Abstract: This invention provides a damper construction for forced-air systems in which the damper may be opened and closed by means of a solenoid or equivalent component against a wide range of volumes and velocity. The main damper panel has a window to one side of an intermediate pivot axis, and a flap hinged to the panel and adapted to close and open the window in either direction across the panel. An air pressure differential across the panel exerts a greater turning moment on the flap side than on the other side when the flap is closed, and exerts a smaller turning moment on the flap side than on the other side when the flap is opened. Flap control means are provided which are switchable to allow the flap to act as a one-way valve in one direction across the panel, or as a one-way valve in the opposite direction of the panel.