Optimizing cleaning schedules for refinery preheat trains requires a robust and reliable simulator, reliable fouling models, and the ability to handle the thermal and hydraulic impacts of fouling. The interaction between thermal and hydraulic effects is explored using engineering analyses and fouling rate laws based on the threshold fouling concept; the potential occurrence of a new phenomenon, thermo-hydraulic channeling in parallel heat exchangers, is identified. The importance of the foulant thermal conductivity is highlighted. We also report the development of a highly flexible preheat train simulator constructed in MATLAB/Excel. It is able to accommodate variable throughput, control valve operation, and different cost scenarios. The simulator is demonstrated on a network of 14 heat exchangers, where the importance of optimizing the flow split between parallel streams is illustrated.