A one-piece multifunction body for a hydraulic valve assembly used in a combined plant for room heating and for the instantaneous production of domestic hot water; the one-piece multifunction body comprising a main hollow body having a substantially polyhedral shape provided with the following faces: - a front frontal face; - a rear frontal face; - a right side face; - a left side face; - an upper face; - a lower face; the main hollow body is further provided with - a first plurality of hydraulic attachments (86A, 87A, 87B) obtained on the faces (,,,,,) of the polyhedron; and - a second plurality of functional attachments (87A, 87B), also obtained on the faces (,,,,,) of the polyhedron; said second plurality of functional attachments (87A, 87B) comprising attachments designed to house a three-way valve; said second plurality of functional attachments (87A, 87B) being designed to be coupled to a plurality of control and interception devices (26, 28A, 29, 30, 33, 35, 38, 61, 62, 63) to control and intercept the water flow inside said main hollow body; said control and interception devices (26, 28A, 29, 30, 33, 35, 38, 61, 62, 63) comprising a three-way valve; wherein at least one type of attachments (86A, 87A, 87B) of said first plurality or of said second plurality (87A, 87B) comprises at least two identical attachments (86A, 87A, 87B) obtained on different faces (,,,,,) of the polyhedron, so as to ensure the most suitable space orientation of said main hollow body said one-piece multifunction body being characterized in that it comprises: - a first type of attachments comprising four identical attachments, two arranged on said rear frontal face and, respectively, two arranged on said lower face; - a second type of attachments comprising two identical opposite attachments, one arranged on said right side face and, respectively, one arranged on said left side face; - a third type of attachments comprising four identical attachments, two arranged on said upper face and, respectively, one arranged on said right side face and one arranged on the left side face. - a front frontal face; - a rear frontal face; - a right side face; - a left side face; - an upper face; - a lower face; the main hollow body is further provided with - a first plurality of hydraulic attachments (86A, 87A, 87B) obtained on the faces (,,,,,) of the polyhedron; and - a second plurality of functional attachments (87A, 87B), also obtained on the faces (,,,,,) of the polyhedron; said second plurality of functional attachments (87A, 87B) comprising attachments designed to house a three-way valve; said second plurality of functional attachments (87A, 87B) being designed to be coupled to a plurality of control and interception devices (26, 28A, 29, 30, 33, 35, 38, 61, 62, 63) to control and intercept the water flow inside said main hollow body; said control and interception devices (26, 28A, 29, 30, 33, 35, 38, 61, 62, 63) comprising a three-way valve; wherein at least one type of attachments (86A, 87A, 87B) of said first plurality or of said second plurality (87A, 87B) comprises at least two identical attachments (86A, 87A, 87B) obtained on different faces (,,,,,) of the polyhedron, so as to ensure the most suitable space orientation of said main hollow body said one-piece multifunction body being characterized in that it comprises: - a first type of attachments comprising four identical attachments, two arranged on said rear frontal face and, respectively, two arranged on said lower face; - a second type of attachments comprising two identical opposite attachments, one arranged on said right side face and, respectively, one arranged on said left side face; - a third type of attachments comprising four identical attachments, two arranged on said upper face and, respectively, one arranged on said right side face and one arranged on the left side face. - a first type of attachments comprising four identical attachments, two arranged on said rear frontal face and, respectively, two arranged on said lower face; - a second type of attachments comprising two identical opposite attachments, one arranged on said right side face and, respectively, one arranged on said left side face; - a third type of attachments comprising four identical attachments, two arranged on said upper face and, respectively, one arranged on said right side face and one arranged on the left side face.