Domestic Gas Pipework: Approved Materials, Sizing, Tightness Testing, and UK Regulations

Domestic Gas Pipework: Approved Materials, Sizing, Tightness Testing, and UK Regulations

Every Gas Safe registered engineer works with domestic gas pipework daily — yet the regulations, approved materials, and testing procedures are frequently misapplied, especially on older properties with non-compliant existing installations. This guide covers the complete picture for domestic gas pipework in the UK: from approved materials and pipe sizing through to isolation valves, tightness testing, and commissioning requirements under BS 6891 and the Gas Safety (Installation and Use) Regulations 1998 (GSIUR).

Note: Only Gas Safe registered engineers may install, commission, or work on gas pipework in the UK. This article is written for those engineers as a technical reference.

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Regulatory Framework

Domestic gas pipework in the UK is governed by three main documents:

• Gas Safety (Installation and Use) Regulations 1998 (GSIUR) — the primary statutory instrument. Reg 5 covers installation and maintenance standards; Reg 26 covers pipework purging and testing.
• BS 6891:2015+A2:2021 — Installation and maintenance of low pressure gas installation pipework of up to 35mm (R1¼) on premises being supplied with gas up to 75 mbar. This is the key technical standard for domestic natural gas installations.
• IGE/UP/2 — Installation Pipework on Industrial and Commercial Premises (referenced where domestic installations border on commercial characteristics).

All gas pipework must be "gastight and adequately supported" under GSIUR Reg 5(1). BS 6891 provides the practical interpretation of what that means — covering materials, jointing, testing, and documentation.

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Approved Pipework Materials

Copper (Most Common for Domestic)

Half-hard drawn copper tube to BS EN 1057 (Table X or Table Y) is the standard material for domestic gas pipework in the UK. It is approved under BS 6891 for internal and — in certain configurations — buried or external runs.

• Use Table X (standard wall) for most domestic applications
• Use Table Y (heavy wall) for buried runs where corrosion risk is elevated
• Copper must not be buried directly in aggressive soils without suitable sleeving or protection
• Joints: end-feed capillary (lead-free solder, BS EN 29453), compression (BS EN 1254-2), or press-fit (approved fittings only)

Fittings must be approved to BS EN 1254. End-feed fittings used on gas must use lead-free solder only — silver solder is not required for domestic gas at low pressure, but lead-free flux-cored solder or non-fluxed solder with separate flux is mandatory.

Corrugated Stainless Steel Tube (CSST)

CSST (e.g. Gastite, TracPipe) is increasingly used in domestic new-build and refurbishment where flexibility and speed of installation are priorities. It must comply with BS 7838 and be from an approved system with its own fittings — do not mix CSST systems or use with non-approved fittings.

CSST requires special earthing and bonding provisions in the UK — lightning strike can cause arcing through the flexible corrugated wall. Always follow the manufacturer's bonding requirements and relevant IET Wiring Regulations (BS 7671) Chapter 54 guidance on equipotential bonding for gas pipework.

MDPE Yellow Pipe (External/Underground)

Medium density polyethylene (MDPE) yellow gas pipe complying with BS EN 1555 is approved for underground external runs from the meter to the building entry point. It is the standard material used by gas network operators and is suitable for any buried domestic gas supply run.

• Yellow MDPE is for external/buried use only — it must not be used inside the building
• Transition from MDPE to copper is made at the gas entry point using an approved meter installation assembly
• MDPE must be at least 375mm below finished ground level (450mm under roads and driveways) per IGEM/G/4

Press-Fit on Gas

Approved press-fit systems (e.g. Conex Bänninger B-Press Gas, Viega Profipress G, Pegler XPress Gas) may be used on domestic gas pipework. Only systems bearing the Gas mark (tested to EN 1775) and approved by the manufacturer for gas use are compliant. Not all press-fit systems are approved for gas — verify the product datasheet before use.

Not approved for domestic gas pipework: standard push-fit plastic fittings (e.g. JG Speedfit, Hep2O), plastic pipe, or lead pipework.

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Gas Pipework Sizing

Correct pipe sizing ensures adequate gas flow to all appliances at sufficient pressure. For domestic natural gas installations under BS 6891, the design pressure drop from meter outlet to the furthest appliance must not exceed 1 mbar under full simultaneous demand.

