22mm Gas Supply Pipe: What Homeowners Need to Know When Overseeing Gas Safe Work

A homeowner planning a kitchen extension finds out, three weeks before the boiler is due to be moved, that the existing 15mm gas pipe will not support the larger combi boiler the architect specified. The Gas Safe engineer quotes for a new 22mm run from the meter, threading through two cavity walls and a concrete floor. The work is not optional. The cost is several hundred pounds. None of this appeared on the architect's drawings or the builder's quote, because gas falls outside their remit. Knowing how 22mm gas pipework actually gets specified, routed, and tested is the difference between a planned cost on day one and a panic call to a second engineer when the kitchen is already half-built.

Why this page exists

Most of the guidance available to UK homeowners about gas pipework is written either for trade engineers (impenetrable to anyone outside the industry) or by boiler-replacement companies (selling a service, not explaining the standard). Neither helps a homeowner who is project-managing an extension, has a Gas Safe engineer on site, and wants to understand whether what they are seeing is correct.

This page covers what the engineer should be specifying and doing, the standards that govern the work, and the questions a homeowner can reasonably ask to verify the job has been done properly. It also flags the common procedural mistakes that cost real money: building over a service pipe, accepting an undersized run, or signing off without a Gas Safety Record in hand.

What 22mm gas supply pipe is

A 22mm gas supply pipe is a length of metal tube that carries natural gas (or LPG in some properties) at low pressure between the gas meter and the appliances inside the building. The "22mm" refers to the outer diameter. In a UK domestic property the pipe is almost always copper, occasionally older threaded steel, and increasingly CSST (corrugated stainless steel tubing, also called TracPipe or Flexigas, supplied as a flexible coil with manufacturer-matched fittings).

The standard that governs domestic gas pipework in the UK is BS 6891:2015+A1:2019 (Specification for installation and maintenance of low pressure gas installation pipework of up to 35mm in domestic premises). For larger or commercial installations, IGEM/UP/2 applies instead. Almost everything in a typical extension falls under BS 6891.

The copper pipe used is the same physical product as ordinary 22mm water pipe sold at every plumbing merchant: half-hard copper to BS EN 1057 R250 in 3-metre straight lengths. The pipe itself is identical to the 22mm copper pipe used for hot and cold water. What changes is the jointing rules, the sealants, the pipe identification, and the testing procedure. A 22mm copper pipe becomes a gas pipe by virtue of how it is installed and tested, not by virtue of being a different product.

Why 22mm is the standard size for modern boilers

Gas pipe sizing is decided by one rule: the pressure drop between the meter and any appliance inlet must not exceed 1 mbar under maximum gas flow, measured per BS 6891. Working pressure for natural gas in a UK domestic property is 20-21 mbar. Lose more than 1 mbar across the pipework and the appliance will not get the gas it needs to fire correctly.

For a modern combi boiler rated at 18 kW or above, a 15mm pipe almost never delivers the required flow over a typical extension-length run (5-15 metres) without exceeding that 1 mbar drop. 22mm copper carries the volume comfortably, which is why most modern combis are quoted with a 22mm minimum supply from the meter to the boiler. The 22mm rule is not a regulation in itself. It is a consequence of running the BS 6891 calculation for the typical combination of boiler size and run length found in a UK home.

For higher-output boilers (35 kW+, system boilers, or any boiler in a long run with multiple bends through a loft) the calculation can require 28mm or 35mm for part of the run. The engineer should be doing this calculation, not assuming. A common and dangerous mistake during reroutes is to drop from 22mm to 15mm on the new section without recalculating; the appliance then misfires under load and the engineer has to come back and redo the run.

The relevant question for a homeowner is not "is this pipe 22mm?" but "did you calculate the pipe sizing for this boiler and this run length to BS 6891?" If the engineer cannot describe the calculation in plain terms, that is a flag.

The 1m rigid copper rule at the boiler

Most major UK boiler manufacturers (Worcester Bosch, Vaillant, Ideal, Baxi) require the final connection to the boiler to be made in rigid copper, typically for the last metre of pipework before the appliance inlet. This is a manufacturer warranty requirement, written into the installation manual, not a clause in BS 6891. Engineers comply with it because failing to do so voids the warranty and can be cited if the manufacturer is asked to assess a fault.

The rule exists for two reasons. Rigid copper is dimensionally stable under heat and vibration, so the joint at the boiler does not work loose over years of expansion and contraction. And it gives the engineer a clean, square pipe to seat the boiler's compression or threaded fitting against, which a flexible product cannot match.

