A drain valve allows for drainage of fluids while preventing backflow of gases from a drainage system through the drain valve. A drain valve can have a first, sealed position in which a component of the valve seals an opening through the valve and a second, open position in which the component of the valve is separated from the opening to allow a fluid to drain through the valve. When the fluid has drained through, the component of the valve can return to the closed position. The valve can include a pressure relief conduit to help equalize pressure across the valve and permit unrestrained draining of fluid through the valve.