Ask Pablo

Summertime, and the living ain't easy in the heat. Should I run the air conditioner or electric fans?

Topics: Environment, Global Warming, Science,

Dear Pablo,

It’s too damn hot to sleep at night! To cool our bedroom on these summer nights, my husband and I can run either three fans or one air conditioner. Which is better for the environment?

Let me first answer the question and then I will discuss some alternatives. Based on additional information from the reader, I learned that each fan uses 50 watts of 120 volt electricity (150 total), while the air conditioner uses 530 watts of 120 volt electricity. Already we can see that the three fans use 28 percent as much energy. If the three fans are truly comparable in the level of comfort they provide, by all means, use the fans!

Fans work in three beneficial ways:

  • Once the sun has gone down, and things begin to cool off, they bring in the outside cool air. They can also vent a hot attic space that might otherwise radiate heat into your living space throughout the night.
  • Stagnant (nonmoving) air creates a boundary layer around your skin. Because air is an excellent insulator (in fact, wall insulation is mostly air), this layer acts to impede the loss of excess heat. Even a light air current from a fan can significantly lessen the thickness of this boundary layer, allowing excess body heat to escape.
  • Sweat creates an evaporative cooling effect that is limited both by the skin’s ability to produce sweat and by the air’s ability to take up excess moisture. Creating airflow allows more air to contact the skin and facilitates evaporation.

  • Air conditioners work in a different way. Inside an air conditioner is essentially a heat pump. This device uses the expansion of compressed gases to absorb heat energy from the indoor air (similar to how a compressed air canister or can of whipped cream gets colder as you use it), and then compresses the gases again to release that heat energy outside. Because a compressor is involved, some of the electricity used is lost to friction, heat and other inefficiencies.

    Fans only use efficient electric motors, and so most of the electricity is converted directly into air movement. Interestingly, since there is some waste — heat generated by the electric motor — a fan in a closed room will gradually warm the room, but not by a significant amount. Using a fan can direct the cooling effects of air movement precisely where it is needed, namely straight at you, whereas an air conditioner works to cool the entire room. This is analogous to lighting. You can either light up an entire room in order to see what you are doing or use task lighting and save a considerable percentage of electricity.



    Fans and air conditioners are not the only options. If humidity is contributing to your misery, you can use a dehumidifier to dry the indoor air and significantly decrease the heat index (the summertime equivalent of the wind chill factor). A machine called a swamp cooler uses the evaporative cooling effect by evaporating water with just a fan and a circulating pump. While it is less efficient than an air conditioner, you can always put a bowl of ice cubes in front of a fan in a pinch. And when all else fails, just get out of the house! Many public places such as coffee shops and bookstores are going to be air-conditioned anyway, so you might as well enjoy an iced tea and a good book until it’s cool enough outside to go home and open the windows.

    In addition to electrical means, there are some other ways to keep your house cool, and that’s by keeping the heat out in the first place. Blinds may keep out the bright sunlight, but once the rays have passed through the glass, much of it turns into heat energy. It is more effective to keep the sunlight out of the window in the first place. Many buildings in Europe have sturdy external louvers to shade out the rays, and “green” architects are taking the sun into consideration when they design window overhangs that stop the summer sun but let in the winter sun.

    Another consideration is installing “low-e” windows. The “e” stands for emissivity and means that the glass inhibits radiative heat flow. If a major remodeling is in your future, you can even consider building materials that have high “thermal mass,” meaning that they stay at a relatively constant temperature rather than fluctuating throughout the day. Tile floors, brick walls and even large sealed water tanks can keep your home warmer in the winter and cooler in the summer.

    Got a question about the environment? Ask Pablo at AskPablo@Salon.com.

    Got a question about the environment? Ask Pablo at AskPablo@Salon.com.

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