Boiler sizing question

[Johnyc86]

The bigger the output of boiler

Does say an 25 kw boiler have a bigger heat exchanger or more surface area in combustion chamber than say an 18??
What exactly makes a boiler haveva higher output??

 

[gmartine]

For most manufacturers there won't be any physical differences between a 18 and 25kw boiler and the hex size will be the same. That only starts changing once you start reaching the much higher outputs as a boiler can modulate so can already operate well below and in between any stated maximum.

Just as a car engine of the same capacity can be tuned or set up to produce different power outputs so can a gas boiler, it's fuelling or the gas rate at which it burns that is the main determinate.