If your engineer is using CSST for the main run, expect to see a transition fitting back to a 1m length of rigid copper before the boiler connection. This is normal and correct. If the CSST runs all the way into the boiler with no rigid section, ask the engineer which manufacturer's installation manual permits that arrangement.

Why push-fit plastic is prohibited

Push-fit plastic pipe (Speedfit, Hep2O, JG Layflat) is an excellent product for hot and cold water and works well on the 22mm push-fit pipe page. It is prohibited inside the property for any gas pipework, without exception, under BS 6891.

The prohibition is absolute. There is no "as long as it's a metre or two from the boiler" exemption, despite what some forum threads claim. The reasoning is straightforward:

• Plastic pipe softens at temperatures well below those reached during a fire. A gas pipe that fails in a fire turns the fire into a feed-and-flame event, which is exactly what the standard is designed to prevent.
• Push-fit joints rely on an O-ring seal compressed by a grab-ring. Over time, gas can permeate plastic and rubber materials in ways that water cannot, increasing the risk of slow leaks that only become detectable through a tightness test.
• Push-fit relies on the user pushing the pipe to a defined depth and trusting visual confirmation. Compression and CSST mechanical fittings are torqued to a measurable specification with the joint visible after assembly.

The only plastic gas pipework permitted in UK domestic installations is PE (polyethylene) service pipe, used by the gas distribution network operator for the underground supply between the street main and the property's external meter box. PE pipe is not permitted inside the building.

If you see plastic pipe connected to gas inside the property, that is a defect. Stop the work, photograph it, and challenge the engineer.

CSST as a modern alternative

Corrugated stainless steel tubing (CSST) under the brand names TracPipe, Flexigas, and similar is increasingly common as an alternative to rigid copper for domestic gas runs. CSST conforms to BS 7838:1996 or BS EN 15266:2007 and is approved for use under BS 6891 (domestic) or IGEM/UP/2 (larger installations).

The advantages from the engineer's perspective are significant. A 30-metre coil of 22mm CSST weighs around 11 kilograms, against roughly 200 kilograms for the equivalent length of rigid copper. There are no soldered joints to make, far fewer mechanical joints to seal, and the pipe routes around obstacles without elbow fittings. A meter-to-boiler run that takes a day in copper can take half a day in CSST.

The disadvantages from a homeowner's perspective come down to one rule: all CSST mechanical fittings must remain accessible. Joints cannot be hidden in walls, under floors, or boxed in. CSST itself can pass through wall voids and cavities (with corrosion-resistant sleeving), but every fitting at every transition point must be reachable for inspection. This is the same accessibility rule that applies to compression fittings on water pipework, but it tends to surprise homeowners who picture CSST as "flexible pipe that goes anywhere".

If your engineer chooses CSST, ask where each fitting will be located and confirm that none will end up boxed in by the kitchen fitter or buried behind a fixed cupboard.

Click to enlarge

How the work gets done (what to expect on site)

Knowing the rough sequence of a gas pipework job lets a homeowner ask sensible questions at the right moments.

Sizing calculation. Before pipe is bought, the engineer calculates the run length, counts the bends and tees, and confirms the pipe diameter required to keep pressure drop under 1 mbar at maximum boiler load. This is a five-minute job for an experienced engineer. If the answer is "I always run 22mm so it'll be fine", the calculation has not been done.

Routing. The pipe route is planned to avoid concealed joints, cavity walls (pipework cannot be installed inside a cavity wall except where it is passing straight through from one side to the other), and electrical cables. Passages through walls are sleeved with a continuous corrosion-resistant sleeve.

Pipe identification. Gas pipework must be identified with yellow tape or markings at intervals along the run, particularly at any point where it could be confused with water pipework. This is required for both maintenance and emergency response. Ask to see the pipe identified before it gets boxed in.

Jointing. Soldered joints on copper gas pipework use a higher-temperature solder than ordinary water plumbing. Threaded joints (typically at meter outlets, isolation valves, and appliance inlets) are sealed with a gas-rated jointing compound like BOSS Gastite, conforming to BS EN 751-2, or with gas-rated PTFE tape to BS EN 751-3. Standard white PTFE tape and water-rated paste (for example, Boss White) are not approved for gas. They must not be used on gas threads, and gas-rated compound must not be cross-applied to potable water joints. Two separate products, separately stocked, separately labelled.

