Abstract: An adjustable louver system for controlling the direct thermal radiation reaching fluid tubes in a direct-fired heater. An angular position of louver blades of the louver system is adjusted by rotating first and second axles attached to the louver blades. The louver blades may be positioned manually or by an electric or pneumatic motor. A hand crank or knob located outside the heater manually turns the louver blades. The motor, which is also located outside the heater, is controllable by a temperature actuator. In some embodiments, the louver blades have pivot pins which fit into slots of a connecting plate. Rotation of one of the louver blade causes the connecting plate to rotate all of the louver blades simultaneously. In some embodiments, the louver blades are vertically positioned and the louver axles fit into holes in upper and lower guide plates. In other embodiments, the louvers are horizontally disposed and the louver axles fit into openings in the heater walls.