On 2020-10-02 22:20:46 +0000, said:

In most cases a 24kW boiler is not permitted to run on 15mm now,
current regs are more demanding re pressure drop. Worcester Bosch do a
pdf:
Technical Training Bulletin: Natural Gas (G20) Supply Pipe Sizing.
that explains how to calculate what's required.

We had a new Vaillant condensing system boiler installed yesterday and
the installer played it safe and put in a 18kw unit as he said the 15mm
gas pipe run was probably quite long and it is about 4 metres in a
straight line from the meter without taking into account the true path
under the hallway/kitchen.. The boiler so far is working fine and we
have hot water and heating after 10 days without and it's comfortably
driving 15 rads in about the same time as the old 24kw unit.

Only hitch was a bleed hole rounded off in one old rad so that's been
taken out of the system for now.