How Does a Central Air Unit Work?
Learn the process behind evenly distributed climate control
By Brad Painting
A central air unit cools an entire building by producing cold air at a central location. This saves energy and produces more evenly distributed cooling than using individual window units. A central air unit may be built into the ductwork in conjunction with a furnace, or it can be a standalone system, as is usually the case with a heat pump.
The parts of a central air conditioner can roughly be divided between those that help generate a cold refrigerant and those that distribute air to the living space. The refrigerant undergoes a thermodynamic cycle with help from a compressor, which moves the refrigerant, and two sets of coils known as a condenser and evaporator used for transferring heat. The distribution of air rests on the functions of the fan (or "blower"), duct systems, and supply and return vents.
The refrigerant acts like a sponge, sucking up heat from the indoors and squeezing it out to the outdoors. This absorption and rejection of heat occurs in the coils and moves through the cycle via the compressor's propulsion. During the refrigerant's cold phase while indoors, it passes through the ductwork, where it absorbs heat from the air propelled by the blower. This movement and pressure created by the blower carries the cold air to the supply vents, where it enters the building interior. The same pressure that forces air into the occupant space also sucks air into return vents and through return ducts so that the process can continually occur.
Every part of a building that is treated as a separate thermal environment, and therefore has its own thermostat, is known in the heating and cooling industry as a "zone." Zones are useful because occupants prefer some rooms, like bedrooms, to be a little cooler than the general living space. This can be accomplished either with one cooling unit for each zone or with one unit overall and motorized dampers within the ductwork that dampen off airflow to certain vents.
Air conditioners obviously lower a building's temperature, but it is seldom known that they invariably lower humidity as well. Because the air unit's coils are very cold, moisture in the air condenses on them as droplets of water. Some air units have built-in humidifiers to compensate for this.
Homeowners should be aware that central air units require some maintenance. Air conditioners contain filters to strain out small particles that the air can pick up from the ductwork. Over time, these filters can become clogged and lower the system's efficiency by 5 percent to 15 percent. The indoor and outdoor coils gather dust and make it difficult to exchange heat with the environment, so they need to be periodically cleaned by an HVAC technician.
About the Author
Brad Painting is a regular contributor to DexKnows and specializes in green building design.
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