Building Regulations Part J still specify a minimum 600mm from a boundary fence or wall (measured in the direction the outlet points, where a pluming control kit is used), although this will almost certainly change later this year. Regardless of the 600mm distance, and all other limitations on flue termination, there is still a duty of care that the plume not cause a nuisance.
The 2.5m figure is for use with the Condensing Boiler Assessment Installation Procedure. Locations where the flue is less than 2.5m from the boundary should not be considered under the procedure, but can still be used if desired by the installer and homeowner.
The 2.5m figure is for use with the Condensing Boiler Assessment Installation Procedure. Locations where the flue is less than 2.5m from the boundary should not be considered under the procedure, but can still be used if desired by the installer and homeowner.