Abstract: The present invention provides apparatus for utilizing heat in the flue of the furnace as a heat exchanger and a humidifying source which comprises a flue with a conduit spaced from and concentric with the flue. Means are provided for forcing air from the upper part of the conduit downwardly between the flue and the conduit in a direction opposite to the normal flow of the gases in the flue. Further means are providing for applying moisture to the outer surface of the upper part of the flue within the conduit and above the means for forcing air into the conduit. The heated and humidified air passes downwardly in the conduit and outwardly therefrom and may be connected to the plenum of the furnace. Baffles may be supplied within the flue and within the conduit in order to increase the turbulence of the air and thus increase the heat exchanger which is provided by the system.