Interpreting results and permissible drops

The two-minute test gives you a reading. What it means depends on what you're testing: a new installation must be, in practice, perfectly tight, while an existing one with appliances connected is allowed a small, defined drop. A smell of gas is never allowed.

Why new and existing are treated differently

A tightness test checks that any leak is below the level that could ever be a hazard, assuming the room is properly ventilated.

• On a new installation it shows the pipework has, in practice, no leakage, within the limits of a short test and the gauge's accuracy.
• On an existing installation the test may be against valves that are old and worn, so a defined maximum leakage is allowed. Appliance manufacturing standards also allow a very small leakage, which can show as a drop on the gauge, especially on an installation with a small volume.

What "no perceptible movement" means

The same rule applies to every let-by test.

Pass or fail

New installations, with or without new appliances, and existing installations with no appliances connected, or with them isolated: pass only if there's no perceptible fall and no smell of gas.

Existing installations with appliances connected and not isolated:

• No perceptible fall, no smell of gas: pass.
• A perceptible fall, within the permitted drop, and no smell of gas: not yet a pass. Isolate all the appliances from the meter and pipework and repeat the test. If that repeat shows no perceptible fall and there's no smell of gas, it passes: the drop has been traced to the appliances, not the meter or pipework.
• A perceptible fall beyond the permitted drop, or any smell of gas: fail.

When you see a drop within the allowance, ask the occupier whether they've noticed a smell of gas.

Isolating the appliances

Isolating usually means closing each appliance isolation valve. Where the isolation is a self-sealing connection on a flexible hose, disconnect the hose.

• If an appliance can't reasonably be isolated, it stays in the retest, and the retest must still show no perceptible movement.
• Where the connection between the isolation valve and the appliance is a long length of pipework, such as a built-in hob, think about leaving that appliance in the retest, or testing that section separately with leak detection fluid or a gas detector. Take account of any current technical bulletins or safety alerts.
• Where an appliance has no isolation valve, consider fitting one.

Once the drop has been shown not to come from the meter or pipework, reconnect the appliances and test every joint you disturbed with leak detection fluid.

How installation volume sets the permitted drop

In Edition 4, the permitted drop for an existing natural gas installation with appliances connected depends on one thing only: the installation volume (IV). You work out the IV (see 6.6 Purging) and read the maximum drop from Table 3 of IGEM/UP/1B. The table has four bands up to the 0.035 m³ limit. The smallest volumes have the largest allowance, and the allowance shrinks as the volume grows.

Example. An installation with a U6 meter, 2 m of 28 mm copper, 6 m of 22 mm copper and 3 m of 15 mm copper has an IV of about 0.0118 m³ (the calculation is in 6.6). That falls in the band above 0.010 m³ and up to 0.015 m³, where Table 3 allows a maximum drop of 2.5 mbar over the two-minute test.

The same principle covers an installation with no meter of its own, such as a flat supplied through its own ECV or AECV: its IV is just the pipework and fittings. Where several dwellings in one building each have their own ECV or AECV, the standard advises testing the whole installation downstream of the valve at the inlet to the primary meter, wherever possible.

ACS CCN1 Issue 16 §6a still describes the allowance in terms of meter type and pipe diameter as well as IV, which reflects Edition 3. Under Edition 4, work out the IV and use Table 3.

Extensions and alterations

Before you start work on an existing installation, test it and note any permitted drop. This first test is mainly to find problems that were already there. When the work is finished, test again: the meter and pipework must be gas-tight, and any drop that comes from the appliances must be within the allowance. Recalculate the IV if the work has changed it.

The new parts of an installation must always show no perceptible movement and no smell of gas. An allowance may still apply to the existing appliances connected to it.

Sources: IGEM/UP/1B Ed. 4 5.2purpose of the test IGEM/UP/1B Ed. 4 4.2perceptible IGEM/UP/1B Ed. 4 5.3.2.1step (g), Table 3 IGEM/UP/1B Ed. 4 5.4extensions IGEM/UP/1B Ed. 4 Appendix 7calculating IV CCN1 Issue 16 §6aassessed

▸Tutor notes

• Give learners a set of readings (a 0.1 mbar fall on an electronic gauge, a visible fall on a water gauge, a fall within the allowance with a smell of gas) and ask for pass, fail or retest, with the reason.
• Ask why a drop that passes as an allowance in Edition 3 now leads to a retest. What could the old approach have missed?
• Have learners work out the IV of an installation they know and find its band in Table 3 using their own copy of the standard.