Take a good look at your ceiling fan and you'll notice -- especially if you have a top-quality one -- that the blades are arranged at an angle. This angle ensure that the blades actually push air when they turn instead of simply creating a static vortex. When it's hot out, you want the fan to push air toward you, but when the temperature is cold, it's better for the fan to push air toward the ceiling, because this improves the circulation of warm air from the heater.
How a Fan Cools
Despite what you might think, a ceiling fan doesn't actually make a room cooler. By circulating air, the fan only makes the room feel cooler, because air movement promotes evaporation of moisture from the body. This is a good thing in summer, when warm temperatures and high humidity make people sweat. But if no one is in the room, the ceiling fan won't create any lingering cooling effect, although the air circulation can help control condensation and resulting mildew. If you don't have a mold problem, save energy by turning off the fan when no one is using the room.
The Ceiling Fan in Winter
Dispelling another possible misconception, a ceiling fan can actually be useful in winter when you need to warm up instead of cool off. Running the fan in the reverse direction draws cooler air from near the floor and blows it toward the ceiling. At the same, the fan moves warm air trapped near the ceiling and pushes it out toward the walls and down to the floor. The overall effect is to evenly distribute warm air that would otherwise collect near the ceiling, making you feel warmer with less output from your heater. This process is particularly effective in rooms with high, peaked ceilings, such as vaulted or cathedral ceilings, where a lot of warm air can collect at the peak. Always run a fan on low speed in winter, to prevent any cooling effect created by strong air currents.
Forward in Summer, Reverse in Winter
A ceiling fan must be a reversible type to run in both directions. Some reversible fans have a small toggle switch on the motor housing. With others, you can change the direction using a wall switch or remote control. Determine the forward direction by looking at the tilt angle of the blades -- air hits the top part of the blade, follows to the lower part and gets pushed into the room. This happens when the blades are spinning counterclockwise. Reversing the fan so the blades spin clockwise pushes air up toward the ceiling and warms the room. You usually don't have to worry about determining directionality. Just use the switch and remember: Forward in summer, reverse in winter.