Busy seeking quotes for the replacement boiler and one guy quoting over
the phone, when I described the layout, was suggesting the pipe from
meter to boiler needs be entirely 22mm. Worcester+'s own conditions
says he. It is 22mm all the way from meter, to the adjacent oven, then
final 2m drops down to 15mm where it goes on to feed our present 18Kw
boiler.

We have had no issues due to pressure drop in the 10 years since
install, at all. The installer ten years ago was at first insistent a
new pipe would need be run from the meter, outside the house to the
boiler. I did the pipe calcs and made clear I would not allow him to do
that, rather I would go elsewhere. He gave in and reused the existing
feed.

The thing is, the branch down from 22mm to 15mm is on the wall, behind
units, so swapping the 15mm to 22mm would be no simple task - but is it
necessary for a Worcester / Bosch 18R1 18Kw boiler's guarantee?