Sizing Method

1. Determine appliance loads — list all gas appliances and their maximum gas consumption in m³/h (from the appliance data plate, or convert from kW input: m³/h = kW ÷ 10.76 for natural gas at calorific value ~10.76 kWh/m³)
2. Apply diversity — BS 6891 allows diversity factors for large installations, but for simple domestic layouts it is common to size for simultaneous use
3. Identify the index circuit — the longest pipe run (by equivalent length) to the appliance with the highest demand
4. Calculate pressure drop — use the BS 6891 Appendix A tables or approved software (e.g. GasSafe, GasCalc). The allowed pressure drop on the index circuit must not exceed 1 mbar total.
5. Select pipe bore — choose the minimum internal diameter that keeps pressure drop within limits at the calculated flow rate

Practical Rules of Thumb

For typical domestic installations with a combi boiler (24–28 kW) as the main appliance:

• 22mm copper is adequate for runs up to around 20–25m supplying a single boiler
• 28mm copper is required for longer index runs, multiple appliances, or high-demand appliances (cookers, fires, boilers >35 kW input)
• 15mm copper is only suitable for short branch runs (up to ~6–8m) supplying individual appliances of modest demand (gas hob under 15 kW, gas fire under 7 kW)

Always size to the calculation — rules of thumb fail on older properties with undersized existing supply pipes or multi-appliance installations.

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Gas Isolation Valves

BS 6891 requires an isolation valve to be fitted:

• At the meter outlet (emergency control valve — usually already installed)
• As close as practicable to each appliance (appliance isolation valve)
• Where a section of pipework cannot be readily isolated from the emergency control valve for maintenance purposes

Approved Valve Types

Gas isolation valves must be to BS EN 331 (manually operated ball valves for gas). These are quarter-turn ball valves with a lever handle and are rated for natural gas and LPG service.

Key points:

• Only use valves specifically rated for gas service — standard plumbing ball valves are not approved for gas
• The valve should have a lever that is in line with the pipe when open and at 90° to the pipe when closed — providing clear open/closed visual indication
• Gas service valves are typically available in 15mm, 22mm, and 28mm compression or BSP thread ends
• Do not use gate valves or standard check valves for gas isolation

A gas cock (also called a gas tap or gas service cock) with a square or slotted head requiring a tool to operate may be used as an alternative in specific situations, but lever ball valves are now standard for new installations.

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Pipework Installation Requirements

Jointing and Sealing

Under BS 6891, permissible jointing methods for copper gas pipework include:

• Capillary solder fittings — end-feed or integral solder ring, using lead-free solder only
• Compression fittings — BS EN 1254-2; non-manipulative (Type A) compression fittings are suitable for gas
• Press-fit fittings — approved gas-rated press-fit systems only
• Threaded joints — PTFE tape or approved jointing compound (gas-rated). Do not use standard PTFE tape on LPG — use approved hemp and PTFE compound or LPG-rated tape

Push-fit plastic fittings, solvent-weld joints, and rubber push-fit connections are not permitted for gas pipework under BS 6891.

Pipe Supports and Fixing

BS 6891 specifies maximum support spacings for copper pipe in gas installations. As a guide (horizontal runs):

• 15mm copper: maximum 1.2m between clips
• 22mm copper: maximum 1.8m between clips
• 28mm copper: maximum 1.8m between clips

Vertical runs allow slightly greater spacing. Support at each side of fittings and changes of direction. Use pipe clips compatible with copper — avoid bimetal contact (e.g. bare steel clips on copper pipe) in corrosive environments.

Pipework Routing

Gas pipework must not pass through or be installed in:

• Cavity walls (unless sleeved in a suitable gas-tight sleeve vented to outside)
• Voids without adequate ventilation
• Beneath solid floors without sleeving (the pipe must be in a continuous sleeve that is sealed at the floor penetration and vented to atmosphere outside the building, or use buried yellow MDPE under a concrete slab with appropriate depth)
• Meter boxes (only the connection to the meter is permitted in the meter box)
• Lift shafts, ducts containing electrical cables (unless separated), or areas where gas could accumulate

Where pipework passes through walls or floors, it must be sleeved and the sleeve sealed to prevent gas tracking into the building fabric.

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Tightness Testing

Tightness testing is mandatory before any gas installation or modification is commissioned, and after any work on gas pipework. The test procedures are specified in BS 6891 Annex B.

Test Equipment

• A calibrated U-gauge manometer or approved electronic gas pressure gauge capable of resolving to 0.1 mbar
• A hand pump or pressurisation adapter

Let-By Test (Tightness of Emergency Control Valve)

Before any tightness test, confirm the emergency control valve (ECV) is not letting by (passing gas when closed):

1. Close the ECV
2. Connect the gauge to the outlet of the ECV
3. If the gauge shows rising pressure when the ECV is closed, the ECV is letting by — stop work and inform the gas transporter

Tightness Test Procedure (BS 6891 Annex B)

1. Pressurise the installation to approximately 20 mbar (or to working pressure if a regulator is fitted and the test is downstream of it)
2. Isolate the test pressure source and allow 1 minute for the pressure to stabilise (thermal equilisation)
3. Record the pressure reading
4. After a further 2 minutes, record the pressure again
5. Pass criterion: No pressure drop — a stable reading within gauge accuracy indicates a gas-tight installation
6. If there is a pressure drop, locate the leak using leak detection fluid or an approved electronic gas detector, repair, and retest

For a new installation or extended pipework, a preliminary test at higher pressure (e.g. 50 mbar) before the final test helps identify larger leaks quickly. Always conclude with the formal 1+2 minute test at working pressure.

