Fouling in heat exchangers is unavoidable and causes performance degradation; hence, occasional cleaning is essential. An effective cleaning strategy requires knowledge of foulants that can be obtained by laboratory tests. However, these are unaffordable for small and medium-scale industries. Model-based methods have been proposed for foulant characterization. However, they fail when exchangers are under temperature control and varying inlet conditions. This paper proposes a simple methodology to directly estimate foulants' characteristics using excess thermal and hydraulic loads. Specifically, it first uses changes in the pressure drop and mass flowrate vis-à-vis the exchanger's clean state to estimate foulant thickness. Subsequently, the estimated thickness is used along with temperature measurements to estimate the thermal conductivity. The methodology is demonstrated on a heat exchanger in two cases– with fouling only on tube side, and on both shell and tube sides. Our results show that the estimates have an error of less than 3% in all cases. A key advantage of the proposed methodology is that it can accommodate setpoint changes and disturbances that are common in industrial practise. © 2022 Institution of Chemical Engineers