Domestic gas tightness testing procedure under IGEM/UP/1B

After any work that could disturb a gas fitting’s tightness, test the affected section straight away, reaching at least the nearest valves upstream and downstream. Altered installation pipework must also be examined and tested before it goes back into use. Make any known escape safe and correct the fault. This applies to existing pipework as well as new work.

What a tightness test proves

First, work out what the gauge is connected to: the meter, the pipework and any appliances included in the test. Isolate the supply, let the pressure and temperature settle, then watch the timed reading. A fall during a valid test must be investigated, but the gauge cannot tell you where the gas is going. Before the reading settles, it can drift without any leak. A pass on the pipework alone does not cover an appliance connection left out of the test.

Let the gauge reach its working temperature, then zero it as its instructions say, in the position you will use it in. Check the hoses and the test-point connection are tight, and keep the gauge and the pipework at a steady temperature during the reading. Preparing the gauge is a separate step from letting the installation stabilise.

No smell of gas proves nothing: a small escape can go unnoticed and still build up over time. Purging and gas rating are separate jobs that answer different questions.

IGEM/UP/1B Edition 4 applies from 1 October 2026. Edition 3 was withdrawn on 30 September 2026. Under Edition 4, installation pipework must be proved tight.

The stages, in order

1. Let-by check. Confirm the emergency control valve holds when closed. Gas passing a closed valve can offset a loss downstream and make the gauge look steady.
2. Stabilisation. Let temperature and pressure settle: gas that is still warming or cooling changes the pressure without any leak. If the gauge is still drifting after the minimum time, keep waiting until it is steady.
3. Timed test. Read the gauge over the prescribed test period.

New pipework, with or without new appliances, must show no perceptible pressure drop over the test period. A perceptible fall fails the test: check the setup and conditions, then find and repair any leak before accepting the work. The allowance for eligible existing appliances never applies to pipework or new appliances, and not being able to isolate the existing appliances does not earn you one.

Edition 4 details for natural gas

• Let-by: with the emergency control valve closed and the pressure at 7 to 10 mbar, watch the gauge for 1 minute.
• Test pressure: 20 mbar. If you cannot reach it, investigate the low pressure. The test can then be run at 18 mbar or more, and the operating pressure is checked once it has passed.
• Stabilisation: 1 minute as a minimum, longer if the gauge is still moving.
• Timed test: reset the pressure if it has moved, then read the gauge over 2 minutes.
• Meter type: no longer sets a permitted drop. Any allowance comes from the fuel and the installation volume, and only for eligible existing appliances.

After a satisfactory test

1. Purge new pipework that contains air, by the set method and amount: under Edition 4, work out the installation volume (IV) and purge 1.5 × IV. Discharge it in a way allowed for that gas, volume and setting, and control the risks of ignition and of gas building up. An open window is not enough to make a discharge point suitable.
2. Commission each appliance before leaving it in use: check the flue clears the products, the air supply is adequate, the operating pressure or heat input is correct, and the appliance works safely. Use the checks given for that appliance: operating pressure, heat input, or both. Pressure does not measure heat input. A sound gas connection is not enough.

When something fails

• Commissioning cannot be completed: disconnect the gas supply to the appliance or installation and seal it with an appropriate fitting. Label it not to be used until it has been commissioned, and tell the responsible person what failed and what you have done. Closing a valve or leaving a warning is not enough.
• A fault on an existing appliance: assess the real risk and follow the unsafe-situations procedure. Immediately Dangerous and At Risk situations need different responses. For an At Risk appliance that turning off would make safer, get permission, then turn it off at the gas isolation valve. If you can't, remove the electrical fuse. Only if neither is possible may you use the appliance's own control, with the warning label attached to it. A room thermostat or timer is not acceptable. Give the required warning label and written notice.
• Spillage on a newly fitted open-flued appliance: stop commissioning and keep the appliance out of use. Find and fix the cause. If you can't, disconnect and seal its gas supply, because commissioning is not complete. Never use a fan or a lower gas rate to hide spillage.
• An escape you cannot stop: warn people and get clear, avoiding ignition sources. From a safe place, call 0800 111 999 for natural gas, or the gas supplier’s emergency service for LPG. Close the emergency control valve and ventilate only if you can do so safely without delaying your exit.

Where these duties come from

• GSIUR regulation 22: after work on installation pipework that could affect its tightness, test it and examine it straight away. Where gas is supplied, then purge it, retest any connection loosened for the purge once it is retightened, and if the pipework is not going into use yet, seal every outlet and test those seals.
• GSIUR regulation 26(9): after work on an appliance, such as installing, servicing or repairing it, examine the flue, the combustion air supply, the operating pressure or heat input (or the combustion performance where those cannot reasonably be checked) and safe functioning. Report any defect straight away to the responsible person and, if different, the owner.