Results obtained from CFD analysis of 3-dimensional natural circulation loops (NCL) that employ carbon dioxide and water as loop fluids are presented for various isothermal wall temperatures of source and sink in the range of 278-341 K. Such a temperature range would be useful in various heat transfer applications of NCL, e.g. air conditioning, solar collectors, extraction of geothermal energy, etc. For the same wall temperature and geometrical parameters, comparison is made between CO2 and water in terms of heat transfer rate. Water is considered at atmospheric pressure whereas CO2 is either in subcritical (liquid) or supercritical state. Liquid CO2 exhibits very high heat transfer rate, approximately seven times higher than water, whereas performance of supercritical CO2 depends on the operating pressure and temperature. Effect of loop operating pressure on the system performance is also investigated. Results show that near pseudo-critical region, CO2 yields very high heat transfer rate, approximately seven times higher than water. Results also show that, due to the presence of bends and local buoyancy effects, fluid parameters such as local velocity and temperature vary in all three dimensions. Validation of simulation results against experimental results reported in the literature with respect to modified Grashof number (Gr(m)) and Reynolds number (Re) exhibit good agreement. Additionally, new correlations are proposed for Re in terms of Gr(m) friction factor (f) in terms of Re, and Nusselt number (Nu) in terms of Re and Prandtl number (Pr). (C) 2012 Elsevier Ltd. All rights reserved.