There is provided an improved heating apparatus of the multiple condensing-vapor substantially air-free type used for uniformly heating moving threadlines of yarn up to 350.degree. C. and which includes a pressurized boiler containing a vaporizable liquid, means for heating the liquid in the boiler to generate hot vapor, means for distributing the hot vapor to the inside of multiple plates where the vapor condenses and thereby heats threadlines of yarn separately moved in frictional contact witn an outer yarn-contacting surface of each hot plate, and possibly means for returning the condensate to the boiler, wherein the improvement comprises having a coating on the inside surface of each said hot plate, said coating having a contact angle with said vaporizable liquid of at least 90.degree..
