From the energy equation,the required pump head rise F-A is equal to the pressure drop due to friction from A-F through the system.But to find this,only the circuit with the greatest pressure drop is chosen.
This is ABCC’DEF, the longest circuit.circuits ABEF and ACDF are ignored.(it is possible in unusual cases that one of these two shorter paths may have a greater pressure drop it is has items with great resistance .such cases must be checked.)