A furnace including an elongated housing having an air intake adjacent one end of the housing and an air discharge vent adjacent the other end of the housing utilizes a substantially U-shaped heat exchanger positioned within the housing and a partition supported within the housing so as to separate the interior of the housing into first and second zones. A fuel burner assembly directs a flame and attending combustion products into one leg section of the heat exchanger so that the heat generated by the flame and combustion products are absorbed through the inside surfaces of both leg sections. The furnace also includes a fan so that a fraction of the air move through the first zone and absorbs heat from the outer surfaces of the one leg section and so that a remainder of the air moves through the second zone and absorbs heat from the other surfaces of the other leg section. The partition provides a first and a second opening through which the first and second zones communicate with one another. The remainder of the air is permitted to mix with a fraction of the air through the second opening before exiting the housing means through the discharge vent with the fraction of the air. As the remainder of air mixes with the fraction of the air within the first zone, it is permitted to flow across regions of the one leg section of the heat exchanger exposed to the highest concentration of heat generated by the flame and products.