the specification discloses an air - to - air , polymeric heat exchanger having a housing and a core removably mounted within the housing . the housing includes four generally identical duct units , which are secured to one another , and a top and a bottom , which are secured to the duct units . the core includes a pair of end walls and a plurality of tubes extending between , and opening through , the end walls . each end wall includes a plurality of tubular projections or pockets extending from the end wall in a common direction . opposite ends of the tubes are secured within the tubular pockets in the opposite end walls . a method of constructing the core includes the step of severing the tubular projections after the tubes are positioned therein to expose the tube interiors through the end walls . preferably , the tubular projections are severed using a hot wire to also fuse the tubes within the end walls .