When to test and purge

A tightness test proves that pipework holds gas. A purge clears the air out of it, so that what reaches the burners is gas and not a gas–air mixture. The law says when both must be done, and IGEM/UP/1B says how to do them on a domestic-sized installation.

What the law requires

Regulation 22 of the Gas Safety (Installation and Use) Regulations sets the duty.

• Test and examine. Straight after any work on installation pipework that might affect its gas tightness, the part you worked on must be tested to show it is gas-tight, and examined to show it was installed as the Regulations require.
• Then protect. Any protective coating goes on the joints only after that test. The Approved Code of Practice explains why: paint or wrapping on an untested joint can seal it for a while and give a false pass.
• Then purge. Where the premises has a gas supply, the pipework that gas can now flow through must be purged straight after the test, to clear out air (and any gas other than the one being supplied) safely.
• Then seal and check. Pipework that isn't going into use straight away must be sealed at every outlet with an appropriate fitting. A connection that was loosened to purge through must be retested once it is tightened, and every seal fitted after the purge must be tested.

The ACoP adds that purging isn't only for when gas is first supplied: it follows any work that breaks into the pipework. Where a supply and an installation are connected for the first time, the person who finally connects them is responsible for the testing and purging. The duty doesn't arise just because the gas was temporarily off, for example because the ECV was shut or an over- or under-pressure valve operated, or because the customer changed supplier.

When IGEM/UP/1B calls for a test

The standard applies a tightness test:

• on new installations, and on new extensions to existing pipework
• when an existing installation is altered, replaced or reused
• before and after any work on an existing installation that may affect its gas tightness
• where there is a known or suspected gas escape
• after a complete loss of supply pressure (upstream of the ECV) or of installation pressure
• for routine testing of existing installations
• immediately before any purge, except when components are being taken permanently out of service.

When it calls for a purge

A purge is needed for new installations and new extensions; when an existing installation is altered, replaced or reused; after a complete loss of installation pressure; and wherever air may have got into the installation.

The standard notes that it may not need to be applied during routine maintenance such as servicing.

Is the installation within scope?

IGEM/UP/1B covers small installations only. For natural gas, every one of these must apply to the section being tested:

• maximum operating pressure at the ECV outlet of no more than 2 bar
• pipe of no more than 35 mm nominal bore (DN32, R1¼)
• a meter with a badged capacity of no more than 16 m³/h
• an installation volume of no more than 0.035 m³
• a nominal operating pressure of 21 mbar at the meter outlet and throughout the section.

Natural gas here includes natural gas blended with up to 20% hydrogen.

Volumes above 0.035 m³ are rare in homes. As a guide, the standard gives 25 m of 35 mm copper with a U6 meter, or 10 m of 35 mm copper with a U16 meter, as still under the limit. If there's any doubt, work out the installation volume (see 6.6 Purging) before you start. Larger installations come under IGE/UP/1, and meter installations tested on their own may come under IGEM/UP/1C.

Who may do it

Anyone tightness testing or purging must be competent, and where GSIUR applies the business must be on the Gas Safe Register. Where the supply pressure is above 75 mbar, the competence needed is set at a higher level. Edition 4 also asks engineers to keep releases of gas to the minimum needed to complete the purge, because of methane's effect on the environment.

Sources: GSIUR 1998 reg. 22test, purge, seal L56 (2018) paras 214–221ACoP IGEM/UP/1B Ed. 4 1.1in force IGEM/UP/1B Ed. 4 1.3other standards IGEM/UP/1B Ed. 4 2.1–2.5scope, hydrogen IGEM/UP/1B Ed. 4 3.3–3.4competence, environment IGEM/UP/1B Ed. 4 5.2.4open ends CCN1 Issue 16 §6aassessed

▸Tutor notes

• Give learners a list of jobs (boiler swap, cooker reconnection, meter exchange, a service, a reported smell of gas) and ask which need a test, a purge, both or neither, and which clause of 2.4 or 2.5 applies.
• Ask why the coating rule exists, then read the ACoP's answer: an untested joint under paint can pass a test and leak later.
• CCN1 §6a also assesses testing pipework larger than 35 mm or over 0.035 m³ (knowledge item 8): use the scope list to show why that falls outside IGEM/UP/1B.