Pipe support. BS 6891 specifies maximum clip spacing for each pipe diameter (typically around 1.8m for 22mm horizontal runs, closer for vertical drops). Pipework must be rigidly secured at both ends. Loose pipe transmits vibration to the boiler and stresses joints over time.

Tightness testing. Before any new gas appliance is commissioned, the engineer performs a tightness test on the pipework. The procedure has two stages. First, a let-by test with the emergency control valve (ECV) closed and all appliance valves shut, watching the manometer for two minutes. Any pressure rise means the ECV is letting gas past, and the engineer must call the distribution network operator before continuing. Second, a working pressure tightness test at 20-21 mbar for 1-2 minutes; the pass criterion is a pressure drop of no more than 1 mbar over the test period. A drop greater than 1 mbar means there is a leak somewhere in the system that must be located and fixed.

Purging. After the tightness test passes, the pipework is purged of air to IGEM/UP/1B procedures before the appliance is fired.

Equipotential bonding. Gas pipework inside a property must be electrically bonded to earth at the consumer unit, as close as practicable to the meter outlet, using a 10mm² bonding cable connected to the gas pipe by an earth clamp. This is required by BS 7671 (the Wiring Regulations) Reg 411.3.1.2, not by BS 6891. Importantly, this is electrical work and falls under Part P of the Building Regulations. It is performed by a qualified electrician, not by the Gas Safe engineer. Most homeowners do not realise the bonding is a separate trade's responsibility, and a common defect is to discover after the fact that no electrician was instructed to verify or replace the bonding when the gas pipework was changed.

Notification and the certificates you should receive

This is the area homeowners most often misunderstand.

Pipework-only work (extending a run, rerouting a pipe, moving a meter outlet) does not require notification to the local authority. It must still be carried out by a Gas Safe registered engineer, but no Building Regulations Compliance Certificate is generated.

Heat-producing appliance installations (fitting a new boiler, replacing a hob, installing a gas fire) trigger a notification requirement in England and Wales. The Gas Safe registered business self-certifies the installation, which generates a Building Regulations Compliance Certificate sent to the homeowner via the Gas Safe Register, typically within 28 days. The local authority is notified within 30 days.

In Scotland and Northern Ireland, there is no equivalent self-certification scheme; building control is engaged via the building warrant process where applicable.

After any gas work, the engineer should also issue a Gas Safety Record (sometimes loosely called a "Gas Safety Certificate") documenting the appliances, the tightness test result, and the engineer's licence number. This is a separate document from the Building Regulations Compliance Certificate. The Gas Safety Record costs around £150–£300 if a second engineer is brought in to inspect work the original engineer refused to register, but is typically a smaller fee (£40 – £150) when produced by the engineer who did the work as part of the job.

Cost expectations

Gas pipework cost has three components: the pipe itself, the engineer's labour and overheads, and any post-installation certificate fees. Rough current ranges (April 2026):

• 22mm copper pipe: around £10 – £10 per metre (Wednesbury copper at Screwfix, Toolstation, Wickes; same product as ordinary 22mm water pipe).
• 22mm CSST: around £10 – £11 per metre (TracPipe and Flexigas coil pricing; per-metre cost approaches copper parity).
• Installed cost (materials plus labour): typically £25 – £45 per metre, with the upper end reflecting routes through concrete floors, cavity walls, or behind finished cabinetry.
• Short section work (under 3m, e.g. extending the supply for a relocated hob): £150 – £700 depending on access.
• Full-house pipework replacement: £2,000 – £6,000.
• Gas Safety Record fee: £40 – £150 when produced by the engineer who did the work.

The labour rate for a Gas Safe registered engineer in 2026 is typically £50 – £100 per hour. A boiler-only connection (existing pipework adequate, just connecting the appliance) is usually £300 – £800.

Two cost traps to watch. First, "the existing 15mm will be fine" quotes from less experienced engineers; if the boiler turns out to be misfiring after install, you pay for the upgrade twice. Second, hidden access difficulty: a quote based on a walk-around tour of the existing layout often does not reflect what is needed once the kitchen is stripped out and the actual route becomes clear. Build a contingency on the gas line equivalent to roughly 30% of the headline quote.

Building over an existing gas pipe (the extension trap)

A specific problem appears repeatedly in homeowner forums during extension projects: the existing gas service pipe (the underground connection from the street main to the meter) runs under the footprint of the proposed extension. You cannot build over a gas service pipe. The gas distribution network operator (Cadent in the south, SGN in Scotland, Wales & West, Northern Gas Networks, depending on your region) will require the pipe to be diverted before construction begins.

