Abstract: An air flow control valve including a valve body having an internal cavity and an inlet and outlet in flow communication with the internal cavity, the axes of the inlet and outlet being aligned at substantially 90° to each other, a valve seat located within the cavity and co-axially aligned with the outlet, the valve seat being located on the distal end of an upstanding tubular pedestal which is formed around the outlet, the internal cavity being configured so as to define an annular space around the pedestal with which the inlet is in flow communication; a flexible generally planar diaphragm mounted above the valve seat, the diaphragm supporting a valve closure member which is adapted to engage the valve seat to close the valve, valve control means for causing the diaphragm to move towards and away from the valve seat to close and open the valve respectiely; and the valve being characterised in that, with the valve oriented with the inlet axis horizontal and the outlet facing downwards, the valve seat is