Gas Pipe Upgrades for New Boiler Installations: Why 22mm Matters
Most modern combis need a 22mm gas supply, not the old 15mm. Here's why correct pipe sizing matters, what an upgrade involves, and what it costs in Chorley & Lancashire.
When we quote for a new boiler, one of the first things we measure is the existing gas pipe. It's an unglamorous detail, but getting it wrong is the single most common reason a brand-new boiler underperforms, locks out, or fails its commissioning gas-rate test. Here's why gas pipe sizing matters on a modern install — and what to expect if your home needs an upgrade.
Why old 15mm pipework isn't enough
Older boilers from the 1990s and early 2000s typically had outputs of 18–24kW and could often be fed comfortably by a 15mm copper pipe. Modern combis are far more powerful — a 30kW combi delivers around 35kW in hot-water mode, and a 35kW unit can peak at 40kW+. That demand requires more gas volume, and a 15mm pipe simply can't carry it without an unacceptable pressure drop, especially over longer runs from the meter.
What the regulations actually say
Under BS 6891 and the manufacturer's installation manual, the working gas pressure at the boiler inlet must be at least 19 mbar with the appliance running at full rate, and the total pressure drop from the meter to the appliance must not exceed 1 mbar. Every Gas Safe engineer is required to verify this with a manometer at commissioning. If the pipe is undersized, the boiler will fail its gas-rate test and legally cannot be left in service.
Symptoms of an undersized gas supply
The boiler locks out intermittently with ignition or flame-loss fault codes. Hot water runs cool when the hob is also lit. The boiler 'hunts' — firing and cutting out repeatedly on the hot-water demand. The new boiler feels weaker than the old one, even though it's higher rated. In severe cases, incomplete combustion can produce elevated CO levels — which is exactly why Gas Safe rules exist.
When does the pipe need upsizing?
A typical 30–35kW combi fed from a meter more than 5–6 metres away almost always needs 22mm copper for at least the main run. Shorter runs (under 3m straight) can sometimes stay in 15mm if the gas-rate test passes, but in our experience, around 70% of pre-2010 properties needing boiler installation Chorley and boiler installation Preston require at least a partial upgrade when we fit a modern combi. Properties with the meter in an integral garage or far from the kitchen almost always do.
What an upgrade actually involves
We trace the gas pipe route from the meter to the boiler, plan the shortest sensible run in 22mm copper (or sometimes 28mm for very long runs or high-output system boilers), and replace the section that's restricting flow. The new pipe is clipped to current standards, tightness-tested at the meter, and the gas rate is re-measured at the boiler. Most upgrades take 2–4 hours on top of the boiler swap and are done the same day.
What it costs in 2026
A short upgrade (under 5m, surface-clipped, accessible route) typically adds £150–£250 to a boiler install. A longer or more awkward run requiring lifted floorboards, drilling joists, or routing through a ceiling void is £300–£500. Very long runs from a detached garage or outbuilding meter can be £600+. We always quote this up-front in the fixed price — never as a 'discovery' on the day of fitting.
How to spot a quote that skips it
If a quote is significantly cheaper than others and doesn't mention measuring the existing gas pipe or pressure-testing at commissioning, walk away. The two classic shortcuts cowboy installers take are (1) leaving an undersized 15mm supply and hoping the homeowner won't notice the lockouts until the warranty period is up, and (2) skipping the manometer test entirely. Both are illegal under Gas Safe rules and both will void your boiler warranty.
Every quote we issue includes a survey of your existing gas supply and an honest answer on whether it needs upgrading. If it does, the cost is built into the fixed price you see online — no surprises on installation day. We also cover boiler installation Leyland on the same fixed-price terms. Build fast online boiler quotes on our home page or call 01257 781481 to book a free survey.