Experiments were carried out in a small scale compartment by introducing different mixtures of air and helium at the top of the compartment to physically model the flow of a buoyant fluid through a horizontal ceiling vent. The ratio of air to helium was used to vary the apparent temperature of the layer. The apparent temperature is the temperature at which air would have the same density as the buoyant layer mixture. In this way a temperature range of 300 to 1100°K was simulated. It was found that when the layer depth was equal to or greater than the vent diameter the buoyant layer was stable and the discharge coefficient was within 15 percent or less of the expected value of 0.60. When the layer depth was less than the vent diameter the layer became unstable and the discharge coefficient decreased to as low as 0.18. It was determined that when the layer depth is less than the vent diameter a weir-type flow condition results where the exiting gases do not fill the entire opening. Previously horizontal vent flow was always treated as orifice-type flow with a discharge coefficient of 0.60.