Abstract: The method and the apparatus according to the invention relate to pasteurization of products in a tunnel pasteurizer. In the application of heating systems comprising water/steam heat exchangers in the zones for heating of sprinkling water, the pipelines and the valves for controlling the water and the steam become very complex and vulnerable, and the number of analog valves for steam become equal to the number of heat exchangers as a minimum. Leakages in analog valves for steam are frequent after some time in operation. According to the invention this problem is eliminated in that hot water is taken out under low pressure from a heating source (7) comprising a heat-insulted storage tank and a shared source of heat, where the water is circulating constantly, said hot water being supplied in a controlled amount directly to each zone (14b, . . .