An auxiliary boiler is provided for substantially reducing fuel consumption of a conventional forced-air heating system. A boiler tank substantially filled with water is connected by hot and cold water lines to a heat exchanger disposed within the cold air duct of the forced-air heating system. A firebox which extends into the boiler tank is adapted to receive combustible material such as wood for heating the water in the tank. A pump directs hot water from the tank through the hot water line to the heat exchanger whereby cool air moving through the cold air duct is preheated as it passes through the heat exchanger. Heating tubes in communication with water in the boiler tank may extend through the firebox for supporting logs therein. Additional heating tubes may extend through a flue directed upwardly from the firebox through the boiler tank.