Gas Purging Procedure: A Step-by-Step Guide for UK Gas Engineers (IGEM/UP/1B Edition 4)

Gas purges are frequently carried out by gas engineers when cutting into pipework, repairing, and recommissioning installations to safely displace the air or gas inside a system.

Whether you’re training as a gas engineer, requalifying, or brushing up your knowledge with the new IGEM/UP/1B Edition 4 changes, this guide walks through the full procedure, the new purge volume calculation, and a worked example you can apply to your next job.

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What is gas purging?

Gas purging is the controlled process of displacing one gas with another inside a pipework system. In practice, that means two things: pushing air out with fuel gas before commissioning a new install, or pushing fuel gas out (often with an inert gas like nitrogen) before working on a live system.

Either way, the goal is to leave the system free of any combustible mixture that could ignite when the next ignition source — a pilot light, a switch, a spark — is introduced.

Two standards govern the work in the UK. IGEM/UP/1B covers small installations with an installation volume up to 0.035 m³, which captures most domestic and small commercial jobs. IGEM/UP/1A applies to larger systems above that threshold. This guide focuses on UP/1B, where most engineers spend their time.

When you need to purge

You purge any time the gas in the pipework can’t be trusted. Common scenarios include:

• New gas installations being brought into service
• Meter exchanges or upgrades
• Pipework alterations, extensions, or replacements
• After a tightness test failure has been rectified
• Recommissioning after the system has been isolated
• Any time pipework has been opened to atmosphere

In short, if you’ve broken the seal, you purge.

What to do before you purge

A purge is the second half of a job. Several checks need to be in place before you open a single valve.

Start with a successful tightness test. If you want a refresher on the procedure or the permissible pressure drop limits under Edition 4, the gas tightness test guide covers it in full. A let-by test on the meter inlet valve should already be done before tightness testing — purge work assumes both have passed cleanly.

You’ll also need to:

• Inform the customer — they need to know they may smell gas, and where to evacuate to if asked
• Turn all appliances off — including pilot lights, where fitted
• Leave electrical switches untouched — flicking a light during a purge is exactly how flashbacks happen
• Ensure no naked flames or smoking — including yours
• Open ventilation — windows, doors, and any vents
• Select the purge point — usually the appliance furthest from the meter
• Have your gas detector ready — you want to know if you’ve created a problem, not guess

Calculating the purge volume

Previously, an E6, U6 or G4 meter feeding pipework up to 28 mm gave you a fixed purge volume of 0.01 m³. However, from 1 October 2026, every small installation under UP/1B uses the same formula:

The 1.5× multiplier exists because one full installation volume of fuel gas can’t guarantee complete air displacement on its own. The extra half gives you the margin for safety and dilution effects.

Working out the Installation Volume

IV is the sum of the internal volumes within the system. We’ve written a full guide about calculating the installation volume of your system in our IGEM/UP/1B Edition 4 guide, but the easiest way is to simply use our Installation Volume Calculator.

Enter the pipe diameter and length per section, add the meter, and you’ll get the installation and purge volume of the system. Updated for IGEM/UP/1B Edition 4, it removes the manual maths on domestic jobs and gives you a result you can record.

Worked example

Take a typical setup: a U6 meter feeding a combi boiler and a gas hob. The boiler runs on 22 mm from the meter, and a 15 mm spur tees off to the hob. Total measured pipework:

• 12 metres of 22 mm copper
• 4 metres of 15 mm copper

The IV calculation:

• 22 mm: 12 × 0.000352 = 0.004224
• 15 mm: 4 × 0.000154 = 0.000616
• U6 meter = 0.008
• IV = 0.004224 + 0.000616 + 0.008 = 0.01284 m³

Then the PV:

• PV = 1.5 × 0.01284 = 0.01926 m³

You watch the meter index pass that volume during the purge.

Gas purging procedure: step by step

With the prerequisites done and the PV calculated, the procedure itself is straightforward. The steps haven’t changed under Edition 4 — only the maths feeding into them.

