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Subject:  Re: ceiling fans Date:  2/4/2004  1:12 PM
Author:  Radish Number:  45688 of 137404


The house has air conditioning. How much of a difference do ceiling fans make in terms of comfort and energy savings?

The principal function of a ceiling fan is to blow air directly on you. When air blows on you, it increases the rate at which heat leaves your body (assuming the air is cooler than you). Humans cannot actually feel temperature; you feel the rate at which heat flows out of or into the skin. That's why when you touch a cool piece of metal it will feel colder than a cool piece of cotton even if they are actually the same temperature.

So, in the summer, when the ceiling fan blows directly on you, you feel cooler. And your skin is cooler, because heat leaves it more rapidly.

However, when you're not in the room, what happens? The fan draws power. Everything that draws electricity creates only heat, light (which turns into heat if it doesn't leave the room), electromagnetic radiation (which also turns into heat if it hits anything conductive), and/or mechanical energy. In the case of the fan, the electromagnetic radiation it creates is miniscule, the fan motor gives off heat, and the mechanical energy is moving air, which turns into heat as it hits the walls and floors.

In other words, when you're not in the room, the fan functions exactly the same as a space heater. It turns electricity into heat. People often don't understand this, and think that a ceiling fan cools a room.

So, in the summer you want to turn on ceiling fans only when they will be blowing directly on people. Turn them off when no one is in the room, as they only add load to your air conditioner.

In the winter, the ceiling fan is not a very efficient space heater. I'm not sure about the claims that you can move hot air down from the ceiling. I suspect that would depend a lot on the height of the ceiling and the positioning of the air-returns (if you have a forced-air heating system). For 8-foot ceiling heights, my experience has been that the air moving across the people below the fan has more of a chilling feeling than the benefit, if any, of moving hot air down. In theory, you run the fan the other direction in the winter to reduce the airflow directly across people, but in my experience air moves across you even when you run the fan the other way.

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