Abstract: A countercurrent heat recovery ventilation system includes an air module assembly and heat exchanger assembly. Air module assembly made from front and back panels connected by two side panels, base plate fixed with the side panels and placed parallel between side panels and a double side radial impeller with a shaft, an electric drive form two hydraulically isolated flow canals with inlet and outlet openings. The heat exchanger assembly comprises a heat-exchanger that could be done as changeable flow side heat-exchanger made as folded corrugated fins or plates, thus each of the both flow passages split in separate flow channels. Every other channel is sealed to flow from flow same direction forcing the flow to be in opposite direction in all adjacent cannels. This forms two hydraulically isolated flow passages with intake and outtake openings connected respectively with outlet and inlet openings of the air blower assembly.