A heater plug is cooled with combustion air for diesel oil-burning heating appliances, especially those provided with a vaporizing burner. The plug is screwed into a burner pipe joint, enclosing but spaced inwardly of the plug. The pipe joint also has inlet for supplying the fuel, and it is fastened to the furnace wall of the heating appliances. The burner carries a continuous lengthwise bore, accessible from one end face and opening into the heater plug coil, the inner end of the bore being connected to the combustion air channel across a proportioning throttle.