Distribution manifold for a fluid, comprising: a containment body internally delimiting a first conduit having a respective first inlet and a respective first outlet, a second conduit having a respective second inlet and a respective second outlet, a connection opening between the first conduit and the second conduit; wherein the first outlet and the second inlet are configured to be connected respectively to an inlet and to an outlet of a treatment or user device of the fluid, preferably to a filter; a valve operatively active in the first conduit and on the connection opening; wherein the valve can be positioned in an open position, in which it closes the connection opening and puts in fluid communication the first inlet with the first outlet to convey all the fluid entering through the first inlet into the treatment or user device, and in a partitioning position other than the open position; wherein, in the partitioning position, the valve opens the connection opening and also puts in fluid communication the first inlet with the first outlet, so that a first part of the fluid entering through the first inlet bypasses the treatment or user device and a second part of the fluid entering through the first inlet passes through the treatment or user device; wherein the valve is a ball valve and can be rotated between the open and partitioning positions; wherein the ball valve has an internal T-shaped cavity comprising a diametral through-hole and a side hole which opens into the diametral through-hole; wherein the connection opening has a calibrated passage section for distributing the inlet fluid in a controlled manner through the first inlet between the first part and the second part when the valve is in said partitioning position. a containment body internally delimiting a first conduit having a respective first inlet and a respective first outlet, a second conduit having a respective second inlet and a respective second outlet, a connection opening between the first conduit and the second conduit; wherein the first outlet and the second inlet are configured to be connected respectively to an inlet and to an outlet of a treatment or user device of the fluid, preferably to a filter; a valve operatively active in the first conduit and on the connection opening; wherein the valve can be positioned in an open position, in which it closes the connection opening and puts in fluid communication the first inlet with the first outlet to convey all the fluid entering through the first inlet into the treatment or user device, and in a partitioning position other than the open position; wherein, in the partitioning position, the valve opens the connection opening and also puts in fluid communication the first inlet with the first outlet, so that a first part of the fluid entering through the first inlet bypasses the treatment or user device and a second part of the fluid entering through the first inlet passes through the treatment or user device; wherein the valve is a ball valve and can be rotated between the open and partitioning positions; wherein the ball valve has an internal T-shaped cavity comprising a diametral through-hole and a side hole which opens into the diametral through-hole; wherein the connection opening has a calibrated passage section for distributing the inlet fluid in a controlled manner through the first inlet between the first part and the second part when the valve is in said partitioning position.