The majority of hi-eff condensing boiler require a 3/4 gas pipe due to the nature of how they operate (using zero governor gas valves)
There are some models that cope with a 1/2 inch line, but have a limited output
and would not be useful for medium to large installations
The allowed pressure drop across a natural gas line (from the meter to the appliance) is 1 m/bar, ng being normative at 20m/bar this would mean minimum of 19m/bar at the appliance.
Traditional boilers required somewhere in the region of 10-14 m/bar to operate
correctly, depending on the appliance so if the pipework had been undersized as long as 14m/bar was available the boiler worked fine (even though the pipework was not to standard)
But the new boilers do not work that way, and the 20m/bar is required (with the correct volume of gas)
In short, a lot of new boilers have been installed on a gas pipe that is undersized, and the boilers are not working correctly, I know as I have shut some of them down!