NOVELTY - Damper for reversing pressurized air flow comprises a housing, an inlet, and exhaust port with several rotating vanes rotating. The vanes are coupled to the inlet and outlet of a centrifugal blower and can direct outside air to the blower intake and direct the blower discharge to aeration process. When reversed to provide suction, the vanes direct the blower discharge through the exhaust port.    USE - For use in the composting process.    ADVANTAGE - The reversal of the aeration mode is effected by a single actuator which makes automation of aeration reversal practical. The automated reversal of aeration increases the probability of reducing pathogens throughout the pile and thus has primary benefit to public health. (1) forms a compact rigid unit with (2), making it easy to install and requires little space and little external ducting as compared with the prior art. (1) has less pressure drop than the prior art, reducing blower horsepower and energy consumption.    DETAILED DESCRIPTION - A damper (1) for reversing the flow of air from a blower (2) having an air inlet (2A) and an outlet (2B) comprises:    (a) a housing (1B) having a central port (3) communicating with (2B) of (2);    (b) an inlet port (4) communicating with (2A) of (2);    (c) an exhaust port (5);    (d) (3), (4) and (5) are in communication with one another through passages; and    (e) first, second, third and fourth vanes rotatably engaged within the ports.    In a first mode of operation, the first vane blocks the passage between (5) from (3), the third vane blocks the passage between (4) from (3). In a second mode when the first, the second, the third and the fourth vanes are rotated, the second vane blocks (3) and directs the air flow of (2) out of (5), connecting (3) to (4). The fourth vane blocks (5). Thus in the first mode of operation (2) receives air through (4) and blows air out of (3) while in the second mode of operation (2) receives air through (3) and blows air out of (5).    INDEPENDENT CLAIMS are also included for the following:    (1) a method of directing the flow of pressurized air in an air distribution system comprising:    (i) connecting (2B) to the air distribution system through (3) in the first mode of operation;    (ii) connecting (4) to (2A) in the first mode of operation;    (iii) providing the four rotatable vanes disposed substantially parallel to one another in (1B); and    (iv) rotating the first, second, third and fourth vanes to the second mode of operation; and    (2) a composting aeration device comprising:    (i) (2) producing a flow of air;    (ii) an air distribution system extending through the material to be composed;    (iii) (1) for reversing the flow of air from (2) to and from the distribution system; and    (iv) (1B).