Strength Test (New or Extended Installations)

BS 6891 requires a strength test for new or extended installations before connection to the gas supply or gas meter. This is typically performed at 1.5× working pressure (up to 100 mbar for natural gas installations under 75 mbar working pressure) using air or nitrogen — never oxygen. The test duration is typically 1 minute with no pressure drop permitted.

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Purging

After a satisfactory tightness test, gas pipework must be purged of air before appliances can be lit. Purging procedures are specified in BS 6891 and GSIUR Reg 26.

Key requirements:

• Purge in a well-ventilated area — gas will be released to atmosphere during purging
• No ignition sources during purging
• Purge at the furthest point from the ECV, or at the appliance connection point, for a minimum volume of gas calculated to displace the pipe volume by a factor of 5× (or as specified in BS 6891 Appendix C tables)
• Test for gas presence at the purge point using an approved gas detector before relighting any appliances
• Record purging in the installation documentation

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Bonding and Earthing

Gas pipework entering a property must be bonded to the main earthing terminal of the electrical installation under BS 7671 Regulation 544.1 (main equipotential bonding). This applies to all gas installation pipework entering the property.

• Bond as close as practicable to the point of entry of the gas service pipe, before any branch connection
• Bonding conductor: minimum 10mm² copper for main equipotential bonding (or half the cross-sectional area of the installation earth, subject to the 10mm² minimum — in practice 10mm² is standard for domestic TN-C-S/PME supplies)
• Connection must be via an approved bonding clamp (not solder) to the copper pipework
• CSST systems have additional bonding requirements — follow the manufacturer's guidance

Supplementary bonding within bathrooms (if required under Reg 415.2) follows different rules — see our guide to Earth Bonding and Equipotential Bonding.

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Documentation and Certification

GSIUR Reg 5(4) requires that gas work is carried out to a standard not lower than that specified in BS 6891. While there is no statutory requirement for a specific form of gas installation certificate for domestic pipework (unlike Part P for electrics), best practice and Gas Safe guidance requires:

• A tightness test record including date, engineer name, Gas Safe registration number, test pressures, and result
• A commissioning record for each appliance
• Landlord Gas Safety Record (CP12) where applicable — required annually for rented properties
• Any building regulations notification required under Local Authority approval where work triggers Building Regulations Part J

Many engineers use the Gas Safe checklist or a firm-specific job sheet. For new boilers, most manufacturers' warranties require a commissioning record to be completed and returned.

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Key Products for Gas Pipework Installations

APM Electricals stocks a full range of approved products for domestic gas pipework work:

• Copper tube — Table X half-hard copper in 15mm, 22mm, and 28mm, sold in 3m and 6m lengths
• End-feed and solder-ring fittings — Yorkshire and generic BS EN 1254 fittings; ensure lead-free solder is used for gas
• Compression fittings — standard Type A compression fittings approved for gas service in copper, including elbows, tees, and straight couplings
• Gas ball valves (BS EN 331) — lever-operated quarter-turn gas isolation valves in 15mm, 22mm, and 28mm compression ends, suitable for natural gas and LPG
• Gas pressure gauges — U-gauge and digital manometers for tightness testing
• PTFE tape and gas jointing compound — gas-rated thread sealants for threaded joints
• Pipe clips — plastic-coated and copper-compatible clips in standard spacing sizes

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Summary

Domestic gas pipework work requires rigorous adherence to BS 6891 and GSIUR 1998. The key principles are:

1. Use approved materials only — copper (BS EN 1057), CSST (BS 7838), yellow MDPE (BS EN 1555) for buried, and approved press-fit systems. No plastic push-fit.
2. Size correctly — calculate pressure drop on the index circuit; keep within 1 mbar total drop under full demand.
3. Install gas-rated isolation valves — BS EN 331 lever ball valves at each appliance and where required for system isolation.
4. Test before commissioning — let-by check, strength test (new installations), then 1+2 minute tightness test at working pressure. No pressure drop = pass.
5. Purge safely — in a ventilated area, with no ignition sources, displacing at least 5× the pipe volume.
6. Bond the gas installation — minimum 10mm² main equipotential bond to the MET, as close to entry as practicable.
7. Document everything — tightness test record, commissioning sheet, CP12 where required.

For Gas Safe engineers managing busy installation schedules, having approved materials on hand matters. APM Electricals holds stock of gas ball valves, copper fittings, and pipework across our site for next-day delivery to trade customers.