A coaxial ground heat exchanger and installation method are disclosed where the heat exchanger has a central core tube that can be provided with a thermally insulating casing and an outer tube that delimits an annular gap which extends radially outwards from the core tube. The core tube and the annular gap can be configured such that a flowable heat transfer medium can flow through. The outer tube is formed by a tubular expandable cover that directly adjoins a wall of a ground heat exchanger borehole when the heat exchanger is installed.