Abstract: A solid fuel heater comprises a plurality of horizontal tubes defining between a vertical partition and an opposed vertical wall of the heater, a tubular grate with given ones of the tubes connected to a primary air source manifold header while others are connected to a secondary air source manifold header for preheating the air. The ends of the tubes opposite the connection to the headers open, respectively, to the upper side of the tubular grate for delivery of primary air and to the lower side for delivery of secondary air. The vertical partition wall forms a hot air passage for a downdraft operation leading to the stack. Preferably, a secondary grate underlies the tubular grate. By operating in a downdraft manner, only the lower ends of the solid fuel are burned, permitting low stack temperatures and long burning operation of the heater whether wood or coal fired.