A heat exchanger for a water heater having a burner includes a tube extending longitudinally along a centerline from a first end adjacent the burner to a second end. The tube defines a passage. Strips extend within the passage and are arranged about the centerline. Each strip includes a base and fins extending from opposite sides of the base such that flue gases from the burner flow from the first end to the second end of the outer tube and in channels between the fins.