A plastic heat exchanger which is simple and inexpensive. The heat exchanger includes an outer duct and a stack of inner ducts, the inner ducts for establishing a first flow path for cooling air or air to be cooled. Each inner duct includes end portions which are stackable upon one another and a middle portion which has a reduced dimension. A seal is placed about each stack of end portions and a second flow path for cooled air or cooling air is formed within the outer duct, between the seals and around the outside of the middle portions of the inner ducts.