This European Standard provides a methodology to calculate/estimate the system thermal loss of water based distribution systems for heating and the auxiliary energy demand, as well as the recoverable part of each. The actual recovered energy depends on the gain to loss ratio. Different levels of accuracy, corresponding to the needs of the user and the input data available at each design stage of the project, are provided in this European Standard by different calculation methods, i.e. a detailed calculation method, a simplified calculation method and a method based on tabulated values. The general method of calculation can be applied for any time-step (hour, day, month or year). Pipework lengths for the heating of decentralised, non-domestic ventilation systems equipment are to be calculated in the same way as for water based heating systems. For centralised, non-domestic ventilation systems equipment, the length is to be specified in accordance with its location. NOTE It is possible to calculate the system thermal loss and auxiliary energy demand for cooling systems with the same calculation methods as shown in this European Standard. Specifically, determination of auxiliary energy demand is based on the same assumptions for efficiency of pumps, because the efficiency curve applied is an approximation for inline and external motors. It needs to be decided by the standardisation group of CEN, whether or not the extension for cooling systems should be made in this European Standard. This is also valid for distribution systems in HVAC (in ducts) and also for special liquids.