DS EN 303-4:1999 Heating Boilers - Part 4: Heating Boilers With Forced Draught Burners - Special Requirements For Boilers With Forced Draught Oil Burners With Outputs Up To 70 Kw And A Maximum Operating Pressure Of 3 Bar - Terminology, Special Requirements, Testing And Marking Pertains to heating boilers with forced draught oil burners up to a nominal heat output of 70 kW. They are operated, either with negative pressure (natural draught boiler) or with positive pressure (p