The heat exchangers refereed to here are for gas forced-air furnace only. A clamshell heat exchanger is constructed of two pieces of flat steel (could be aluminized, could be bare, could be stainless steel, etc…). The two pieces of sheet metal are stamped with a concave shape. The two concave sides are pressed together, and are either welded or folded over each other to create a cavity inside. It’s inside this cavity that the gas is burned. A tubular heat exchanger is constructed of a single or more tubes. It’s inside the tube that the gas is burned. The main differences are wall thickness and seem length. Wall thickness: the wall thickness of the tubes used in tubular heat exchangers is larger than than the wall thickness of clamshell heat exchangers. Thicker wall means it will take more punishment before the wall fails. Seam length: The seem length of a tubular heat exchanger is less than 1/10th of the clamshell design. Seams are typically where heat exchanger failures occur. Thus,