Thermally Activated Building System (TABS) provides not only better thermal comfort but also good indoor air quality. It can be coupled with passive cooling systems as it can operate at a relatively higher water temperature. Even though TABS is a promising energy-efficient and eco-friendly system, the influences of various design and operating parameters on the indoor thermal comfort of a building with TABS are not well understood. Hence, the influences of cooling surfaces (area), shading, natural ventilation and ceiling fan on the performance of cooling tower based TABS were investigated in an experimental room of dimensions 3.5 m × 3.5 m × 3.15 m. The increase in the number of cooling surfaces decreased (favourable) the thermal comfort indices and advanced the time at which their maximum and minimum were reached. The average predicted percentage of dissatisfied was 89% if roof alone was cooling. This reduced to 20% if all the surfaces of the building were cooled. Shading of the roof reduced the maximum operating temperature of the indoor space by 0.9 °C when all the surfaces of the building were cooled with TABS. The use of ceiling fan increased the indoor operative temperature marginally. However, the increase was less than the neutral temperature offset of 2.6 °C that was achieved by higher air movement. Thus, the use of the fan results in a better indoor thermal comfort. Natural ventilation advanced the occurrence of the maximum temperature of indoor air by 1½ and 2¾ hours for fan off and fan on cases respectively. It also advanced the extrema of the other indoor comfort parameters. The cooling tower based TABS was able to achieve thermal comfort even in unfavourable warm and humid tropical climates under certain operating conditions. © 2018 Elsevier Ltd