This Valentine's Day we’re asking - Does the size of your pipe really matter? 2025

It’s true, size does matter, especially when it comes to getting you hot this February.

In this blog we’ll be exploring new size regulations for boiler gas pipes, and telling you how if the job needs it, our engineers can lay some pipe, no problem at all.

Valentine’s Done Right

The sensory rush of hot water dancing down the back of your neck, soothing steam matching the temperature of your skin, the cool sting of the shower glass on the palms of your hands…

Not quite the same if you both have chattering teeth and the water is half frozen.

You may need a thicker pipe to finish the job.

What is a Gas Pipe?

A gas pipe or ‘boiler gas pipe’ is a branch line from the mains building line or ‘gas supply pipe’.

Boiler gas pipes split off from this mains connection and run to the gas powered appliances around the home, i.e. a boiler.

Gas pipes deliver gas from the meter to be used as fuel by the boiler to generate central heating and domestic hot water.

Boiler gas pipes are commonly constructed from copper or alternatively some types of pipework can be constructed of steel, brass or aluminium.

Why Does Size Matter?

Gas pipes can vary in size (inc. 15, 22, 28 or 35mm) often depending on the distance between the gas meter and the boiler, the boiler’s brand/model and the size of the boiler.

Petite Combis Need Big Pipes

Older properties are often fitted with 15 mm gas pipework, which was commonly sufficient for traditional boilers with lower or more gradual gas demand.

Modern combi boilers, while physically compact, typically require a higher volume of gas over a shorter period of time.

This is because they heat water instantly, rather than storing it in a cylinder.

Delivering hot water on demand places greater pressure on the gas supply, particularly during peak use.

System and regular boilers heat water more gradually and store it in a cylinder.

As a result, they usually place less immediate demand on the gas supply, meaning existing 15 mm pipework can sometimes remain suitable, depending on the installation.

For a combi boiler to operate efficiently and safely, the gas pipework must be correctly sized to deliver sufficient flow and pressure.

In many domestic installations, this assessment results in upgrading the gas supply to 22 mm, but the correct pipe size depends on factors such as boiler output, pipe run length, and overall system design.

Undersized pipework does not usually burst under pressure, but it can restrict gas flow, leading to poor boiler performance, ignition faults, or safety shutdowns.

This is why correct sizing is essential and always confirmed by a Gas Safe engineer during installation.

How iHeat’s Engineers Can Plug the Gap

Any changes to gas pipework must always be carried out by a qualified professional.

All iHeat engineers are Gas Safe registered and assess gas pipe sizing as part of every boiler installation or conversion.

During an installation, engineers calculate whether the existing gas pipework can deliver the required flow and pressure for the chosen boiler.

Where the existing 15 mm pipework is found to be insufficient, it is upgraded to a larger diameter, commonly 22 mm, to meet manufacturer specifications and Gas Safe requirements.

In some installations, particularly where pipe runs are short or boiler outputs are lower, existing pipework may already meet the required standards and not require upgrading.

Decisions are always based on measured pressure and flow, not assumptions.

iHeat’s approach is to upgrade gas pipework only when it is necessary for the boiler to operate safely and efficiently.

This ensures compliance with manufacturer guidance, maintains warranty validity, and avoids unnecessary work while prioritising safety and performance.

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