5 n7 A valve assembly for a combined plants adapted for providing both heating and sanitary hot water comprising a gas-fired boiler (1), a heat exchanger (2), a water/water heat exchanger (17) that can be included through a three-way valve (4) in the circuit of the heating water, at least one radiator (R), and a pump (P) for circulating the heating water, such assembly being formed by two sub-assemblies or valve units. The first unit (I) comprises an integral metal block in which there are formed or mounted said three-way valve (4), a pipe fitting (3) for the connection with the main heat exchanger (2), a fitting (5) for the connection with the radiator (R), an outlet fitting (9) for the domestic water, two pipe fittings (22, 23) for the connection with the water/water heat exchanger (17), a pressure switch (15) and a by-pass valve (16) with an associated outlet fitting (10). The second unit (II) comprises an integral metal block in which there are formed or mounted a pressure difference device (14), a pipe fitting (6) for the connection with the radiator (R), an inlet fitting (8) for the domestic water, an outlet fitting (7) towards the pump (P), two fittings (32, 3) for the connection with said water/water heat exchanger (17).