Increasing energy demand for domestic space cooling in Kerala, India, necessitates innovations in the passive systems. Earth tube heat exchangers (ETHE) are gaining popularity in the architectural practice of the region. Owing to the higher specific heat of water and the other underlying principles of the ETHE, there is a possibility of developing a similar system by replacing earth with water as the heat exchange medium. A conceptual water tube heat exchanger (WTHE) system was designed, which consists of a 45 m long PVC pipe staggered in an underground reinforced concrete rain water harvesting tank connected to a house. Performance of the system was assessed using the ‘Psychrometric Chart+Duct Calculator V4.3 simulation tool. Later, this system was developed on site and its performance was analysed. The simulation predicted a maximum difference of 4.7°C between the inlet and outlet air temperatures. However, a difference of 8.4°C was achieved by the proposed WTHE system built.