The ventilation equipment for enclosed spaces or office rooms is specified according National Building Code of India published by the Bureau of Indian Standards. Natural ventilation peri-odically together with mechanical ventilation is recommended to remove pollutants. The need to study fire and smoke behavior inside a completely closed room with air intake and exhaust vents becomes important in case of low or no mechanical ventilation service. An experimental study on unsteady heptane pool fires of different sizes in their initial stages was conducted in a cubical fire test chamber of 27 m3 inside dimensions. The compartment was naturally ventilated with a typical configuration of a vertical intake on a side wall and an exhaust vent at the ceiling leading into a duct. Three circular pans of diameters 0.34, 0.47 and 0.61 m were employed to generate the fire with n-heptane as fuel on a bed of water. Temperatures, wall heat fluxes and mass loss rate were measured. The flame was visualized using a video camera through a tempered view glass. The total heat transfer to the ceiling and wall increased with the increase in fire size as the flames became taller in the initial stages (3-4 minutes) with sig-nificant increase in case of 500 kW fire. The smoke layer was observed at about mid height (1.5 m) above floor. The leaning behavior of flames was seen due to naturally induced air inflow. The wall heat flux of about 50 kW/m2 obtained indicate hazardous environment for further flame spread. A fourfold increase in mass loss rate was observed with just 2.5 times increase in fire size inside the ceiling vented compartment.