Abstract: A heater for heating a stream of air has a burner assembly to heat the stream as it flows along a flow path and a primary heat assembly positioned in the flow path and coupled to the burner assembly for conducting gaseous combustion products. The primary heat exchange assembly has an array of U-shaped heat exchange tubes for conducting hot combustion products generated by the burner assembly to an array of second U-shaped heat exchange tubes for combustion products discharged from the first heat exchange tubes, the second tube having a smaller diameter than the first and being positioned in the space defined by the array of first U-shaped heat exchange tubes. Combustion products leaving the primary heat exchange assembly are passed through a secondary heat exchange assembly before being exhausted.