Abstract: A hot-water heating system has a heater for producing hot water, a heat radiator, a supply pipe for supplying them hot water from the heater to the heat radiator, a return pipe for returning the heat radiator to the heater, and a circulation pump for circulating the hot water from the heater successively through the supply pipe, the heat radiator, and the return pipe. A supply pipe temperature sensor for detecting the temperature of the hot water flowing through the supply pipe is mounted on an outer surface of the supply pipe. A return pipe temperature sensor for detecting the temperature of the hot water flowing through the return pipe is mounted on an outer surface of the return pipe. A control unit controls the circulation pump to reduce an output rate thereof as the difference between the temperatures of the hot water detected by the supply pipe temperature sensor and the return pipe temperature sensor is reduced.