If the architect's drawings do not call out the gas service pipe location, the homeowner needs to. Contact the network operator early in the planning phase. Pipe diversion typically takes 6-12 weeks from request to completion and costs several hundred to several thousand pounds depending on the route. Discovering the conflict mid-build will cost weeks of programme delay and potentially require ripping out finished work.

The same caution applies to the meter location. If the kitchen extension demolishes the wall the meter is mounted on, the meter must be relocated before that wall comes down. Meter relocation lead times are similar (8-12 weeks). Coordinate this with the network operator at the earliest planning stage, not the week the builder starts.

Questions to ask your Gas Safe engineer

A homeowner overseeing gas work cannot do the work, but can ask the questions that confirm it is being done properly. The following list is reasonable to put to any engineer quoting on an extension project. An engineer who refuses to engage with these questions is not the right engineer.

• "Can I see your Gas Safe ID card and check the licence number?" Every Gas Safe registered engineer carries a card with a unique 7-digit number and a hologram. The reverse lists qualifications. For boiler work, the card must show "central heating boiler" or equivalent. The number can be verified at gassaferegister.co.uk or by texting "Gas" plus the number to 85080.
• "Have you sized the pipe to BS 6891 for this boiler and run length?" The answer should describe the calculation: appliance flow rate, run length, number of fittings, pressure drop result.
• "Where will the joints be located, and will all of them remain accessible?" No fitting (CSST or compression) can be in a wall void, under a floor, or boxed in.
• "Are you using rigid copper for the final connection at the boiler?" The 1m rigid copper rule is written into most boiler manufacturer manuals.
• "Are you using gas-rated jointing compound or gas-rated PTFE?" The product should conform to BS EN 751-2 (compound) or BS EN 751-3 (tape). Standard white plumber's PTFE is not gas-rated.
• "What pressure did the system test at, and what was the pressure drop?" Pass criterion is no more than 1 mbar drop over 1-2 minutes at 20-21 mbar working pressure.
• "Will I receive a Gas Safety Record at the end of the job?" Always yes. Get it in writing.
• "If you're installing a new boiler, when will the Building Regulations Compliance Certificate arrive?" In England and Wales: within 28 days from Gas Safe Register.
• "Has the equipotential bonding been checked or replaced?" This is electrician's work. Confirm whether the engineer is leaving a note for an electrician or whether you need to book one separately.

Common mistakes (homeowner perspective)

Assuming push-fit can be used on gas. It cannot, ever, anywhere inside the property. Plastic push-fit on gas is a defect that fails any subsequent gas safety inspection.

Allowing joints to be boxed in. The kitchen fitter installs base units across the route of a gas pipe. The next gas safety check finds inaccessible joints. The units have to come out. The original error is on whoever signed off the gas job, but the cost lands on the homeowner.

Not knowing the 1m copper rule. The engineer fits a flexible CSST connection straight to the boiler, the manufacturer voids the warranty when the boiler develops a fault, and the homeowner pays for repairs that should have been free.

Cross-applying water-rated jointing compound to gas threads. Boss White (water-rated) and BOSS Gastite (gas-rated) look similar on the shelf and have similar names. They are not interchangeable. If you see one tin being used for both, stop the work.

Skipping the equipotential bonding. No electrician is booked. The bonding from the old pipework was disconnected when the meter was moved. The next certified inspection flags the missing bonding as a defect.

Building over an undisclosed service pipe. The architect did not mark the gas service pipe on the drawings. The foundation trench cuts within inches of it, or builds over it. Either the network operator forces a diversion mid-build (weeks of delay) or the violation is discovered at sale, at which point the seller pays for remediation.

Accepting "I always run 22mm" without a calculation. For a 35 kW system boiler in a long loft run, 22mm may not be enough. The boiler misfires; the engineer comes back and upgrades part of the run to 28mm; the homeowner pays for the visit twice.

Where you'll need this

Gas supply pipework is touched at three points in any extension or renovation involving heating or cooking work:

• First fix plumbing - the engineer drops or extends gas runs to the new boiler position and to any gas hob location while walls are open and accessible.
• Second fix plumbing - final connections to the boiler and hob, tightness testing, commissioning, and certification.
• Utility meter relocation - if the extension affects the meter position or the underground service pipe route, the gas distribution network operator must be involved 8-12 weeks ahead of construction.

These tasks appear in any extension, conversion, or major renovation that moves gas appliances or alters the gas supply. The links above point to a kitchen extension project tree because that is where most homeowners first encounter this work, but the principles apply across all UK domestic gas projects.