The duty to test after gas work, the let-by, stabilisation and timed stages, and the purging, commissioning or make-safe steps that follow.
Work that might disturb a gas fitting’s tightness calls for a prompt test of the affected section, reaching at least the nearest valves upstream and downstream. Altered installation pipework must also be examined and tested before return to use. Make any known escape safe and correct the fault; the duty applies to existing pipework as well as new work.
What a tightness test proves
Start by identifying what the gauge is connected to: meter, pipework and any appliances included in the test. Isolate the supply, allow pressure and temperature to settle, then observe the timed reading. A fall during a valid test needs investigation, but one gauge reading cannot locate the loss; drift before stabilisation can occur without a leak. A pipework-only pass does not clear an appliance connection excluded from the test.
The absence of a gas smell proves nothing: a small escape can go unnoticed yet still build up over time. Purging and gas rating are separate procedures that answer separate questions.
IGEM/UP/1B Edition 4 takes effect on 1 October 2026, and Edition 3 is withdrawn on 30 September 2026. Under Edition 4, installation pipework must be proved tight.
The stages, in order
- 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.
- Stabilisation. Let temperature and pressure settle: gas still warming or cooling changes the pressure without any leak. Waiting the minimum time is not enough if the gauge is still drifting as the pipe warms; the stability conditions must be met too.
- 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 locate and repair any confirmed leakage before accepting the work. An allowance for eligible existing appliances never transfers to pipework or new appliances, and being unable to isolate existing appliances does not create 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 that cannot be reached, the low pressure has to be investigated; the test may be run at no less than 18 mbar, 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
- Purge new pipework that contains air, by the prescribed method and quantity: under Edition 4, calculate the installation volume (IV) and purge 1.5 × IV. Use a discharge arrangement permitted for the gas, volume and setting, and control ignition and accumulation risks. An open window alone does not make a discharge point suitable.
- Commission each appliance before leaving it in use: the flue clears the products, the air supply is adequate, the operating pressure or heat input is correct, and the appliance functions safely. Checking pressure or heat input confirms it burns at its design rate. A sound gas connection on its own is not enough.
When something fails
- A defect you cannot correct on the visit: isolate or cap the affected part so it cannot be used, and tell the responsible person what is wrong and what you have done. Switching it off at its own controls, or only labelling it, leaves it able to be turned back on.
- Spillage on a newly fitted open-flued appliance: stop commissioning and keep the appliance out of use. Find and resolve the cause; if that cannot be done, make it safe under the unsafe-situations procedure. A fan or lower gas rate must not be used to hide spillage.
- An escape you cannot stop: warn people, get clear and call 0800 111 999 from a safe place, avoiding ignition sources. 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.