district heating, although they have higher heat capacities than water, as they are expensive and have environmental issues. At customer level the heat network is usually connected to the central heating system of the dwellings via heat exchangers (heat substations): the working fluids of both networks (generally water or steam) do not mix. However, direct connection is used in the Odense system. Typical annual loss of thermal energy through distribution is around 10%, as seen in Norway's district heating network. Heat metering The amount of heat provided to customers is often recorded with a heat meter to encourage conservation and maximize the number of customers which can be served, but such meters are expensive. Due to the expense of heat metering, an alternative approach is simply to meter the water – water meters are much cheaper than heat meters, and have the advantage of encouraging consumers to extract as much heat as possible, leading to a very low return temperature, which increases the efficiency of power generation. Many systems were installed under a socialist economy (such as in the former Eastern Bloc) which lacked heat metering and means to adjust the heat delivery to each apartment. This led to great inefficiencies – users had to simply open windows when too hot – wasting energy and minimising the numbers of connectable customers. Size of systems District heating systems can vary in size. Some systems cover entire cities such as Stockholm or Flensburg, using a network of large 1000 mm diameter primary pipes linked to secondary pipes – e.g. 200 mm diameter, which in turn link to tertiary pipes that might be of 25 mm diameter which might connect to 10 to 50 houses. Some district heating schemes might only be sized to meet the needs of a small village or area of a city in which case only the secondary and tertiary pipes will be needed. Some schemes may be designed to serve only a limited number of dwellings, of about 20 to 50 houses, in which case only tertiary sized pipes are needed. Pros and cons District heating has various advantages compared to individual heating systems. Usually district heating is more energy efficient, due to simultaneous production of heat and electricity in combined heat and power generation plants. This has the added benefit of reducing greenhouse gas emissions. The larger combustion units also have a more advanced flue gas cleaning