In order to move a fluid through a network at a specified flow rate, when the coolant outlet is higher than the inlet, the difference in elevation between the points needs to be taken into account. Accounting for elevation differences is especially important if the fluid is an incompressible fluid with a high fluid density.
If the coolant inlet is at a lower elevation than the coolant outlet, positive pressure is required to move the coolant through the circuit. However, if the coolant outlet is lower than the inlet, gravity assists the flow, and the pressure required at the inlet to move the fluid at a specified flow rate is reduced by the elevation difference. In some instances the total pressure required to move the coolant at a given flow rate through a circuit may depend more upon the ground elevation profile than the actual pressure drop due to frictional and minor losses. Usually with no external energy added to a circuit, the fluid will always flow from the point of higher elevation to the point of lower elevation irrespective of the highest point within the circuit. The fluid will readily flow uphill in the circuit, provided it is powered by the downhill fall of the circuit to a lower elevation than the inlet. This is known as the siphoning effect.