This document is applicable to electronic control equipment for heating systems with water as the heating medium and a supply water temperature up to 120 °C. This control equipment controls the distribution and/or the generation of heat in relation to the outside temperature and time and other reference variables. This document also is also applicable to controllers that contain an integrated optimum start or an optimum start-stop control function. Safety requirements on heating systems remain unaffected by this document. The dynamic behaviour of the valves and actuators are not covered in this document. A multi-distribution and/or multi-generation system needs a coordinated solution to prevent undesired interaction and is not part of this document.