The Clyde Tunnel in Glasgow, Scotland, is currently undergoing refurbishment. This refurbishment includes the installation of a new tunnel lining / fire protection system in both tunnel tubes. This lining system has already been shown to protect the tunnel structure from high temperatures However, there is a gap at the lower edge of the lining to allow any water accumulated behind the lining to run into a drainage channel within the main tunnel void. There were concerns that this small gap might lead to the exposure of the structural members to very high temperatures in the event of a fire in the tunnel. A heat transfer analysis was carried out. This analysis consisted of the evaluation of the thermal field inside the tunnel using Computational Fluid Dynamics (CFD) computations. This identified the locations in the tunnel which were most likely to be at risk, and predicted the conditions at those locations in the event of a ‘worst case’ fire scenario. On the basis of this information a bounding heat transfer analysis was carried out that allowed the establishment of the temperature evolution of the structural elements. CFD tools were not used for the heat transfer analysis because the complexity of the geometry made the uncertainty too large to justify a detailed CFD study. Instead, this bounding analysis allowed the establishment of the maximum possible temperatures of the structural elements. It was found that, even in a ‘worst case’ scenario fire, the temperature of the structural elements would not exceed critical levels for several hours of maximum fire exposure. The design of the lining / channel is held to be more than satisfactory in this case.