In the kitchen environment, pollutant fumes of the cooking process are released into the ambient air by the convection plumes. The practical problem is to compute the requested extract air flow rate to maintain good indoor air quality in an energy efficient manner. In the most accurate design method, the design of a kitchen ventilation system is based on the flow rate of the thermal plume. In this method, the amount of heat carried in a convective plume over a cooking appliance at a certain height is calculated. The heat load is then assumed to be a point heat source and the velocity and temperature profiles are approximated to be Gaussian distributed. In commercial kitchens, the location of the extraction point is at a height of 0.9-1.4 m above the heat source where the convection flow is not yet fully developed. This paper demonstrates that the generic plume equation, derived in the region of complete flow similarity, is not accurate in this intermediate zone. However, it gives a reasonable accuracy for practical applications when an individually adjusted empirical factor of the virtual origin is applied. The power intensity of the heat gain has a much more significant effect on the plume characteristic than the previous studies indicate. The plumes are narrower and the spreading angle is smaller with higher heat gains. (C) 2006 Elsevier B.V. All rights reserved.