Abstract: A downdraft furnace comprises a combustion component associated with a heat exchange component. The combustion component includes a fuel chamber, a primary combustion chamber and a secondary combustion chamber operatively interconnected by a constricting throat opening. Log support structure is provided to carry split logs and at an angle of approximately thirty degrees from the horizontal. The support structure and the logs define a primary air flow passage directed toward the throat opening through which primary air flow is directed at the bottom portions of the fuel segments. The inlet includes closure means to restrict and vertically vary slightly the path of primary air flow. Passage means are provided to introduce secondary air directly at the throat opening. The inlet includes closure means to restrict and vertically slightly vary the path of primary air flow.