A preliminary method to estimate the plume chimney height for a still ambient condition is presented here. The method predicts a negligible plume chimney effect on the laboratory-size test exchanger Measurements from an industrial-size heat exchanger (2.4 x 60 m(2)) in forced-draft mode have been compared with simulations incorporating the new method. The simulated heat toads agree with the 11 measurements to within +25/-40%, compared to an average of +70% if calculated by an existing formula and -75% by assuming negligible effective plume chimney height. Agreement between the new method and the measured exchanger performance did not appear to be affected by wind speeds in the range 2-6 m/s.