This European Standard specifies rules for design, calculation and installation for preinsulated bonded pipe systems for buried hot water distribution and transmission networks (cf. figure 2) with pipe assemblies in accordance with EN 253, for continuous operation with hot water at various temperatures up to 120 °C and occasionally with peak temperatures up to 140 °C and maximum internal pressure 25 bar (overpressure). Application rule: For larger pipe dimensions and pressures below 25 bar wall thickness bigger than specified in EN 253 can be required for straight pipes, bends and tees. The principles of the standard can be applied to preinsulated pipe systems with pressures higher than 25 bar, provided that special attention is paid to the effects of pressure. Adjacent pipes belonging to the network (e. g. pipes in ducts, valve chambers, road crossings above ground etc.) can be designed and installed according to this standard. The standard assumes use of treated water, which by softening, demineralisation, deaeration, adding of chemicals, or otherwise has been treated to prevent internal corrosion and deposits in the pipes. This standard is not applicable for such units as: pumps, exchangers, boiler installations, tank installations, consumer installations. However, the full functional ability and durability of such units should be ensured in consideration of the impacts from the district heating system and other impacts occurring from the conditions under which they have been installed. Guidelines for product quality inspection and in situ tests of joints are given in Annex A of EN 448:2003, Annex D of EN 253:2003, Annex A of EN 488:2003 and Annex B of EN 489:2003. Guidelines for welding of polyethylene casing are given in Annex B of EN 448:2003. The estimation of expected life with continuous operation at various temperatures is outlined in Annex B of EN 253:2003.