The awareness in society concerning environmental impacts due to the energy consumption in buildings is steadily growing. As a result, the general demand for buildings with low environmental impact, and especially the energy consumption, is very likely to increase even more in the future. Today’s building concepts will probably not be consistent with future demands for energy efficient buildings with an optimal indoor environment. Buildings with significantly lower energy demands for space heating can be achieved by different means. However, in the Nordic climate there is still a resultant need for additional space heating to ensure an optimal thermal indoor environment. Today, hydronic floor heating is by far the most common heat supply system for space heating in new residential buildings in the Nordic region. A building design concept based upon a building with a low heat demand, which is supplied by a hydronic floor heating system, is considered in this paper. The water temperature supplied to the floor heating system is only a few degrees above the desired room temperature. A low supply temperature offers an ultimate flexibility when the primary heat source is selected. Furthermore, in comparison with today’s floor heating systems, the low temperature system will more or less be self-regulated due to the small temperature difference between floor surface and indoor air. The process of self-regulation is a highly dynamic thermal process. A state-of-the-art numerical simulation model is applied to study this process. Especially the temperature fluctuations in the indoor operative temperature will be studied. Furthermore, a tepid floor construction, due to the operation of the floor heating system, will decrease the ability to react on internal heat loads, a reduction of the buildings heat capacity occurs. This phenomenon is also studied in this paper by means of the simulation tool.