Abstract: A heat exchanger unit (10) between flue gas and water in a boiler comprises internally first paths for the water and second paths for the flue gas. Walls (23) separating the first and second paths comprise fins (22) projecting therefrom in the second paths to be licked by the flue gas and perform heat transfer between the flue gas and the walls of the water paths.Each fin (22) has substantially flat lateral faces (24,25) mutually inclined to taper downward and an upper end (27) and lower end (26) rounded to radius together said lateral faces (24,25).