The subject of fly-ash collection and disposal on spreader-stoker-fired boiler units has received considerable study ever since this method of fuel burning came into extensive use. The J. I. Case Company has carried on considerable study and experimentation on this subject in connection with a rehabilitation and modernization program of its power plants started in 1941. This work along with the over-all performance records obtained from the Tractor Works Plant of the company has received considerable attention from engineers both in this country and abroad. In November, 1949, a new 100,000-lb-per-hr boiler unit was put into operation which incorporated, in so far as known, the first system of gravity return of fly ash from the dust collector to the boiler furnace. This installation was called to the attention of Bituminous Coal Research, Inc. (BCR), by W. S. Major, then a development engineer with that organization. As a result of his initiative and efforts, a test program was initiated to investigate the over-all results obtained with this system under the joint sponsorship of the Detroit Stoker Company, BCR, and the J. I. Case Company. Battelle Memorial Institute was engaged to supervise and conduct the tests. The purpose of this paper is to report some of the test results obtained.