Abstract: A prefabricated ground coil assembly for use in association with a ground source heat pump comprises a flexible pipe configured as a double spiral. The outermost turns of the spiral are provided with an inlet connection and an outlet connection for heat exchange fluid, respectively. The turns of the spiral are interconnected by flexible constraining means so that the assembly is extendible axially from a nested annular configuration to an elongate coniform configuration in which the turns of the double spiral are supported in predetermined space relation by the flexible constraining means. The assembly is easy to install, and can be installed in a relatively shallow pit which occupies only a small ground area.