The report presents the findings of an investigation into the main heat exchanger performance and the modifications now being carried out on the Grimethorne pressurized fluidized-bed combustor. The heat exchanger was designed as part of the steam/water circuit to cool the exhaust gas entering with a temperature in the range of 750 to 950/sup 0/C to approximately 300/sup 0/C at the outlet. During hot commissioning, it was noted that the gas outlet temperature from the heat exchanger was appreciably above design although the inlet temperature was substantially below design. An analysis of the calculated heat transfer coefficients derived from several hot commissioning runs for the tube banks within the exchanger, showed them to be more than 50% below the design values. Inspection of the exchanger revealed the tubes to be covered with dust deposits, which acted as insulation and accounted for the reduction in performance, apart from the difference in design and actual inlet conditions. The report provides a description of the heat exchanger in terms of its function and design. Operational aspects up to the completion of Test Series 1 are outlined, and details are given of the modifications that are to be carried out prior to Test Series 2. A brief outline of the auxiliary heat exchanger and its performance is included.