Computer-implemented relational optimisation method for a heat exchanger of a ventilation and air-conditioning system, wherein optimisation is performed by varying at least two dimensions of the heat exchanger starting with an initial design of the heat exchanger based on its characteristic data, for example volume flow, exhaust air temperature, efficiency of the heat recovery, outside air conditions, and incident flow area, resulting in design data such as height and width as the actual incident flow area of the heat exchanger, installation depth of the heat exchanger, heat transfer area of the heat exchanger, heat transfer coefficient of the heat exchanger, and/or pressure drop at the medium sides, in order to determine a discrete optimum for efficiency under the present project-specific framework conditions, for example by varying a dimensionless characteristic with a corresponding degree of temperature change from case to case and comparing the effort required for this from case to case to the possible benefit of the heat recovery, and wherein the relational optimisation method comprises at least the method steps of: changing of the cross-sectional area of the heat exchanger by varying the height and/or the width of the heat exchanger and determining the associated heat transfer characteristics by means of an approximation as well as the pressure drop resulting from the changed cross-sectional area, changing of the heat transfer area of the heat exchanger by varying the depth of the heat exchanger, and subsequently searching for a discrete economic and/or ecological optimum for heat exchanger efficiency under the selected cross-sectional areas and inserting the optimised heat exchanger into the ventilation and air-conditioning system, wherein the input-side cross-section of the heat exchanger is determined in a first calculation loop and the installation depth of the heat exchanger in a second calculation loop by way of iteration, wherein the cross-sectional area of the heat exchanger is varied from a minimum starting value to a maximum final value to be selected in a step width to be specified in the first calculation loop, wherein k-numbers resulting relationally for each individual value are determined based on the initial design, wherein the depth of the desired heat exchanger is varied from an initial depth value to a final depth value in a depth step width to be specified in the second calculation loop, wherein the respective k-number from the first calculation loop remains unchanged in the second iteration loop, and wherein the k-number is the heat transfer coefficient. changing of the cross-sectional area of the heat exchanger by varying the height and/or the width of the heat exchanger and determining the associated heat transfer characteristics by means of an approximation as well as the pressure drop resulting from the changed cross-sectional area, changing of the heat transfer area of the heat exchanger by varying the depth of the heat exchanger, and subsequently searching for a discrete economic and/or ecological optimum for heat exchanger efficiency under the selected cross-sectional areas and inserting the optimised heat exchanger into the ventilation and air-conditioning system, wherein the input-side cross-section of the heat exchanger is determined in a first calculation loop and the installation depth of the heat exchanger in a second calculation loop by way of iteration, wherein the cross-sectional area of the heat exchanger is varied from a minimum starting value to a maximum final value to be selected in a step width to be specified in the first calculation loop, wherein k-numbers resulting relationally for each individual value are determined based on the initial design, wherein the depth of the desired heat exchanger is varied from an initial depth value to a final depth value in a depth step width to be specified in the second calculation loop, wherein the respective k-number from the first calculation loop remains unchanged in the second iteration loop, and wherein the k-number is the heat transfer coefficient.