Abstract: A hot-air furnace has a long-flame burner for combusting gas or liquid fuel with a combustion chamber connected to the burner and having its length (l) and width (w.sub.1) in relationship of w.sub.1 <l. A heat exchanger is located above the combustion chamber and has internally a gas flow guide plate which guides combustion gas flow discharged from the combustion chamber to the heat exchanger. The heat exchanger has a width (w.sub.2) and length (l) in the relationship of w.sub.2 <l. An exhaust port for exhausting the combustion gas flow is located at the front or rear of, right or left-hand side of or on the top side above said heat exchanger. A casing has a drum integrally connecting the combustion chamber and the heat exchanger, an air flow guide and directing plate which covers the drum, a radiant heat absorber plate outside the combustion chamber, and a blower above or below the drum.