A heat exchanger unit (13) for recovering heat from the flue gases from a cylinder steam boiler (10), said heat exchanger unit comprising a flue-gas tube (14) and a heat-exchanging tube (16) which is supported concentrically inside the flue-gas tube (14) by a bent inlet pipe (19) extending from a lower opening (20) in the peripheral wall of the flue-gas tube (14) to an inlet opening (21) in a lower outwardly bulging end wall (22) of the heat-exchanging tube (16), and an outlet pipe (23) extending from an outlet opening (24) in an upper part of the peripheral wall of the heat-exchanging tube (16) to an upper opening (25) in the peripheral wall of the flue-gas tube (14). The inlet pipe (19) has a first, horizontal, straight end portion (19A), a second, upwardly directed, straight end portion (19B) and also an intermediate, curved portion (19C). The bending angle (α) of the curved portion exceeds 90° and the other end portion (19B) extends with an inclination determined by said bending angle (α), towards the centre of curvature of a partially spherical portion of the lower end wall (22) of the heat-exchanging tube (16), in which wall the inlet opening (21) is arranged eccentrically in relation to the central longitudinal axis of the heat-exchanging tube (16).