An experimental investigation was carried out to obtain benchmark data for clamshell heat exchangers in residential gas furnaces. Detailed flue-gas and surface temperature distributions were obtained from the furnace tests. It was found that flue-gas temperatures were non-uniform at the same cross-section. The highest surface temperature was found near the middle of the first U-bend. The surface temperatures were not uniform on the perimeter of the same cross-section. In contrast to general expectations, the surface temperatures did not decrease monotonously along the direction of gas flow, which may be explained by the complex flow field in the furnace cabinet.