A direct-fired make-up furnace comprises a cabinet which houses an air blower. A gas fired burner located in a sloped passage, which is formed by walls of a return air compartment and a by-pass air compartment, radiates heat at an angle relative to the blower. The return air compartment substantially blocks direct radiation of the burner. An exit opening of the by-pass air compartment is located between the burner and the blower.