How to read let-by, stabilisation and the timed test, and when a pressure drop is a pass.
A gauge reports pressure, not a complete safety verdict. Interpret it with a valid test setup, the correct test boundary and the applicable procedure. The following distinctions use IGEM/UP/1B Edition 4, effective from 1 October 2026.
Start with a valid measurement
Passing isolation can add gas and hide a loss elsewhere. Temperature change can raise or lower the reading without identifying a failed component. Establish the specified isolation and stable conditions before timing. If conditions change, investigate; do not subtract an invented temperature allowance or accept a rise as proof of tightness.
Pipework has no leakage allowance
New and existing installation pipework must be proved tight. This includes the fitted meter in the normal test from the ECV; a test from an AECV may have a different boundary. The meter's presence, age or mechanism does not justify leakage.
With existing appliances connected, investigate an observed loss by isolating the appliances where reasonably practicable and retesting the pipework and meter. Any perceptible loss on that retest is unacceptable. Correct or make safe the affected section and obtain a satisfactory retest.
When appliances remain connected
If isolation is not reasonably practicable, the connected test must show no perceptible movement. Do not use an appliance allowance to excuse a loss that could still be in the pipework. Pipework and new appliances also have no permissible loss.
Where the pipework has been proved tight and eligible existing appliances are connected, assess their loss against the applicable fuel/volume limits. No smell of gas is also required; a smell or loss beyond the applicable limit requires action.
Volume is not the meter badge
Installation volume is the internal gas-holding volume within the relevant boundary. The meter's maximum flow rating is not that volume. For an eligible appliance-loss comparison, a loss below the smallest permissible limit for the fuel does not require an IV calculation solely for that comparison. Where the relevant comparison needs IV, calculate it and use the published table. Check boundary cases in the standard rather than assuming a rule at equality.
A larger volume can show less pressure movement for the same lost quantity; that is why the applicable band matters. It does not mean larger pipework is allowed to leak.
Keep the test and return-to-use decisions separate
Use the prescribed test pressure, which must not be assumed identical to appliance operating pressure. A tightness result does not certify combustion or flue performance. Complete the required purging and recommissioning checks and record the conditions and result before handover.
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