Abstract: The efficiency of a heat exchanger used in applications where heat conduction between rows of conduits located from front to back of the heat exchanger and provided with common fins (42) is increased where the fins (42) are abutted to adjacent tube runs (22,24,26) in each row and extend from the front (28) to the back (30) of the heat exchanger so each fin (42) is common to each of the rows (22,24,26). Heat flow interrupters (56,58) are located in each fin (42) at locations aligned with spaces (27) between the runs (22,24,26). Each heat flow interrupter (56,58) is defined by a slit (62) extending completely through the fin 42 which is characterized by the absence of the removal of any material of which the fin (42) is made at the slit (62).