Low-temperature district heating permits the use of energy resources formerly discarded or unclaimed. However, its investment may be capital intensive because of equipment oversizing and temperature peaking required by conventional equipment. In classical design philosophy, flow is minimized in order to compensate some of the oversizing costs and to improve the utilization rate of the energy source. This paper shows that oversizing has diminishing returns; there exists an optimum split between equipment oversizing and thermal peaking. This article identifies the attributes and limitations of low-temperature district energy systems and describes the factors affecting the feasibility. Equipment oversizing characteristics in series and parallel arrangements, such as capacity improvement, temperature drop, and energy loss in the circuitry, are analytically expressed. A new design strategy is developed for optimum building retrofitting for low-temperature applications. A design example shows that district heating can be substantially improved by using cascaded, hybrid HVAC systems with alternative equipment.

Units: SI