Tempering furnace for glass sheets

The invention relates to a tempering furnace for a glass sheet, which has a conveyor for the glass sheet, first convection blow means over the conveyor to heat the glass sheet by hot air jets blown onto its top surface, whereby the first convection blow means include a blower pressurising air sucked from the tempering furnace, air channels to lead air from the blower to blow enclosures which have, in their bot-tom surface, first blow openings from which air is discharged as jets towards the top and/or bottom surface of the glass sheet, electric elements inside the blow enclosures, warming up air, and second convection blow means with which pressurised air external to the tempering furnace may be led to second blow nozzles from which air is discharged as jets towards the bottom surface of the glass sheet, and the electric elements form a matrix-like separately-adjustable electric element field, whereby the heating effect of the air jets of the first convection blow means onto the glass sheet is arranged to be