In order to maximize bird cooling in a tunnel-ventilated house, 48" exhaust fans need to operate at maximum efficiency. Dirty fan blades and shutters can reduce exhaust fan output by 30% or more. If fan screens become dusty and feather covered, fan output can be reduced as much if not more. If the belts slip, the blades turn slower and the amount of air the fans move is adversely affected as well. For instance, if the fan blades are spinning 10% slower than they should, then the fan will move 10...% less air. All of these factors add together to reduce a grower's ability to provide adequate cooling during hot weather. In fact, in the average tunnel-ventilated house, a 30% reduction in air speed would reduce the air speed from 400 ft/min to approximately 280 ft/min. This loss in air speed would result in the effective temperature (taking into account windchill effect) rising between three and six degrees.