In attempting to use the 4-1 mixture for Boiler No. 6 in the Virginia Polytechnic Institute Heating and Power Plant, the problem of maintaining proper depth of field bed had arisen. This problem seemed to warrant an investigation, which was subsequently made. The fuel bed depth that will give the best results for a boiler depends on many factors, and it is best determined by boiler tests. Therefore, the authors decided to investigate the effect of the field bed depth on the boiler performance and to determine the optimum field bed depth, when the 4-1 mixture was used and the load was 50,000 pounds of steam per hour. This investigation consisted of the collection and evaluation of data obtained from the boiler tests at various field bed depths. During the boiler tests, some of the variables were held as constant as possible in order to obtain comparative results. The fuel bed depths tested were 2-¾, 4, and 5-½ inches. The optimum depth of fuel bed is that fuel bed which would give the highest boiler efficiency and good boiler control. The results of the tests showed that the boiler efficiency remained approximately constant between 2-¾ and 4 inches of the fuel bed depths and decreased as the fuel bed depth was increased above 4 inches. But, at 2-¾ inches of the fuel bed, close attention was necessary to control the boiler, whenever the load swung, or there was a change of the quality of fuel fed into the furnace. The authors, therefore, conclude that Boiler No. 6 should have a fuel bed depth between 3 and 4 inches deep when the 4-1 mixture is used and the load is about 50,000 pounds of steam per hour. / Master of Science