A device for heating wrapped batches of products and a packaging installation provided with such a heating device, specifically having a manifold of heating elements with lines of orifices and a blocking device for the orifices using a mobile plate having for each line of orifices openings, at least two successive openings having different lengths, to go the direction of movement a) from a position blocking at least a first line of orifices and opening at least a second line of orifices, b) to another position opening the first line of orifices whilst maintaining open the second line of orifices.