What is a Gas Supply Pipe for Boilers in the UK

The gas supply pipe feeding your boiler is just as important as the boiler itself. If it’s undersized, outdated, or poorly routed, even the best boiler on the market won’t perform properly — and in some cases, the installation won’t meet UK gas safety regulations at all.

We see this regularly at Boilers2Go: homeowners upgrading to a modern, high-efficiency boiler, only to find the existing gas pipe quietly holding the whole system back.

This guide explains what the gas supply pipe actually does, how to spot when it needs upgrading, what it typically costs, and how a Gas Safe engineer carries out the work properly during a boiler installation.

What Does the Gas Supply Pipe Do?

The gas supply pipe carries gas from your meter to your boiler — and often to other appliances such as a hob or gas fire.

For a boiler to run safely and efficiently, that pipe needs:

• The correct diameter

• A sensible route (not excessively long or full of tight bends)

• Minimal pressure loss between the meter and the appliance

In practical terms, most modern boilers need the gas pressure drop from meter to boiler to stay within around 1 mbar.

Older homes often have 15 mm copper pipework from the meter. That was usually fine for older 15–18 kW boilers, but modern combi boilers — especially 24–35 kW models — typically need 22 mm pipework for at least part of the run to deliver enough gas under load.

Signs Your Gas Supply Pipe Needs Upgrading

You won’t always see the pipe, but you’ll often notice the symptoms.

You’re Installing a Modern High-Output Boiler

Most manufacturers now expect a minimum 22 mm gas supply for modern combi boilers, particularly above 30 kW. If your existing pipe is 15 mm for any significant distance, an upgrade is very likely.

Boiler Faults or Poor Performance

Common warning signs include:

• Flame failure or low gas pressure fault codes

• Boiler locking out during heavy hot water use

• Weak hot water or radiators that never reach full temperature

These issues often come down to gas starvation, not the boiler itself.

Long Pipe Runs or Multiple Appliances

If the boiler is far from the meter, or the same pipe feeds a boiler, hob, and gas fire, the combined demand can exceed what the pipe can safely supply.

Old or Non-Standard Pipework

Red flags include:

• Long runs of 15 mm pipe from the meter

• Old steel or lead pipe sections

• DIY-looking joints, unsupported pipework, or awkward routing

Any of these should prompt a proper assessment by a Gas Safe registered engineer.

Cost of a Gas Pipe Upgrade for a Boiler (UK)

Costs depend heavily on access and routing, but typical UK prices for 2025/26 look like this:

Simple upgrade (short run, good access):
£200–£400 when done alongside a boiler install

Moderate upgrade (several rooms, some floor lifting):
£400–£700

Complex upgrade (long runs, concrete floors, difficult access):
£700–£1,000+

Prices increase with:

• Distance from meter to boiler

• Concrete or tiled floors instead of floorboards

• External or loft routing

• Multiple gas appliances sharing the same line

At Boilers2Go, we always assess gas pipework at survey stage and include any required upgrade in the quote upfront — so there are no surprises on installation day.

How a Gas Safe Engineer Upgrades a Gas Supply Pipe

This work must be carried out by a Gas Safe registered engineer. A proper upgrade follows a clear process.

1. Survey and Calculation

The engineer inspects the existing pipework, measures total length, counts fittings, checks the boiler’s kW rating, and factors in other gas appliances. Pipe sizing is calculated using recognised gas sizing tables or software — not guesswork.

2. Route Planning

Routing is agreed with the homeowner, whether under floors, along skirting, through loft spaces, or externally clipped. Access requirements and making good are discussed in advance.

3. Isolation and Safety

Gas is isolated at the meter and the system is made safe before work begins.

4. New Pipe Installation

New copper pipework — usually 22 mm, sometimes 28 mm — is run from the meter toward the boiler. Pipework is clipped correctly, sleeved through walls, and reduced in size near the boiler only where appropriate.

5. Pressure Testing and Purging

The new pipework is pressure-tested for leaks, purged safely, and checked to ensure working pressures meet both regulations and manufacturer requirements.

6. Boiler Commissioning

The boiler is gas-rated, commissioned, recorded in the Benchmark logbook, and handed over with documentation.

Common Gas Pipe Mistakes We See

When we’re called to fix poor installs, the same issues crop up repeatedly:

• Assuming 15 mm pipe is “fine because it was before”

• Guessing pipe size instead of calculating it

• Ignoring pressure drop under high demand

• Routing pipes for convenience rather than efficiency

• DIY additions by non-Gas Safe individuals

These mistakes can lead to unsafe installations, higher running costs, nuisance breakdowns, and invalidated warranties.

How Homeowners Can Sense-Check Their Gas Pipe

You can’t size a gas pipe yourself — but you can ask the right questions.

Visual Checks

Look at the pipe leaving the gas meter. If it’s 15 mm for any distance, that’s a concern for modern combis. Watch for poor joints or unsupported runs.

Questions to Ask Your Installer

• Will my gas pipe need upgrading for this boiler?

• What size pipe will be used from meter to boiler?

• Have you calculated pressure drop properly?

A competent engineer will talk confidently about pipe sizing, pressure drop, and compliance — not brush it off.

Regulatory Background

Gas work must comply with the Gas Safety (Installation and Use) Regulations 1998, alongside Building Regulations requirements and manufacturer instructions. You don’t need to quote legislation — just ensure your installer is Gas Safe and prepared to evidence compliance.

The Boilers2Go View: Don’t Cut Corners on the Gas Pipe

At Boilers2Go, we treat the gas supply pipe as part of the boiler system, not an optional extra. A high-end boiler on an undersized gas pipe is like a performance car running through a restricted fuel line — you’ll never get what you paid for.

When we quote, we:

• Assess existing gas pipework properly

• Include any required upgrades upfront

• Commission and test everything to current UK standards

If you’re unsure whether your gas pipe is up to scratch, a professional survey is the safest place to start.

Want a boiler quote that actually includes everything you need?
Boilers2Go can inspect your gas supply, size it correctly, and provide a fixed-price installation with no hidden pipework surprises.