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Height of the fan relative to the ceiling. If a fan is too close to the ceiling, the airflow is restricted; that is, the fan will not be able to draw as much air through its blades as it has the potential to do. For this reason, "hugger"-style fans (those which mount directly to the ceiling without the use of a downrod) are all inherently disadvantaged. The distance that a fan should be mounted from the ceiling is directly correlated with its air-moving potential; no fan should be mounted with its blades closer than 24 inches to the ceiling, however that figure is often far greater with industrial fans. Unfortunately, this is often impossible in household situations due to the fact that a minimum ceiling height of nine feet would be required to meet safety codes ("blades must be mounted a minimum of seven feet from the floor", and 8 or more feet is typically desired).

And another:
To achieve maximum cooling, a ceiling fan has to move lots of air. The distance the fan is from the ceiling and the tilt or pitch of the fan blades must be optimized. If a ceiling fan is too close to a ceiling, say six inches or less, it can't easily get the needed air to push down towards the floor. Just the same, the fan blades must have a tilt to them so they cut through the air and push it down. Airplane and ship propellers operate the same way. They slice through air or water to push objects that weigh many tons.