Temporal limitations for concern for the future

The invention relates to a method for heating glass sheets to be tempered or heat-strengthened, in which method glass sheets are heated in a preheating furnace (1) by applying a hot-air blast and convection heating produced thereby to the opposite sides of a glass sheet and the preheated glass sheet is transferred from the preheating furnace (1) into a radiation heating furnace (2) for heating the glass sheet to a tempering temperature. During the convection heating of a glass sheet, the rotating speed of a hot-air fan (5) is increased while adjusting the heating effect of heating resistances (6) so as to maintain the temperature of blasted air substantially constant. Thus, the diminishing temperature difference between glass and air is compensated for by controlling the coefficient of heat transfer.