Abstract: A heat exchanger serving for the heating of an organic transfer liquid by combustion gases from a burner has a metallic housing in the shape of an upright cylinder with a widened upper part; the transfer liquid flows through an inner coil extending over nearly the full height of the housing and in series therewith through an outer coil surrounding the inner coil in the widened upper part. The two coils are enveloped by combustion gases, generated within or outside the housing, which rise from its bottom to the top inside the inner coil and then descend around the outer coil to an exit port provided near the lower end thereof. The top, bottom and upper cylinder part are overlain by conduits carrying a cooling fluid, preferably the same transfer liquid; in a preferred embodiment, these conduits are designed as half-pipes welded onto the outer housing surfaces to form cooling channels.