Abstract: Hot gases from a fuel burner in a combustion chamber are exhausted to a flue at the top of the furnace. Air to be heated is drawn into the furnace through an upper inlet, passes through a chamber around combustion gas flow tubes, and then downwardly through sidewall flow spaces along the outside of the combustion chamber. Partially heated air from the sidewall spaces flows to one end of the furnace and then through air tubes above the burner to a plenum at the other end of the furnace where the so heated air is withdrawn by a fan. The fuel can be a solid fuel such as lignite, and the heated air can be used for grain drying.