Does a New Boiler Need a Bigger Gas Pipe?

Often, yes, especially when a combi replaces an older heat-only boiler. A combi heating a running shower draws much more gas at full rate than a boiler that only heated radiators, and a gas pipe that was adequate for the old boiler can be too small for the new one. Whether yours needs upgrading is decided by measuring the gas pressure at the boiler with it firing at full rate, not by the pipe's size alone.

Why a new boiler can need more gas than the old one

A boiler's gas demand follows its maximum input, and a combi's maximum is set by hot water, not heating. Heating a flowing stream of cold mains water instantly takes far more power than keeping radiators warm.

So a house that ran happily on a modest regular boiler for decades can have its gas pipe pushed hard by a combi of a typical size, even though the heating demand has not changed. The same applies, to a lesser degree, when a larger system boiler replaces a small one. How a combi boiler works explains why its output is rated higher than a heat-only boiler for the same house.

Is 15mm gas pipe enough for a combi boiler?

Sometimes, over a very short run with few bends. In many homes it is not, and 22mm from the meter is the common upgrade.

There is no single pipe size that is right for every combi. What matters is how much the pressure drops between the meter and the boiler when everything is running. The factors are:

• The boiler's maximum gas rate. Higher-output combis need more gas at peak.
• The length of the pipe run from the meter to the boiler. Longer runs lose more pressure.
• The number of elbows, tees and fittings, each of which adds resistance.
• Other appliances on the same pipe, such as a gas hob or gas fire, which can be running at the same time as the boiler.
• Diameter changes along the way. A run that starts at 22mm and drops to 15mm for the last stretch behaves like a smaller pipe.

How an engineer checks whether your pipe is big enough

A Gas Safe registered engineer measures the working pressure at the meter and at the boiler with the boiler, and ideally the other gas appliances, running at full rate. The gap between the two is the pressure drop.

Industry guidance for domestic installations allows only a small pressure drop, typically no more than 1 mbar, between the meter outlet and the appliance. If the measured drop is larger, the pipe is undersized for that boiler. On a survey, before the boiler is fitted, the engineer works this out from the pipe route, length and fittings; on the day, the measurement at commissioning confirms it and is recorded on the Benchmark checklist.

What happens if the gas pipe is too small

An underfed boiler cannot reach its full output and may not burn cleanly. You tend to notice it on hot water first.

Typical symptoms are hot water that runs warm rather than hot when the boiler is at full demand, temperature that wobbles when the hob is also on, and lockouts or ignition faults under load. It also matters for the warranty: manufacturers expect the gas supply to meet their stated inlet pressure, and a fault caused by an inadequate supply is an installation problem rather than a boiler one. This is one reason a price given without anyone looking at the gas route should be treated with caution, as covered in comparing boiler quotes.

What upgrading the gas pipe involves

Upgrading usually means running new, larger pipe from the gas meter to the boiler position. How much work that is depends almost entirely on the route.

In a house where the meter and boiler are close, or the route runs through a basement, under an accessible floor or along a cupboard, it can be a modest addition to the installation. Where the pipe has to cross several rooms, go through solid floors or climb floors of a terraced house, it takes longer and can involve lifting floorboards and some making good. Some routes are run externally where that is neater and allowed. Moving the boiler at the same time changes the route too, which is covered in moving your boiler during a renovation.

The meter end: when the supply itself is the limit

In most homes the pipework after the meter is the problem, not the meter. Occasionally, the meter or the incoming supply is the limit.

Standard domestic gas meters have a maximum flow rate. A large combi running alongside a range cooker and a gas fire can approach it in some homes. The pipework after the meter is the installer's responsibility; the meter belongs to your energy supplier and the pipe that feeds it belongs to the gas network, so any change at that end is arranged through them, not by the boiler installer. If the survey suggests the supply is marginal, you should be told before the boiler is chosen, so the output can be matched to what the supply can actually deliver.

Should the gas pipe be on the quote?

Yes. If the gas pipe needs upgrading, the route and the work should be written into the quote, not discovered on installation day.

Our combi boiler replacement survey checks the gas pipe size and route alongside the mains water flow, and the fixed written quote says plainly whether an upgrade is included. If your existing pipe is adequate, it will not be replaced for the sake of it. Book a free survey to have your gas supply checked before you choose a boiler.