Does a new boiler need a bigger gas pipe?

It comes up on a lot of surveys and it catches people out, because it is the one item that can appear on a quote after somebody has already looked round the house. The boiler is going in the same spot as the old one, everything else is like for like, and then the installer says the gas pipe needs upgrading.

The first thing worth saying is that this is a real requirement, not an upsell. The second thing worth saying is that the rule everyone repeats about it is wrong.

The 22mm rule does not exist

Search this question and you will be told, confidently and in several places, that Gas Safe require a 22mm supply from the meter to any combi boiler. There is no such regulation. Pipe diameter is not set by the type of boiler at all.

What does exist is a limit on how much gas pressure the pipework is allowed to lose on the way. BS 6891, the standard for domestic gas installation pipework, says that for natural gas the design pressure loss between the outlet of the meter and the point where the appliance connects must not exceed 1 mbar at the design maximum flow rate. For LPG the figure is 2 mbar.

Pipe size is the answer to that sum, not the starting point. Sometimes the answer is 22mm. On a short straight run to a modest boiler it can still be 15mm. On a long run feeding several appliances it can need to be larger than 22mm in sections. The size is an output, and anyone who tells you the size before they have measured the route has not done the calculation.

Source: Gas Safe Register, Registered Gas Engineer: low pressure and under-sized pipework

What the sum actually uses

Gas Safe Register's own technical guidance sets out the method. The engineer starts from the maximum gas rate of the appliance or appliances to be connected, then measures the route the pipe will take from the meter to the boiler position, then adds an allowance for every fitting on that route to get what is called the equivalent length. Every elbow and every tee behaves like extra metres of pipe, because every change of direction adds friction.

That equivalent length is then read against the standard's tables to find the smallest pipe size that keeps the total loss inside 1 mbar. The same guidance notes that an allowance may need to be made for future appliances too, so a run that only just passes today is a run that will fail the moment somebody adds a gas hob.

This is why two identical boilers in two identical houses can get different answers. One has a meter in a box on the kitchen wall and a metre of pipe to the boiler. The other has the meter at the front of the house and the boiler at the back, upstairs, round two corners and along a joist. Same appliance, very different sum.

Why "it lights, so it must be fine" is the wrong test

This is the part that almost nobody explains to homeowners, and it is the reason a corner cut here does not show up for years.

Gas Safe Register give the worked example themselves. Imagine a boiler installed on existing pipework that loses 5 mbar instead of 1. On the day, with 20 mbar at the meter outlet, the appliance sees 15 mbar. The manufacturer's instructions say the boiler will work safely down to 14 mbar, so the engineer signs it off and it runs perfectly.

Then comes a cold evening when the whole network is at peak demand. Pressure in the street falls to 20 mbar, the meter itself absorbs 4 mbar of that, and only 16 mbar reaches the meter outlet. Take off the same 5 mbar loss across the undersized pipe and the boiler is now getting 11 mbar, below the manufacturer's minimum, on the coldest night of the year.

That is the whole point of the 1 mbar allowance. It is headroom for the worst conditions, not a design target for the best ones. A boiler that is starved of gas under load does not usually stop politely either. It struggles to light, drops out mid-cycle, or throws an ignition fault, and those symptoms get blamed on the boiler for years.

Source: Gas Safe Register, Registered Gas Engineer: low pressure and under-sized pipework

Swapping like for like does not mean the pipe is fine

The common assumption is that if the old boiler ran on this pipe, the new one will too. Three things break that assumption.

The demand may have gone up. On a combi, the maximum gas rate is set by the hot water side, not the heating side, because that is when the burner runs flat out. Going from an older combi to a higher output one to get a better shower increases what the pipe has to deliver, even though the heating load on the house has not changed at all.

The old installation may never have been right. Plenty of pipework was sized by eye, or was correct for the appliance it was originally fitted for and quietly inherited by two boilers since. Nothing about a boiler running for fifteen years proves the pipe was ever inside the 1 mbar limit.

And the pipe may not only be feeding the boiler. A hob, a gas fire and a boiler all running at once is the design condition, not an unusual event, and the calculation has to hold with everything on.

How to check it was actually done

You do not need to understand the tables to hold an installer to this. You need the Benchmark checklist, which every new gas boiler should be commissioned against and which must be completed in full by the person who commissioned it and handed to you to keep.

The checklist has fields for gas inlet working pressure in mbar, recorded in central heating mode and, on a combi, in hot water mode as well. The commissioning guidance is explicit that gas supply pressure and gas rate are checked before the combustion checks are done at all. If those boxes are blank, the pressure at the appliance under load was not proved.

So the question to ask when the job is finished is a simple one: what was the working pressure at the boiler at full rate, and is it written on the Benchmark. It takes ten seconds and it tells you a great deal about the standard of the installation.

Source: HHIC Benchmark, gas boiler system commissioning checklist; Benchmark code of practice for installation, commissioning and servicing

What makes a gas run expensive, and what makes it easy

We will not put a figure on it, because the figure is almost entirely about your house rather than about the pipe. What drives it is the route.

An easy job is a short run in accessible space: meter box on an outside wall, boiler on the other side of it or a couple of metres away, pipe run in a garage, a cupboard or along a wall that can be boxed. A hard job is a long run that has to cross the house, lift flooring, pass through joists, or be chased and made good afterwards, and the making good is often the bigger half of the work.

Locally it splits fairly neatly. A lot of the estate stock around Milton Keynes and the newer parts of Northampton was built with the meter box on the flank wall and the boiler in the kitchen, so the run is short and often already adequate. The Victorian and Edwardian terraces closer to Northampton town centre are the opposite: meter in a cupboard or box at the front, boiler moved to a back kitchen or a first floor bathroom at some point in the last thirty years, and a long indirect route between the two with several changes of direction. Those are the houses where this comes up.

The one thing worth doing is asking about it before you accept a quote rather than after. Any installer who has surveyed properly can tell you whether the existing pipe passes, and a quote that does not mention gas pipework on a house like that has either checked it and been satisfied, or not checked it at all. It is a fair question to ask which.

Getting it priced properly

If you want the gas supply assessed as part of the quote rather than discovered on the day, tell us about the job and we will put you in touch with three vetted local installers. Give them the boiler position, the meter position and a rough idea of the route, and the answers you get back will be worth comparing.

It is also worth saying plainly: if a pipe upgrade is genuinely needed, it is needed. It is the difference between a boiler that works in November and a boiler that works in February.