Exergy is destroyed when work is degraded by friction and turbulence and when heat is transferred through finite temperature differences. Typical HVAC systems use a combination of high quality energy from combustion and electricity to overcome relatively small temperature differences between the building and the environment. It is possible to achieve the heating/cooling necessary to maintain comfort in a building without these high quality energy sources and their high potential-energy destruction. A low-exergy heating and cooling system seeks to better match the quality of energy to the loads of the building and thus to minimize exergy destruction and increase the exergetic efficiency of the building’s heating and cooling system. The method described here for low exergy building system design begins by minimizing overall heating and cooling loads using a tight, highly-insulated envelope and passive solar design strategies. Next a low-exergy heating and cooling system is designed that uses hydronic radiant heating and cooling in floors, along with high thermal mass. The large surface area of the floors enable low fluid flow rates and relatively small temperature differences to achieve heat transfer rates that would traditionally be driven by high temperature differentials and flows. The building uses a solar wall to passively drive ventilation requirements and earth tubes to condition the ventilation air. High thermal mass in the floor reduces peak loads and eliminates the need for solar thermal storage tanks. Thus, this paper begins to explore the practical limits of low-exergy design.