An approximate method is presented for calculating the effective length of a flat plate heat pipe when a strip heater is partially covering the evaporator section. For a specific width of the strip heater, the optimal position to achieve a minimum effective length is found when the heater is placed symmetrically at the centre of the evaporator section. In this position, a higher value of the capillary heat transport limit can be achieved as compared to the case where the heater is placed on one side of the evaporator section.