In accordance with the present invention, there is provided a tubular heat exchanger wherein a central tube is employed to enclose an internal heated fluid which ascends inside the central tube. The heated fluid is then radially disbursed as it converges toward the internal surface of a top cover of the heat exchanger assembly, being forced outwardly under the top flange by the updraft force of the heated fluid, which forces the heated fluid downwardly apportioning the heated fluid around equally spaced axially arranged heat transfer tubes surrounded by an outer tube which are in sealed engagement with the top flange and attach at the bottom in sealing engagement with the exhaust collection manifold. The heated fluid descends around the tubes and enters an exhaust manifold exiting through an exhaust pipe. The heat transfer fluid enters a lower plenum and flows around the heat exchanger core into an upper plenum.