A conveyor type or tunnel type dishwashing machine comprising - at least one rinsing zone, where one or more rinsing arms are placed, which spray the dishes with water optionally added to rinsing agents, - at least one pre-rinsing zone, where one or more pre-rinsing arms (2, 3) are placed, which spray the dishes with re-circulating liquid containing rinsing agents and detergent residues and by means of a first pump, - at least one collection tank (VR1, VR2), which is placed below at least one of said rinsing and pre-rinsing arms (1, 2, 3), - one or more washing zones, with respective one or more washing tanks, where one or more washing arms are placed, which spray the dishes with recirculating liquid, coming from a respective washing tank, which is added to detergent and which is sent to said washing arms by means of a further respective pump, - at least one drying zone, configured to evaporate water from the dishes through a flow of hot air coming from the outside and conveyed in a transition zone of said dishes, - said at least one collection tank (VR1, VR2) and said one or more washing tanks being connected one with each other through a decanting system, so that said tanks are filled with liquid one after the other, starting from said at least one collection tank (VR1, VR2) of said rinsing and pre-rinsing zones, wherein said first pump, which is connected to said pre-rinsing arms (2, 3), has a first flow duct, equipped with at least one valve or throttle, inside of which a portion of liquid contained within said at least one collection tank (VR1, VR2) flows, said portion of liquid being sent to a pre-washing zone or in correspondence of a drainage conduit of said dishwashing machine, characterized in that said portion of liquid is sent to a heat exchanger of the drying zone and from here inside a second flow duct, which flows in said pre-washing zone or in correspondence of said drainage conduit of said dishwashing machine. - at least one rinsing zone, where one or more rinsing arms are placed, which spray the dishes with water optionally added to rinsing agents, - at least one pre-rinsing zone, where one or more pre-rinsing arms (2, 3) are placed, which spray the dishes with re-circulating liquid containing rinsing agents and detergent residues and by means of a first pump, - at least one collection tank (VR1, VR2), which is placed below at least one of said rinsing and pre-rinsing arms (1, 2, 3), - one or more washing zones, with respective one or more washing tanks, where one or more washing arms are placed, which spray the dishes with recirculating liquid, coming from a respective washing tank, which is added to detergent and which is sent to said washing arms by means of a further respective pump, - at least one drying zone, configured to evaporate water from the dishes through a flow of hot air coming from the outside and conveyed in a transition zone of said dishes, - said at least one collection tank (VR1, VR2) and said one or more washing tanks being connected one with each other through a decanting system, so that said tanks are filled with liquid one after the other, starting from said at least one collection tank (VR1, VR2) of said rinsing and pre-rinsing zones,