Abstract: A high efficiency furnace is provided for a forced air heating system having a furnace housing which is open at its lower end to cold air return means for the heating system and is open near its top to hot air exhaust means for the heating system. The furnace includes a burner housing contained within the furnace housing and adapted to receive and to burn a combustible fuel. A heat exchanger housing is positioned on top of the burner housing and the interiors of the burner and heat exchanger housings communicate with each other by means of a pipe. An exhaust means is provided for exhausting combustion products from the interior of the heat exchanger housing and comprises a substantially horizontal tube extending through the interior of the heat exchanger housing. The tube is open at one end to a flue outside the furnace housing and at the other end includes an opening open to the interior of the heat exchanger housing.