Replace worn fan belts. Fan belts do not stretch. A fan belt rides in the motor and prop pulleys on its sides. Over time, the sides of the belt wear and as a result it becomes thinner. The thinner a belt becomes the lower it will ride in the motor and prop pulleys and the slower the fan blades will rotate. The slower the fans spins the lower amount of air the fan will move. It is not uncommon for a worn belt to result in a loss of fan capacity of 25%. Since the relationship between air speed and... wind chill is exponential, a 25% loss of wind speed can reduce wind chill of possibly 50%! Keep in mind that though automatic belt tensioners reduce belt slippage and therefore increase belt life they do not eliminate the need to replace belts on a regular basis. At a minimum fan belts should be replaced once a year to insure maximum air moving capacity and therefore bird cooling.