Abstract: A heat exchanger for exchanging heat between an inside airstream flowing within an interior of a building structure and an outdoor airstream flowing outside of the interior of the building structure includes a heat pipe system comprising a refrigerant. A first heat pipe assembly is installed within the interior of the building structure such that heat is transferrable between the first heat pipe assembly and the inside airstream flowing within the interior of the building structure. A second heat pipe assembly is installed outside of the interior of the building structure such that heat is transferrable between the second heat pipe assembly and the outside airstream flowing outside of the interior of the building structure. The heat pipe system is configured such that the inside airstream remains within the interior of the building structure and the outside airstream remains outside of the interior of the building structure.