How Does a Central Air Unit Work?

The Equipment

Central air conditioning units are made up of three basic parts. There is the condensing unit, which is normally placed outside of the house or on the roof. Next there's the fan unit, which contains both a fan and fan coils. In most homes, the fan unit is combined with the furnace to work with both heating and cooling. Finally, there is the duct work system, which transfers the air through the house. There are generally two sets of ducts. One set, called the supply ducts, takes cool air from the furnace and distributes it through the house. The second set, the return ducts, take warm air from the home and send them back to the compressor so the air can begin the cooling cycle.

Cooling the Air

A central air unit cools the home by removing heat from the air. During this process, warm air in the home is drawn through the return ducts into the furnace. As it passes through the fan coils, it is cooled by the refrigerants running through the coils. The heat from the air is trapped in the coils, where it heats the refrigerants, transforming them into a high temperature gas. These refrigerants are then sent through pipes to the outdoor compressor. The compressor releases the heat from the refrigerant into the atmosphere, which compresses the material back into a liquid. The cooled liquid is then sent back to the fan unit, where it cools more air, repeating the cycle.

Choosing a Central Air Unit

Central air conditioning units are measured in terms of capacity, which is rated in British Thermal Units (BTU's). The higher the BTU rating on the unit, the more powerful it is. This doesn't mean that you should get the unit with the highest BTU's however. If you do, you could end up paying a lot for cool air you don't really need. Instead, choose a unit that's appropriate for your home. Take the total square footage of your home and multiply it by 20. The value you calculate will give you an appropriate BTU value to aim for. For instance, if you live in a 1,000 square foot home, you should look for a central air unit that is around 20,000 BTU (source: Energy Star).

Resources

• Energy Star Guide to Sizing an Air Conditioner