Step 1: Prepare the system

Confirm all appliances are off. Confirm the meter inlet valve is closed. Have your leak detection fluid (LDF), gas detector, and purge point access ready. Clear ignition sources from the area.

Step 2: Open the purge point

The purge point is typically a burner control on the appliance furthest from the meter. Loosen the appropriate fitting if the appliance doesn’t allow a clean purge through its burner. Confirm ventilation and that no ignition source is nearby before opening anything.

Step 3: Open the gas supply slowly

Open the meter inlet valve smoothly — never snap it open. Read the meter index immediately, then watch it as the calculated PV passes through.

A gas detector running at the purge point gives you real-time confirmation of when fuel gas reaches the far end.

Step 4: Ignite, or vent safely

For small systems with an IV up to 0.02 m³, the gas is normally ignited at a burner once enough volume has passed and a steady flame is achieved. For systems with an IV greater than 0.02 m³ but still within UP/1B scope, the gas must be continuously ignited at the vent point during the purge — which is where a purge stack and a competent second pair of hands earn their keep.

Step 5: After the purge

Once the calculated PV has passed, close the purge point and check for a steady flame on the appliance you ignited. Apply LDF to every disturbed joint and watch for bubbles.

Recommission appliances per the manufacturer’s instructions. Record the IV, PV, test pressures, and observations on the appropriate certificate — start a free trial to create your certificate on the GES gas certificate app.

Common mistakes to avoid

The procedure is unforgiving of small errors. The ones that catch engineers out — and that get flagged in Gas Safe inspections — tend to be the same handful:

• Skipping or rushing the tightness test — purging an unsound system is dangerous, and any subsequent leak gets blamed on the work just done
• Miscounting branches in the IV calc — easy to forget the spur to the hob or the dropper to a fire
• Operating electrical switches during the purge — including a thermostat or the doorbell going
• Ignoring the >0.02 m³ vent-point ignition rule — handling larger small-installation purges as if they were domestic
• Not rechecking disturbed joints with LDF — the purge confirms gas is in the system, not that it’s contained
• Failing to record IV and PV — without those numbers on the certificate, you can’t demonstrate compliance later. Start a free trial to copy results straight from our Purging & Installation Volume Calculator into a digital certificate.

Frequently asked questions

What is the gas purging procedure?

Gas purging is the controlled process of displacing air or fuel gas from a pipework system using fuel gas or an inert gas, so the system can be brought safely into service or worked on. It follows a successful tightness test and uses a calculated purge volume to confirm full displacement.

What is the purge volume formula?

Under IGEM/UP/1B Edition 4, the formula is PV = 1.5 × IV, where IV is the Installation Volume (the internal volume of pipework from meter outlet to furthest appliance) and PV is the Purge Volume. This single formula now applies across all installations in UP/1B scope.

How do you calculate Installation Volume?

Add the internal volume of every section of pipework between the meter outlet and the most distant appliance, including all branches. Use the standard volume-per-metre figures from BS 6891 for copper tube — 0.145 L/m for 15 mm, 0.32 L/m for 22 mm, 0.539 L/m for 28 mm.

Do you need to purge after a tightness test?

Yes — if the test was carried out on a system containing air, or if the system was opened during fault-finding, the air has to be purged before the gas is used. Tightness testing confirms the system is sound; purging confirms the system is full of fuel gas, not air.

Does this procedure apply to LPG installations?

IGEM/UP/1B covers small Liquefied Petroleum Gas/Air, Natural Gas and LPG installations. The PV = 1.5 × IV calculation applies across these fuel types, with the LPG-specific safety considerations covered in the standard.

What’s the difference between UP/1A and UP/1B?

UP/1B covers small installations up to 0.035 m³ Installation Volume — most domestic and small commercial work. UP/1A covers larger installations above that threshold. The PV = 1.5 × IV principle applies in both, but UP/1A includes additional procedural requirements that come with larger purge volumes.