The invention is a heat exchanger (1; 10), particularly suited to be used in condensation boilers, comprising: a tubular jacket (2) that internally delimits a first duct (3) developed between an inlet section (3a) and an outlet section (3b) that define an outflow direction (F) for a first fluid; a second duct (4; 11) arranged in thermal contact with the tubular jacket (2) and developed mainly outside the first duct (3) between an inlet mouth (4a; 11a) and an outlet mouth (4b; 11 b) for a second fluid; heat exchange elements (5, 5') projecting from the tubular jacket (2) towards the inside of the first duct (3); a thin body (7) arranged in the first duct (3) parallel to the outflow direction (F) and in thermal contact with one or more heat exchange elements (5).