Abstract: A heat exchanger cell for a furnace is constructed of first and second cell divisions, each cell division having a general concave configuration and a rim extending around at least a portion of a circumference of the cell division. The first and second cell divisions are positioned adjacent each other and the rim of one cell division is folded over the rim of the other cell division and overlaps opposite sides of the rim of the other cell division forming a seam joining the first and second rims. A crease is formed in the seam joining the cell division rims and is spaced from a terminal edge of the seam. A plurality of indentations are formed in the seam traversing the crease and spaced from the terminal edge of the seam. The crease and plurality of indentations formed in the seam enable the seam to resist relative movement between the rims of the two cell divisions due to alternating heating and cooling of the cell divisions.