The TermoDeck hollow core slab system is a versatile energy storage technique for controlling the environment within large and medium sized buildings. It utilises the hollow cores within pre-cast concrete floor slabs as ventilation ducts to produce an environment which is thermally stable. Although many TermoDeck systems have successfully been installed in Scandinavia, the United Kingdom and in other northern European countries, the thermal performance of the system is not fully understood. This paper presents the results of a theoretical study, using a numerical model, into the thermal performance of the TermoDeck system. In particular, the role of the bends in the system is investigated and the conclusion reached that their impact on overall heat transfer is minimal. It is also concluded that greater thermal attenuation is achieved by using a five-core pass system in comparison with a three-core system.