Abstract: A heat exchanger for use with exhaust pipes. A shroud inlet communicates with a rear chamber, which communicates with a front chamber via a center plate vent. The front chamber communicates with a shroud outlet. Ambient air entering the rear chamber is routed around at least one J-pipe in the rear chamber, and around a collector in the front chamber, thus being twice heated prior to exiting the heat exchanger through the shroud outlet. The double heating provided by this design increases the efficiency of the heat exchanger. The collector is attached to the center plate by means of a full-penetration fillet weld, and the J-pipes are attached to the center plate, a front plate, and a pipe assembly baffle by stitch welding external to the J-pipes, thus strengthening the integrity of the hermetic seal between exhaust gasses and ambient air passing through the heat exchanger.