The invention relates to a heat box for thermally conditioning preforms (10) that consist of a thermoplastic material and are provided for blow moulding, in which two lateral walls (32) and one base wall (34) delimit a heat tunnel through which the preforms (10) are conveyed, said heat tunnel comprising at least two parallel heat channels, and a heating unit (14) being arranged between two adjacent, particularly parallel heat channels and comprising a plurality of rod-shaped heating elements (16) that extend longitudinally in the conveyor direction of the preforms (10).