An experimental investigation comparing Monodraught’s Classic and X-air windcatcher natural ventilation systems, in a proven wind tunnel, using 1:10 scale models of the systems, is presented. The University of Nottingham hot wire technique, developed initially for measuring the reverse flow percentage in ventilation stacks, is used for the comparison. The experiments investigate the effect of wind speed and direction, the effect of local obstructions, and different roof geometries. The windcatchers perform well in a variety of conditions, and the X-air design is found to give a significant improvement on the Classic design.