## 1 Introduction  
The heating systems of South Korea have evolved into various forms with economic development. Before the 1980s, the primary fuels for residential heating were wood thicket and briquette. However, after the second oil crisis and in accordance with the South Korean government's energy diversification policy, natural gas was introduced in order to lower dependence on oil consumption. Since the 1980s, with economic development, heating fuel has started to change from oil to natural gas. Because of this, the heating system was mainly changed to an individual heating system (IHS). Natural gas was supplied to the metropolitan area for the first time with the establishment of an acquisition base and a pipeline network. Since then, it has spread to metropolitan areas, and the number of households that have been supplied with IHS was 17.75 million as of 2017 [1]. Two oil crises caused an energy crisis in the 1970s. This also led the South Korean government to recognize the need for fundamental energy-saving measures in the residential, commercial, and industrial sectors. As a result, since the mid-1980s, the government introduced a district heating system (DHS) that can dramatically improve energy efficiency compared to a central heating system or an IHS. At the time of the introduction of the combined heat and power (CHP) generation-based DHS, the South Korean government made an important goal to secure eco-friendly features such as saving energy and improving environmental pollution in dense residential areas. Thus, as new towns have expanded since the mid-1980s, DHS has become a heating system that can satisfy economic efficiency and convenience in the construction of apartments, which are large-scale public housing buildings with high a population density. As of 2017, DHS is supplied to 2.8 million households. This is about 16.4% of the total number of households in South Korea. The DHS is provided through a district energy business. The district energy business refers to a business in which energy (heat or heat and electricity) is produced by one or more concentrated energy production facilities such as CHP plants or incinerators. It is supplied and sold to a large number of users in residential, commercial, or industrial complexes [2]. There was no data comparing IHS and DHS as in this study, but information on DHS supply in several countries is provided only by Euroheat and Power [3]. Due to their high energy efficiency and reduction of greenhouse gas emissions, DHS are also distributed in many countries, and Euroheat and Power [3] data are the only data that provide information on the rate of prevalence of DHS in residential sector as far as the authors know. In general, the rate of prevalence of DHS in countries that operate large-scale DHS was high. For example, in Europe, the rate of prevalence of DHS in the residential sector was 90% in Iceland, 90% in Sweden, 65% in Denmark, 38% in Finland, 15% in Austria, and 13.8% in Germany as of 2017. In addition, countries with large-scale DHS have a high proportion of CHP. As of 2017, the share of CHP in DHS was 70% in Finland, 59% in Austria, and 83% in Germany [3]. As an exception, in Iceland, which uses a lot of geothermal heat, the share of CHP is low, and in Sweden and Denmark, which have a high dependency on waste heat, CHP has a relatively low share of 46% and 51%, respectively. On the other hand, the rate of prevalence of DHS in countries that operate small-sale DHS was relatively low. For instance, the rate of prevalence of DHS in residential sector was 2% in UK as of 2013 and 3% in the US as of 2011. In Japan, no information on the rate of prevalence of DHS was given, but the area of DHS supply increased by 3.6% in 2013 compared to 2009. In China, 98% of the total DHS was distributed in the northern regions, and the rate of prevalence of DHS in that region was about 55% in 2013. In countries with small DHS, there is no information on the share of CHP in DHS.