Thermally activated building system (TABS) can be operated at relatively higher water temperature. Hence, it can be coupled with passive cooling systems. This paper investigates the influences of three design parameters on thermal comfort of TABS using COMSOL Multiphysics, a computational fluid dynamics (CFD) tool. For the same inlet velocity, an increase in the pipe inner diameter from 9 to 17 mm decreased the operative temperature (OT), a thermal comfort index, by 1.8°C. An increase in the pipe thermal conductivity from 0.14 to 1.4 W/mK reduced the average OT by 2.5°C. However, a further increase in thermal conductivity had no significant influence. For cooling pipes embedded at a constant depth, an increase in the thickness of both roof and floor from 0.1 to 0.2 m delayed and reduced the maximum OT by 48 minutes and 0.3°C, respectively. © 2019, Indian Academy of Sciences.