The supply of heat, either in the form of steam or hot water, from a central source to a group of buildings. As an industry, district heating began in the early 1900s with distribution of exhaust steam, previously wasted to the atmosphere, from power plants located close to business and industrial districts. Advent of condensing-type electrical generating plants and development of long-distance electrical transmission led to concentration of electrical generation in large plants remote from business districts. Most district heating systems in the United States rely on separate steam generation facilities close to load centers. In some cities, notably New York, high-pressure district steam (over 120 lb/in.² or 830 kilopascals) is used extensively to feed turbines that drive pumps and refrigerant compressors. Although some district heating plants serve detached residences, the cost of underground piping and the small quantities of heating service required make this service generally unfeasible.