This document describes a method of establishing a guarantee diagram for the performance characteristic values of a heat exchanger working under such conditions that mass-flow and temperature of the media vary little from those at rating conditions. The equations used for establishing this guarantee diagram are generally valid but here they were obtained particularly for finned heat echangers. The numerical data used for the given example are only valid for a cross-flow water-air heat exchanger. This document is available in English, French and German.