Energy tunnels have emerged as systems that can contribute to the production of clean, renewable thermal energy. Nevertheless, so far applications have been related almost exclusively to new tunnelling projects. Accordingly, no systematic methodologies for the heat exchange instrumentation of existing tunnels have been proposed up until now. Starting from the valuable experience gained from different energy tunnel testbeds worldwide, this paper proposes two approaches that would allow the thermal activation of the existing heritage of tunnels. Different solutions are conceived for both approaches to fit various existing tunnel decay contexts and diverse levels of refurbishment necessity. These are illustrated, outlining characteristics and advantages, describing expected installation details and issues and analysing the possibility of real implementations. The geothermal potential of such solutions is assessed through thermo-hydraulic numerical modelling. Finally, with the aim of investigating their economic attractiveness and profitability, a brief economic analysis is drawn up, considering the geothermal energy produced and the costs involved in installing and running the systems.