Judging a tightness test result: what the gauge is telling you

The gauge (manometer) shows the pressure held inside the pipework, not how much gas is flowing. A steady reading means pressure is being held; a falling one means it is being lost. Whether that is a leak depends on a valid test. These rules follow IGEM/UP/1B Edition 4, effective from 1 October 2026.

Make the reading valid

Complete the stages in order: let-by check, stabilisation, then the timed test. The tightness test procedure explains each stage in more detail.

• Prepare the gauge. Use a suitable manometer with sound hoses and connections. Follow its instructions for letting it adjust to the temperature and for zeroing it in its final position. Keep it steady and at a stable temperature during the test. This does not replace letting the gas in the installation stabilise.
• Stop gas use. A burner or pilot using gas causes a fall that looks like leakage. Turning appliance controls off is not the same as isolating appliances for a pipework-only retest.
• Let-by. Gas passing the closed emergency control valve can offset a loss downstream and make the gauge look steady. Closing the valve leaves gas held beyond it: unequal pressures either side of a regulator or shut-off device can still move the reading, so set up the test as the procedure says to allow for them. If the reading rises again and again once those effects and temperature drift are ruled out, the valve is probably passing. Confirm it and fix the cause before you rely on the test.
• Stabilisation. In the closed test section, warming raises the gas pressure and cooling lowers it, so temperature can hide a leak or look like one. Pressurising can warm the gas, and as it cools the pressure can fall with no leak. Allow the set stabilisation time before timing. If the sun is warming the pipe and the gauge drifts, wait until conditions are stable. Never subtract an earlier drift afterwards.
• An unexplained rise while pipework warms is not a diagnosis yet: check temperature, isolation and setup first.
• Test pressure. Use the pressure the procedure specifies. A higher pressure of your own choosing can damage the meter or appliance parts, and the test pressure is not just the appliance operating pressure.

Pipework has no leakage allowance

New and existing pipework must be proved tight: no perceptible movement over the test period. That means no movement beyond 0.25 mbar on a water gauge or typical electronic gauge, or 0.2 mbar on an electronic gauge reading to one decimal place.

• The normal test from the emergency control valve includes the meter. A meter's presence or age never creates an allowance.
• Example: on a stable zero-drop test with let-by ruled out and the gauge and connections checked, compare 20.0 mbar at the start with 18.4 mbar at the end. The 1.6 mbar loss fails this exercise; trace and rectify the escape.

Existing appliances connected

If a loss appears with existing appliances connected, isolate them where reasonably practicable and retest the pipework and meter. Any perceptible loss on that retest fails. If isolation is not reasonably practicable, the connected test must show no perceptible movement.

Only once the pipework is proved tight can you compare an eligible existing appliance's loss with the limit for its fuel gas and installation volume (IV), from the published table. Never pick the limit from the meter badge (U6, U16), the pipe length or a percentage of test pressure. IV is the volume of gas held inside the test section; a meter's m³/h rating is a flow, not a volume. The same leak shows a smaller pressure change in a larger volume, which is why the limit depends on volume.

A within-limit result passes only with no smell of gas. A smell always means investigate and make safe.

For natural gas the limit gets smaller as the volume grows, because every band is based on the same leak rate of about 19 ml a minute:

• IV up to 0.005 m³: 8 mbar
• IV over 0.005 and up to 0.010 m³: 4 mbar
• IV over 0.010 and up to 0.015 m³: 2.5 mbar
• IV over 0.015 and up to 0.035 m³: 1 mbar

LPG and LPG/air have their own tables. A loss of up to 1 mbar is inside every natural-gas band, so you only need to work out the IV for this comparison when the loss is more than 1 mbar.

With all three conditions met (pipework and meter proved tight, loss within its limit, no smell of gas) the test is passed. The appliance's small loss does not have to be eliminated, and the appliance can stay connected where a site-specific risk assessment supports it.

After the reading

• Trace a leak with approved leak-detection fluid or an electronic detector, never a naked flame.
• If the failure cannot be resolved, do not commission. Disconnect the gas supply and seal it with an appropriate fitting. Label it not to be used until commissioning is complete, and tell the responsible person.
• Any work that breaks into the installation, such as a meter change, needs a fresh tightness test.
• A pass proves gas-tightness only, not combustion or flue performance.
• After a pass, remove the gauge, reseal the test point, restore the gas and reopen any appliances you isolated, then check every disturbed joint with leak-detection fluid. Include the pipe between each isolated appliance and its isolation valve, which the pipework retest did not cover. There must be no smell of gas.