Gas pipework

Why sizing matters more than people think

Gas leaves the meter at a regulated pressure. Every metre of pipe and every fitting between the meter and an appliance causes a pressure drop, and the drop increases sharply as pipe diameter reduces. There is a maximum permissible drop between the meter and the appliance, and staying within it is a calculation based on the total gas demand, the pipe length, the diameter and the number of fittings.

When pipework is undersized, the appliance furthest from the meter does not receive its required pressure when everything is running. A boiler starved of gas will not reach its rated output, may lock out under load, and will burn less efficiently. Critically, none of that shows up as an obvious fault – the boiler works, just not properly, and the fault gets blamed on the appliance.

This gets missed most often when a new, higher-output boiler replaces an older one on existing pipework. The old appliance was within the capacity of that pipe; the new one is not. It is a straightforward check with a pressure gauge under load and it should be part of every boiler quote. See gas supply and pipe sizing.

What we install

• New appliance supplies – boilers, cookers, hobs, fires and heaters, sized for the appliance and for what else is on the run.
• Upgrading undersized existing runs, typically when a new boiler needs more than the old pipe can deliver.
• Extensions and alterations to serve a new kitchen, conversion or extension.
• Relocating a meter position in co-ordination with the gas transporter, and the pipework that follows from it.
• Replacing old or corroded pipework, particularly older steel that has been in place for decades.
• Emergency control valve and isolation work, so appliances can be isolated individually.
• Tightness testing on existing installations, whether as part of other work or as a check in its own right.

How gas pipe has to be run

Materials and jointing

Copper with appropriate fittings, or steel, installed to the relevant standards. Certain fittings and jointing methods permitted on water are not permitted on gas, and the distinction is not cosmetic.

Protection and routing

Pipe must be protected where it could be damaged, sleeved where it passes through walls, and supported at the correct intervals. It must not be run in unventilated voids where a leak could accumulate – which is why gas pipe in a sealed floor void or a boxed-in duct needs ventilation at both ends. Pipe in a cavity wall is generally not permitted other than to cross it directly by the shortest route.

Electrical bonding

The gas installation must be electrically bonded at the correct point near the meter, with conductors of the right size. This is a genuine safety requirement and it is very commonly found missing or undersized on older installations – and it is routinely picked up during a safety check.

Testing

Any new or altered gas installation must be tightness tested and purged before being brought into use, and the results recorded. There is no version of this that is optional or skippable.

Cookers, hobs and fires

Appliance connections have their own requirements. A freestanding cooker is normally connected with a bayonet fitting and an approved flexible hose, with a stability chain or bracket fitted – the bracket matters, because a cooker that tips when a door is loaded puts strain on the connection. A built-in hob is a rigid connection with an accessible isolation valve.

Gas fires bring ventilation and flue requirements of their own, and a flue that was adequate for an open fire is not necessarily adequate for a gas appliance. Any appliance in a room also affects, and is affected by, extract ventilation elsewhere – a powerful kitchen extractor can depressurise a room enough to affect a flued appliance, which is why ventilation is assessed as a whole rather than appliance by appliance.