District heating (less commonly called teleheating) is a system for distributing heat generated in a centralized location for residential and commercial heating requirements. The heat is often obtained from a cogeneration plant, although dedicated facilities called heat-only boiler stations are also used. A district heating plant can provide higher efficiencies and better pollution control than localized boilers.

National variation
In most Russian cities, district-level Heating and Electricity Stations produce more than 50% of the nation's electricity and simultaneously provide hot water for neighboring city blocks. They mostly use coal and oil-powered steam turbines for cogeneration of heat. Now, gas turbines and combined cycle designs are beginning to be widely used as well. A Soviet-era approach of using very large central stations to heat large districts of a big city or entire small cities is fading away as due to inefficiency, much heat is lost in transportation tubes because of leakages and lack of proper thermal insulation.

Scandinavian homes also use district heating for their hot water and heating needs.

District heating accumulator tower and workshops on the Churchill Gardens Estate, Pimlico, London. This plant once used waste water piped from Battersea Power Station on the other side of the River Thames. (January 2006)In Denmark district heating covers more then 60% of space heating including hot water demand. Most major cities in Denmark have big district heating networks including transmission networks operation with up to 125 degC and 25 bar pressure and distribution networks operating with up to 95 degC and between 6 and 10 bar pressure. The heat comes mainly from waste inceneration and combined heat and power plants. The efficiency of the powerplants can be raised to nearly 100% if suplying heat along with electricity compared to 30% when producing electricity only. The largest system in Denmark is in the metropolitan area operated by CTR I/S and VEKS I/S

In the United Kingdom, district heating also became popular after World War 2, but on a restricted scale, to heat the large residential estates that replaced areas devastated by the Blitz. The photo (right) shows the accumulator for one such plant at Pimlico, just north of the River Thames. This once relied on hot waste water from the now-disused Battersea Power Station on the far shore of the river. It is still in operation, the water now being heated locally. Many other such heating plants still operate on estates across Britain. Though they are said to be efficient, a frequent complaint of residents is that the heating levels are often set too high - the original designs did not allow for individual users to have their own thermostats.

In Italy, district heating is used in some towns (Bergamo, Brescia, Reggio Emilia).

In North America, district heating systems fall into two general categories. Those that are owned by and serve the buildings of a single entity are considered institutional systems. All others fall into the commercial category. Consolidated Edison of New York (Con Ed) operates the largest commercial district heating system ever built. [1] The system has operated continuously since March 1882 and serves Manhattan Island from the Battery through 96th Street. In addition to providing space and water heating, steam from the system is used in numerous restaurants for food preparation, process heat in laundries and dry cleaners, as well as to power absorption chillers for air conditioning.

History
District heating traces its roots to the hot water-heated baths and greenhouses of the ancient Roman Empire. District systems gained prominence in Europe during the Middle Ages and Renaissance, with one system in France in continuous operation since the 14th century. Across the Atlantic, the U.S. Naval Academy in Annapolis began steam district heating service in 1853.

Although these and numerous other systems have operated over the centuries, the first commercially successful district heating system was launched in Lockport, New York, in 1877 by American hydraulic engineer Birdsill Holly, considered the founder of modern district heating.

The future of many of these systems are in doubt; the same kind of problems many district heating operations in former Soviet Union and Eastern Europe have today, many North American steam district heating systems began to experience in the 1960s and 1970s. In North America, the owners, in many cases power utilities, completely lost interest in the district heating business and did not provide sufficient funding for maintenance of the systems. The systems and the service to the customer started to deteriorate. The result was that, after some years, the systems started to lose customers; the reliability for heat supply went down and finally the whole system closed down. For example, in Minnesota in the 1950s there were about 40 district steam systems; today only a few remain.[2]

Diffusion of district heating
Penetration of district heating (DH) into the heat market is very different by country. Penetration in influenced by different factors, including environmental conditions, availability heat sources and economic and legal framework.

In the year 2000 the percentage of houses supplied by heat from district heating in some European countries was as follows:

Iceland 95%, Estonia 52%, Poland 52%, Denmark 51%, Sweden 50%, Slovakia 40%,
Finland 49%, Hungary 16%, Austria 12.5%, Germany 12%, Netherlands, 3%, UK 1%
Source: Sabine Froning (Euroheat & Power): DHC/CHP/RES a smile for the environment, Kiev 2003

On Iceland the prevailing positive influence on DH is availability of easily captured geothermal heat. In most East European countries energy planning included development of cogeneration and district heating. Negative influence in The Netherlands and UK can be attributed partially to milder climate and also to stiff competition from natural gas supply.

District cooling
The opposite of district heating is district cooling. Working on broadly similar principles to district heating, district cooling delivers chilled water to buildings like offices and factories that need to be cooled. In winter-time, the source for the cooling can often be sea water so is a cheaper resource than using electricity to run compressors to generate the cooling effect. Helsinki has a district cooling system based on these principles are the idea is now being adopted in other Finnish cities.

In August 2004, Enwave Energy Corporation, a district energy company based in Toronto, Canada, started operation of an innovative system that uses water from Lake Ontario to cool a variety of downtown buildings, including office towers, the Metro Toronto Convention Centre, a small brewery and a telecommunications centre. The process has become known as Deep Lake Water Cooling (DLWC). It will provide for over 40,000 tons (refrigeration) of cooling – a significantly larger system than has been installed elsewhere. Another unique feature of the Enwave system is that it is integrated with the city of Toronto’s system for local drinking water supply, bringing benefits to the city in terms of improved drinking water supply.