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Why Must I Open The Window When Using A Fan But Close It When Using An Air-Conditioner?

By: Ebenezer Sarfo Adu

We all are aware that we live in a Country of a hot climate, and which is why almost everyone wants to have air-conditioning units inside their homes. However, the Air-conditioner is one of the high energy-consuming appliances, making it expensive to use. Most of us will therefore go for fans to cool our rooms for economic reasons. Fans and air-conditioners operate with different principles and this can be confusing for some users.

The air-conditioner extracts heat from a confined space, by cooling the air in that environment. This is how the Air-conditioner works:

An Air-conditioner (AC) has a compressor inside the unit which is connected to a cooling tube. The compressor compresses air in the room which is stored in the unit. Due to the compression, the air becomes hot and develops high pressure. This high pressured hot gas runs through a set of cooling tubes so it can dissipate the heat, eventually turning the gas into liquid. This liquid runs through an expansion valve, and in the process evaporates to become low pressured gas. This cool gas runs through a set of tubes in a coil that allows the gas to absorb heat and cool down the air surrounding the coil of the tubes. A fan blows the air over the set of cool tubes, blowing the cool air into the room. This reduces the temperature of the room, making it cool.

From this, we learn that like in a vicious cycle, the AC takes in air from the room, processes it into cold air, and feeds the room back with it. It is therefore advisable to keep the room enclosed to reduce the amount of air that enters the room to be cooled just like a refrigerator.

The AC is designed to operate as a closed loop. If you have the windows open, you are drawing in humid, warm air from outside and forcing your AC unit to "condition" that air as well. If you close the windows, the AC unit recirculates the air in the house, extracting moisture and cooling it much more efficiently.  In addition to cooling efficiency, leaving the windows open will also put stress on the AC, and causing it to consume more electrical energy.

On the other hand, Fans do not actually cool air (if anything, electric fans warm the air slightly due to the warming of their motors), but work by evaporative cooling of sweat from the human body and increased heat convection into the surrounding air due to the airflow from the fans. Thus, fans may become ineffective at cooling the body if the surrounding air is near body temperature and contains heat. Fans are mainly used to circulate air in the room. This is the reason why the windows have to be opened to bring fresh air into the room for the fan to work effectively.

The cooling sensation that you feel when you stand under a fan is also quite interesting. It feels as if the room is becoming cooler, but in fact, it's your body that is cooling. The reason it feels cooler is because the downdraft of air is actually helping to evaporate the perspiration on the surface of your skin and pushing away the body heat that you expel.

You'll also only want to use your fan when you're in the room. As discussed above, your ceiling fan doesn't actually make a room cooler. The cooling effect is felt on your body, not in the actual temperature of the room, so leaving your fan on in an empty room is a sure way to miss out on the energy saving tips, or the economic benefit of using a fan instead of an Air-conditioner. 


   About the Author

Ebenezer Sarfo Adu is an Electrical Engineer and a Strategist with the Corporate Planning Division.

He is currently the Team Lead for ECG's Digital Media Strategy. 




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