Stack Effect Calculator Chimney Shape Square Circle Area of Flue: square feet. inches. centimeters. Height difference: feet. Indoor Temperature: C Outdoor Temperature: C Flow Rate: CFM*. * Cubic Feet per Minute Heat Transfer Rate: BTUH, which is about Watts. That’s enough energy lost every hour to: Run a tumble dryer for hours Leave a hair dryer on for hours Run a grill for hours Or run a electric heater for hours * Based on average power consumption of each appliance. Source This tool is not available on mobile devices. Please check back on desktop.