In many European cities and some older American cities, electric utilities use the waste heat from the boilers or heat exchangers to keep buildings warm in the cooler months of the year. Hot water is circulated through pipes through nearby buildings, and cooler water returns to the utility. This is known as district heating because an entire region can be served by this system. In the United States, 5.6 x 1017J (0.56 EJ) of energy was distributed as district heating in 1995.
Savings are possible because of economy of scale (one central heating system instead of many individual systems), the reduced maintenance per unit of output characteristic of a larger system, and because the waste heat would have to be disposed of by other means were it not used for this useful purpose. District heating is only feasible when there is a large but compact region having a relatively large population density through at least part of the day. This is more characteristic of European and Asian conditions than American or African ones.