Abstract: An apparatus for the storage of thermal energy and for the distribution of such thermal energy to the interior of a building. The preferred system utilizes prestressed, precast concrete decking panels with hollow cores through which a fluid conducting conduit extends to form a grid within the decking panels. Selected cores have spaced ports communicating with face surfaces of the panels to provide an air flow passage through the selected cores. Fluid from a primary heat storage is controllably circulated through the conduit so that the fluid and the panels provide a secondary storage as well as a heat radiator for continuous even heating while the air flow passages permit forced or gravity flow heating of air for rapidly compensating for an extreme or sudden heat loss.