In the paper main emphasis is placed on both single-and double-pipe prefabricated ducts, which are the most popular pipe systems currently used in Europe and Scandinavia. The following facts affecting heat losses are dealt with: temperature conditions, i.e., temperature of outgoing and returning water as well as outdoor temperature, thermal conductivity of the insulating material and the earth, installation depth of the carrier pipes, heat transfer coefficient from the earth to the air, eccentricity between the carrier pipe and the jacket pipe, and some factors that vary with time. Attention is also paid for one special fitting, i.e., anchor point element.

At the end of the paper some examples of laboratory and field tests are described.