NOVELTY - The furnace has a housing defining an interior, and an expansion bowl (50) placed in the interior. A burn pot (30) includes a set of holes (33) for directionally venting pre-heated inlet air into the burn pot to create a combustion vortex of the inlet air in a burn pot interior. A fan (92) blows the inlet air through a preheat module (40) into the burn pot. A bio-fuel feed unit includes an internal storage bin and a rotating scoop arm for depositing bio-fuel into a combustion unit (20) such that the bio-fuel is passed vertically through the combustion vortex inside the burn pot interior.    USE - Bio-fuel burning furnace for burning shredded, granulize, extruded or pelletized bio-fuel in energy production application. Uses include but are not limited to residential, commercial and industrial heating, ventilation and air-conditioning (HVAC) and military applications.    ADVANTAGE - The preheat module uses a portion of thermal energy from the combustion of bio-fuel in the burn pot to preheat the inlet air to temperature above ignition point of the bio-fuel, so that preheated oxygen rich inlet air spiraling in the combustion vortex provides an ideal environment for efficient and complete combustion of the bio-fuel. The furnace can burn the bio-fuel with moisture content above twenty-five percent and the bio-fuel with high content of inert and non-combustible contaminants without extinguishing flame. The combustion vortex is strong such that the bio-fuel is fully ignited before settling at a bottom part of the burn pot so as to produce elemental ash and gases. The directional venting of the inlet airflow into the burn pot creates an insulating swirling curtain of the inlet air for acting as a protective thermal barrier centering the combustion vortex and spacing the superheated combustion vortex from side walls of the burn pot. The components of the furnace can be repaired by a user with basic tools and without specialized knowledge or skill. The feed unit with the rotating scoop arm mechanism provides a maintenance free fuel delivery mechanism and eliminates the problem of conventional auger fed bio-fuel furnaces. The expansion bowl is made of thermal insulating material aerated concrete (ACC) so as to reduce thermal energy loss through conduction. The combustion unit is made of aerated concrete as an insulating thermal lining, thus reducing weight of the furnace and improving structural integrity and thermal efficiency of the furnace.    DETAILED DESCRIPTION - An INDEPENDENT CLAIM is also included for a method for burning bio-fuel in a furnace.    DESCRIPTION OF DRAWING(S) - The drawing shows a side sectional view of a combustion unit performing air flow through a combustion unit.    Combustion unit (20)    Burn pot (30)    Holes (33)    Preheat module (40)    Expansion bowl (50)    Fan (92)