A high temperature fouling test unit is provided. The fouling test unit includes a flow tube with a fouling probe received within the flow tube. The fouling probe includes a heated section. An annular passage is defined between the fouling probe and the flow tube. An impeller is mounted within the flow tube for inducing fluid flow within the annular passage. A plurality of temperature responsive devices monitor a rate of heat transfer and a change in fluid temperature through the annular passage.