A stack of heating surface elements includes a first heating surface element (4) having first (10), second (12) and third (14) zones arranged sequentially along a primary gas flow direction (A). The first zone (10) includes a herringbone structure, the second zone (12) includes a flat structure, and the third zone (14) includes a plurality of corrugations extending in the primary gas flow direction (A). The corrugations have flat peak and trough regions. The stack also includes a second heating surface element (36), where the second heating surface element includes a plurality of corrugations extending in the primary gas flow direction (A).