A heating apparatus for supplying heated air to a grain dryer or the like. The apparatus features a biomass furnace with a burn chamber for combustible biomass material, a chimney having a lower end in fluid communication with the burn chamber interior, and air ducting that has a fresh air inlet, an output end connected to the dryer, and directs heated exhaust air from the chimney to the dryer. An airflow control system is configured to both control airflow to the dryer through the air ducting, and control a temperature of said airflow by varying a ratio between the fresh ambient air and the heated exhaust air. The burn chamber has a chain gate with overbed and underbed circulation fans that generate airflow up through the chain grate, as well as tumbling air currents above the chain grate for thorough combustion and cleaner exhaust that won't contaminate the grain.