District heat systems can contribute to the achievement of social and environmental targets and energy security. District heat production systems (DHSs) vary in size depending on heat demand, which is influenced by several factors such as local climatic conditions and the sizes of the communities they serve. In this study, we analyzed options for minimum-cost DHSs of different sizes under different environmental and social cost scenarios. We calculated the production cost and primary energy use of district heat for minimum-cost options by considering a value of cogenerated electricity equivalent to the value of electricity produced in minimum-cost standalone condensing power plants. We varied the size of DHSs from 100 to 1800 GWh(heat) per year to investigate how size influences the minimum-cost compositions of production units and district heat production costs. We determined that the optimal composition and cost of district heat production is dependent on the size of the system, the overall load factor of heat demand and the technologies considered for both DHSs and reference power plants. In general, cogenerated district heat is more energy-efficient than district heat from heat-only production and also more cost-efficient, except for small DHSs, for which cogenerated district heat is more costly than heat-only production. The cost and primary energy use of district heat production is dependent on environmental and social cost scenarios; however, this dependence is reduced if a DHS is cost-minimized and based on cogenerated units. (C) 2014 Elsevier Ltd. All rights reserved.