On 03/03/2021 10:24, Bob Eager wrote:
On Wed, 03 Mar 2021 09:42:44 +0000, Muddymike wrote:

When our old gas boiler was replaced at our last house the engineer said
he would have to take up the parquet flooring right through the length
of our hall to install a new pipe. We objected telling him to find
another route.

He then agreed to test the gas flow through the old pipe and guess what?
Yes it was more than adequate to feed the new boiler.

Our guy (who we have used for years and trust) said he was a bit worried
about the flow rate. He measured and calculated, then found that a small
part of the run was in 15mm pipe. The rest was in steel pipe.

He changed the 15mm part (which was not disruptive) and it's all fine.



Surely the actual connection at the boiler is always 15mm, and
the installation instructions that I have seen say that the last metre
or so can be 15mm pipework.

Just because the OP can see 15mm ?iron pipe does not mean it is
15mm all the way from the meter. What size of pipe exits the meter ?.