Abstract: A heat exchange system and related method for retrofitting an existing bore with the said system, the system being adapted to be used within a bore formed within the ground, and being independent of formation fluids, the system comprising a first pipe and a second pipe together forming a fluid path, wherein substantially a lower end in use of each of the first pipe and the second pipe are in fluid communication with each other and with a sump, so that fluid can flow between the first pipe and the second pipe via the sump within the said bore, the system further comprising a pump adapted to drive heat exchange fluid through the said pipes and a heat exchange unit adapted to transfer thermal energy to or from the said heat exchange fluid.