Abstract: A hot air furnace has an outer housing and a firebox positioned within the housing. A space between the outer housing and the firebox defines a pre-heating chamber which communicates through an air distribution blower to a lower chamber and an upper chamber within the housing. The walls of the firebox are hollow and open into the lower chamber and the upper chamber so that air extracted by the distribution blower from the pre-heating chamber and directed into the lower chamber is heated as it is channeled up through the hollow walls of the firebox prior to entering the upper chamber. A smoke chamber is located within the upper chamber and it serves to further heat the air entering the upper chamber. A combustion air blower is provided and it together with the distribution blower is automatically controlled by a thermostat located within the space to be heated.