is escaping. You test a new installation before it is commissioned, after any alteration or repair that opens the pipework, before reconnecting a supply, and any time you suspect a leak. On a new installation a strength test to IGEM/UP/1A precedes the tightness test.

The domestic procedure below is the tightness test to IGEM/UP/1B for a typical low-pressure natural gas installation with a U6/G4 diaphragm meter. LPG, larger meters and higher operating pressures follow different procedures and figures.

The let-by test

The let-by test checks that the emergency control valve (ECV) is holding — that it is not itself letting gas by into the installation while you test. With appliances isolated and the test gauge connected, the ECV is turned off and the gauge watched for a short settling period.

The gauge must not rise. Any rise means the ECV is passing gas (letting by), which would mask a leak and invalidate the tightness test — the valve must be dealt with before you go any further. Only once the let-by is clear do you proceed to pressurise for the tightness test.

A rise means let-by

During the let-by test the gauge should hold steady. Any upward movement means the emergency control valve is passing gas — stop, resolve the valve, and do not attempt to interpret a tightness result until the let-by is clear.

Stabilisation and the timed test

With the installation charged to the nominal test pressure (around 20 mbar for a domestic natural gas system) and the ECV turned off, the pressure is left to stabilise. This settling period lets the gas temperature equalise so that a drop from cooling gas is not mistaken for a leak — a short wait before timing begins is essential, especially on larger or newly-charged installations.

The tightness test proper is then timed — commonly two minutes for a domestic installation — and the pressure drop over that period is read from the gauge. The installation passes only if the drop is within the permitted figure for that meter, installation volume and test pressure.

The permitted pressure drop

The maximum permitted pressure drop is not a single universal number. It is determined by the IGEM/UP/1B procedure from the meter type, the installation volume (which affects how much a given leak moves the pressure) and the test pressure, and it also allows for the resolution of the gauge you are using. For a standard domestic installation on a U6/G4 meter the procedure yields a small permitted drop over the two-minute test.

The practical rule is to read the permitted drop from the current IGEM procedure/ready-reckoner for the actual installation, using a properly ranged and calibrated gauge (a water/electronic manometer reading in mbar), rather than working to a remembered number. If the measured drop exceeds the permitted figure, the installation has failed.

Read the figure, don't recall it

Because the permitted drop depends on meter, volume, test pressure and gauge, the correct pass/fail figure comes from the IGEM/UP/1B procedure for the specific installation — not from memory. Use a calibrated manometer and record the actual test pressure and drop.

Failing, finding leaks and recording

If the test fails, the installation is not sound: locate the leak (leak-detection fluid or an approved detector on joints and fittings, never a flame), repair it, then repeat the full test from the let-by. Never sign off a marginal or failed test, and never leave an installation live if soundness cannot be established — treat it under the Gas Industry Unsafe Situations Procedure.

When the installation passes, record it: the test pressure, the stabilisation and test durations, the measured drop and the pass result, on a gas tightness test certificate. That record is your evidence the installation was proven sound before it was recommissioned.

Frequently asked questions

What test pressure is used for a domestic tightness test?

A domestic natural gas installation is tested at its nominal operating pressure of around 20 mbar. The let-by test is done first, then the installation is charged to test pressure and left to stabilise before the timed test.

What is the let-by test checking?

It checks that the emergency control valve is holding and not itself letting gas past into the installation. If the gauge rises during the let-by test, the valve is passing gas and must be sorted before the tightness test can be interpreted.

What is the maximum permitted pressure drop?

There is no single figure. The permitted drop is derived from the IGEM/UP/1B procedure using the meter type, installation volume, test pressure and gauge. For a domestic U6/G4 meter it is a small drop over the timed test — always read it from the current procedure for the specific installation.

Why do I need a stabilisation period?

Freshly charged gas can be at a slightly different temperature to the pipework. Left to settle, the pressure equalises, so a genuine timed test does not misread cooling gas as a leak. Skipping stabilisation gives false fails.

What do I do if the installation fails?

Locate the leak with leak-detection fluid or an approved detector (never a flame), repair it, and repeat the whole test from the let-by. Do not sign off a marginal result and do not leave the installation live if soundness cannot be established.

From guidance to action

Gas tightness test certificateRecord the test pressure, drop and pass result, signed on site.Gas pipe sizing calculatorSize the installation pipework for the required flow and pressure drop.