An apparatus for mixing hot water and cold water, such as to form a flow of mixed water for a sanitary water plant, and for carrying out an anti-legionella disinfection, the apparatus comprising: - an inlet conduit of cold water, destined to be hydraulically connected to a source of cold water supply; - an inlet conduit of hot water destined to be hydraulically connected to a source of hot water supply; - a delivery conduit of mixed water destined to be connected to a delivery line of mixed water, located downstream of the apparatus, of a sanitary water plant; - a mixing device provided with a first inlet hydraulically connectable to the inlet conduit of cold water, with a second inlet hydraulically connectable to the inlet conduit of hot water, and with an outlet of mixed water, the mixing device further comprising a mixing chamber into which the first and second inlets flow, carrying respectively a flow of cold water and a flow of hot water, and from which the outlet of mixed water exits, carrying a flow of mixed water, the mixing device being configured such as to supply from the outlet of mixed water a flow of mixed water having a determined delivery temperature that is intermediate between the temperature of the cold water and the temperature of the hot water; the apparatus being characterized in that it comprises: - a valve device, provided with a third inlet hydraulically connectable to the outlet of mixed water, with a fourth inlet and with a delivery outlet, hydraulically connectable to, or coinciding with, the delivery conduit; - a bypass conduit branching from the inlet conduit of hot water at a point upstream of the mixing device and hydraulically connecting the inlet conduit of hot water with the fourth inlet of the valve device; wherein the valve device is configured and predisposed to operate selectively at least between a first configuration, in which it hydraulically connects the third inlet to the delivery outlet such as to send mixed water coming from the mixer device to the delivery line of the sanitary water plant and contemporaneously closes the inflow of hot water to the fourth inlet, and a second configuration in which it hydraulically connects the fourth inlet to the delivery outlet such as to send hot water coming from the bypass conduit to the delivery line of the sanitary water plant and contemporarily closes the inflow of mixed water to the third inlet, wherein in the first configuration the apparatus enables sending the mixed water to the delivery line of the water plant and in the second configuration it enables sending only hot water to the delivery line of the water plant such as to carry out an anti-legionella thermal disinfection treatment of the plant. - an inlet conduit of cold water, destined to be hydraulically connected to a source of cold water supply; - an inlet conduit of hot water destined to be hydraulically connected to a source of hot water supply; - a delivery conduit of mixed water destined to be connected to a delivery line of mixed water, located downstream of the apparatus, of a sanitary water plant; - a mixing device provided with a first inlet hydraulically connectable to the inlet conduit of cold water, with a second inlet hydraulically connectable to the inlet conduit of hot water, and with an outlet of mixed water, the mixing device further comprising a mixing chamber into which the first and second inlets flow, carrying respectively a flow of cold water and a flow of hot water, and from which the outlet of mixed water exits, carrying a flow of mixed water, the mixing device being configured such as to supply from the outlet of mixed water a flow of mixed water having a determined delivery temperature that is intermediate between the temperature of the cold water and the temperature of the hot water; - a valve device, provided with a third inlet hydraulically connectable to the outlet of mixed water, with a fourth inlet and with a delivery outlet, hydraulically connectable to, or coinciding with, the delivery conduit; - a bypass conduit branching from the inlet conduit of hot water at a point upstream of the mixing device and hydraulically connecting the inlet conduit of hot water with the fourth inlet of the valve device;