Abstract: A redundant blower unit is installed in a rack above another unit containing heat-emitting electrical elements (such as CPU cards) in order to draw air through the underlying unit and out to atmosphere. Four blowers in the blower unit draw air from a plenum, each blower discharging into an individual exhaust duct leading to the exterior. Two blowers are positioned at a first tier relative to the bottom of the plenum and the other two at a second tier higher than the first tier. If one blower fails, its exhaust duct may malfunction as an air inlet, in which case the incoming air is mixed with exhaust air and discharged through the other blower in the same tier as the failed blower. Thus the efficiency of the two blowers in the other tier is not impeded.