Abstract: This invention pertains to apparatus and a method for utilizing waste heat as produced in furnaces, particularly boilers. This apparatus and method is equally useful in hot air furnaces. After heating the water or air the heat that passes into the plenum is caused to pass over and around a heat exchanger which moves air by a blower means through this exchanger into the house or basement. This air can be humidified, if desired. The exhaust stack during the combustion period is open to the chimney. At other times it is closed to prevent heat escape from the boiler. A collecting means is provided for moisture that may collect on the heat exchanger.