A clamshell heat exchanger comprises upper and lower clamshell plates assembled together and sealed at the peripheral edges thereof, the assembled plates defining at least three internal passageways communicating in a serpentine configuration. The passageways include an inlet passageway having an inlet port for receipt therethrough of combustion gases, an intermediate passageway and an exhaust passageway having an exit port for the discharge of combustion gases, all such passageways lying generally parallel to each other. In one arrangement of the heat exchanger, the upper and lower clamshell plates define an air gap between the inlet passageway and the intermediate passageway, with the intermediate passageway and the exhaust passageway being joined by a secured flattened portion of the upper and lower clamshell plates. In another arrangement, instead of an air gap, the inlet passageway and the intermediate passageway are also joined by secured flattened portions of the upper and lower clamshell plates. Turbulent flow structure is provided by dimpled surfaces projecting inwardly into the intermediate and exhaust passageways and a longitudinally extending rib projecting into the intermediate passageway. A drain shunt, defined by a generally tubular channel, communicates between the intermediate passageway and the exhaust passageway to allow drainage of condensation from the heat exchanger when the heat exchanger is disposed in any orientation.