A granular biomass burning furnace for use with any appropriate granular biomass, such as grains, cherry pits, etc. The furnace includes a three stage heat exchanger, a fuel injector, a fuel stirrer, an ash ejector, a wash down system, a three stage air inducer, a fuel igniter, and supporting components. The unit includes a computer controller which controls all aspects of the operation of the unit based on information from sensors located throughout the unit. The unit includes a smart logic thermal controller to adjust the output heat of the unit via a variable speed air inducer. The three stage heat exchanger system includes a spiral water jacket surrounding the burn pot, a plurality of heat exchanger baffles in the unit, and a fine finned heat exchanger at the top of the unit. The air inducer provides air to the burn pot from three directions to promote complete combustion.