Regenerative glass furnaces have continuous glass production but are cyclic in thermal operation. The length of the thermal cycle affects the thermal efficiency of the furnace. We summarize the results of 11 experiments performed on a 33 ton/day end-port type regenerative glass furnace. Six flame-direction reversal times, ranging from 15 to 40 min, were used. The furnace thermal efficiencies were determined to be in the range 41.8-43.8%, with longer flame direction reversal times yielding higher efficiencies. Two alternative methods were used to determine the optimum flame-direction reversal time and both resulted in an optimum cycle time of 40-45 min.