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.