This ÖNORM is applicable to water based surface heating and cooling systems in residential, commercial and industrial buildings. The methods apply to systems integrated into the wall, floor or ceiling construction without any open air gaps. This standard is part 3 of a series of standards. The aim of the present Standard is to give a guide for the design for use of renewable energy sources and for the use of Thermo-Active-Building-Systems (TABS). The tool allows calculation of peak cooling capacity of a thermo-active system, based on heat gains (solar, internal loads, ventilation). This method also allows calculation of the energy demand on the water side (system) to be used for sizing of the cooling system, like chiller, fluid flow rate, etc. Steady state heating capacity is calculated according to method B or E, ref. Part 1 of this series of standards.