In article , Robin
writes
On 02/03/2021 23:57, Fredxx wrote:

snip
If you know the kW of your boiler you would be able to calculate the
size of pipe you actually need. In domestic situations with limited
pipe runs 22mm copper pipe is a safe bet, plus to save embarrassment
to Gas Safe installer Gas Safe have made a blanket requirement to
combis where they must be piped in 22mm. Though I would be happy to
have the last sectionin 15mm.


I don't want to give false hope to the OP who may well need a new gas pipe.

But I've seen before on some sites that "Gas Safe regulations now state
that gas supply pipes from the meter to a combi boiler must be at least
22mm". Smelt to me like a way to get more work and avoid the bother of
calculating pressure drops. Gas Safe have no power to make
regulations. And while the Gas Safety (Installation and Use)
Regulations were amended in 2018 I can't see anything in them about a
minimum pipe size. Have I missed something?


Only that when combi boilers first appeared a lot of installers used the
existing 15mm which was then found to be inadequate.
--
bert