A heat exchanger (10) comprises a front wall (20) and a back wall (30) to form a space (40) for a flue gas such that a fluid flowing through a channel (60, 70) formed in the front wall (20) and back wall (30) can exchange heat with the flue gas, in use. The entire back wall (30) extends along a first plane (P1). The back wall (30) is provided with a back fin (120). The front wall (20) includes a lower portion (22) and an upper portion (24). The lower portion (22) extends upwardly along the back wall (30). The upper portion (24) extends upwardly from the upper end of the lower portion (22), extends outwardly away from the back wall (30) so as to form a combustion space (42) of a flammable gas between the upper portion (24) and the back wall (30), and is provided with a front fin (110). The front fin (110) and the back fin (120) are arranged symmetrically with respect to a virtual line (C2) along which the flammable gas is to be injected into the combustion space (42).