The heater is primarily intended for infra-red drying of travelling sheet material such as printed or lacquered paper. Parallel elongate heating elements are contained in a housing having a front window and a rear openable cover carrying a ceramic reflector behind the heating elements, which reflector also serves to insulate the cover against heat from the elements. End portions of the elements are supported in rearwardly directed clips whereby elements removal and replacement can be readily effected at the rear of the housing. The clips are carried by opposed support bars which with adjacent side walls of the housing define cooling air ducts, the bars having openings for directing cooling air onto the end portions of the elements or to enable the latter to extend into the ducts for direct cooling. The front window is provided with air venting slots parallel with the heating elements and positioned opposite spaces between the latter. The elements and slots are inclined to the direction of travel of the sheet material.