An air-to-air heat exchanger comprising: a first housing comprising two opposite end walls (111, 112), two opposite side walls (113, 114), a floor, and a ceiling, two heat exchange units (160, 180) comprising air channels (161, 162, 181, 182) extending between the side walls (113,114) of the first housing, the two heat exchange units (160, 180) dividing the interior of the first housing into a middle section situated between the two heat exchange units (160, 180) and two end sections (150, 190) situated between the respective end wall (111, 112) of the housing and the respective heat exchange unit (160, 180), the heat exchanger comprises further: a side duct (130, 140) at each side of the housing extending along the respective side wall (113, 114) of the housing and comprising a first end (130A, 140A) and a second opposite end (130B, 140B), characterized in that said side duct (130, 140) being divided by a partition wall (119, 120) extending between the first end (130A, 140A) and the second opposite end (130B, 140B) of the side duct (130, 140) into an upper compartment (131, 141) and a lower compartment (132, 142), said side duct (130, 140) comprising an inlet opening (135, 145) at the first end (130A, 140A) of the side duct (130, 140) in one of the compartments (131, 132, 141, 142) and an outlet opening (136, 146) at the second end (130B, 140B) of the corresponding compartment (131, 132, 141, 142) in the opposite side duct (130, 140). a first housing comprising two opposite end walls (111, 112), two opposite side walls (113, 114), a floor, and a ceiling, two heat exchange units (160, 180) comprising air channels (161, 162, 181, 182) extending between the side walls (113,114) of the first housing, the two heat exchange units (160, 180) dividing the interior of the first housing into a middle section situated between the two heat exchange units (160, 180) and two end sections (150, 190) situated between the respective end wall (111, 112) of the housing and the respective heat exchange unit (160, 180), the heat exchanger comprises further: a side duct (130, 140) at each side of the housing extending along the respective side wall (113, 114) of the housing and comprising a first end (130A, 140A) and a second opposite end (130B, 140B), characterized in that said side duct (130, 140) being divided by a partition wall (119, 120) extending between the first end (130A, 140A) and the second opposite end (130B, 140B) of the side duct (130, 140) into an upper compartment (131, 141) and a lower compartment (132, 142), said side duct (130, 140) comprising an inlet opening (135, 145) at the first end (130A, 140A) of the side duct (130, 140) in one of the compartments (131, 132, 141, 142) and an outlet opening (136, 146) at the second end (130B, 140B) of the corresponding compartment (131, 132, 141, 142) in the opposite side duct (130, 140). a side duct (130, 140) at each side of the housing extending along the respective side wall (113, 114) of the housing and comprising a first end (130A, 140A) and a second opposite end (130B, 140B), characterized in that said side duct (130, 140) being divided by a partition wall (119, 120) extending between the first end (130A, 140A) and the second opposite end (130B, 140B) of the side duct (130, 140) into an upper compartment (131, 141) and a lower compartment (132, 142), said side duct (130, 140) comprising an inlet opening (135, 145) at the first end (130A, 140A) of the side duct (130, 140) in one of the compartments (131, 132, 141, 142) and an outlet opening (136, 146) at the second end (130B, 140B) of the corresponding compartment (131, 132, 141, 142) in the opposite side duct (130, 140).