Thermal performance of Earth Air Tunnel Heat Exchanger (EATHE) under transient operating conditions in winter season has been evaluated for predominantly hot and dry climate of Ajmer (India) using experimental and Computational Fluid Dynamics (CFD) modeling. Effect of time duration of continuous operation and thermal conductivity of soil on thermal performance of EATHE under transient conditions has been analyzed. Results show that the transient thermal performance of EATHE is significantly dependent on thermal conductivity of the soil and duration of its continuous operation. Under steady state condition, a rise of 19.6 degrees C is obtained in air passing through EATHE having 0.1 m diameter and 60 m length, at 5 m/s flow velocity, whereas, the transient analysis shows that for soil having thermal conductivity of 0.52 Wm(-1) K-1, the heating of air reduces from 19.4 degrees C to 17.2 degrees C, after 24h of continuous operation. Heating effect after 24h of operation for soil thermal conductivity of 2.0 Wm(-1) K-1 and 4.0 Wm(-1) K-1, reduced from 19.6 degrees C to 19.2 degrees C and 19.6 degrees C to 19.5 degrees C, respectively. The analyzed cases have shown the range of derating to be as minimal as 0% to as high as 64%, which if ignored while designing may lead to poor performance of earth air heat exchangers. (C) 2013 Elsevier B.V. All rights reserved.