An air-gas mixer device for premix burner equipment of boilers, particularly of condensing boilers, comprising: flow guide means for guiding an air-gas mixture to the intake section of a centrifugal fan with an impeller driven in rotation about a first axis, the guide means including a tubular Venturi effect conduit structure with contiguous converging and diverging portions (6a, 6b) developed in an axially symmetric manner about a second axis, a section being provided between the portions of conduit into which section the gas flow is delivered for mixing with the air flow supplied through the converging portion of the conduit, the conduit being in communication with the intake section of the fan, downstream of the diverging portion with respect to the direction of the flow, the impeller of the fan being housed in a scroll manifold including a housing for receiving the impeller and a cover plate to close the housing, a through aperture being formed in the cover plate to put the Venturi effect conduit into communication with the intake section of the fan, characterized in that the first axis of rotation of the fan does not coincide with the second axis of longitudinal development of the Venturi effect conduit, the offset between the first and second axes (X, Y) being chosen so as to allow the mixer device to have a valve unit mounted on it for supplying the gas flow into the Venturi effect conduit with a configuration in which the projection of the overall dimensions of the Venturi effect conduit and of the valve unit, in the direction of the axis of rotation, lies substantially and predominantly within the projection of the cover plate of the fan in the direction of the first axis. flow guide means for guiding an air-gas mixture to the intake section of a centrifugal fan with an impeller driven in rotation about a first axis, the guide means including a tubular Venturi effect conduit structure with contiguous converging and diverging portions (6a, 6b) developed in an axially symmetric manner about a second axis, a section being provided between the portions of conduit into which section the gas flow is delivered for mixing with the air flow supplied through the converging portion of the conduit, the conduit being in communication with the intake section of the fan, downstream of the diverging portion with respect to the direction of the flow, the impeller of the fan being housed in a scroll manifold including a housing for receiving the impeller and a cover plate to close the housing, a through aperture being formed in the cover plate to put the Venturi effect conduit into communication with the intake section of the fan, characterized in that the first axis of rotation of the fan does not coincide with the second axis of longitudinal development of the Venturi effect conduit, the offset between the first and second axes (X, Y) being chosen so as to allow the mixer device to have a valve unit mounted on it for supplying the gas flow into the Venturi effect conduit with a configuration in which the projection of the overall dimensions of the Venturi effect conduit and of the valve unit, in the direction of the axis of rotation, lies substantially and predominantly within the projection of the cover plate of the fan in the direction of the first axis.