An experimental study of the fuel savings obtainable with a residential, gas-fired, cast iron boiler operating with boiler pump overrun was conducted. A unique "thermostat"-controlled, fixed-load, steady-state cycling test procedure was employed. The seasonal fuel saving obtained with pump overrun, control setpoint of 95 F (35°C) outlet water temperature, was determined to be 9.7% . The associated seasonal energy cost saving was determined to be 7 .3%. The energy cost saving calculations were based on a 100 watt pump and a 3 to 1 electric/gas energy cost ratio. The data indicated that the savings are only slightly influenced by variations in the pump control setpoint over a 95-105 F (35-410C) range. The cycle per hour rate of the "thermostat" had little effect on the savings.