The present investigation has been carried out in order to increase the efficiency of the RETAL-type boiler, used in the Cuban sugar mills. Test methods generally used in the evaluation process and further adjustment of the boilers operation have been analyzed, pointing the attention on the importance of the stoichiometric ratio and steam power on the overall efficiency. Important general rules have been extracted from the complete regular tests following ASME and GOST methodologies, and, as a result, a simplified test code has been obtained. Boiler design optimization has also been achieved determining the optimum waste heat recovery scheme from both, thermal and economical viewpoints. As a result, the optimal stack gas temperature has been calculated as well as the range of the optimal value for the excess air fraction. Their influence on the efficiency has been analyzed and the total costs determined. Once the total costs are included in the analysis, the most efficient low-temperature heat recovery scheme results to be the combination of an economizer followed, in the direction of the exhaust gas flow, by an air heater. (C) 2003 Elsevier Science Ltd. All rights reserved.