Single-phase natural circulation loops are used in many industrial systems like nuclear reactors, geothermal systems, solar water heating systems and process industry. The present paper presents a 1-D model for simulating the startup from rest of water cooled single-phase natural circulation loops. A pseudo-conductivity model was developed previously by the authors to account for heat diffusion caused by local convection currents and the results of numerical simulation for loops having horizontal heater and horizontal cooler were presented. In the present study, the model has been used to simulate the start-up of loops having different heater and cooler orientations and the applicability of the model as a general tool for simulating start-up of rectangular natural circulation loops is assessed. The paper presents more insight into the behavior of pseudo-conductivity models developed to simulate start-up of these loops from state of zero flow.