In many parts of the world, heating is of primary importance, while the climate makes the need for cooling inconsequential. In other parts of the world, such as the American South, cooling systems are a perceived necessity of life. The combination of heating, ventilating and air conditioning (HVAC) a dwelling or commercial building is epitomized by the use of central heating and air conditioning units.
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Central heating systems are nothing new. The Romans pioneered the concept in the first century. Their hypocaust system allowed furnace-heated air to travel through passageways under floors and into the rooms through pipes. In the 18th century, Swedes were the first to incorporate hot water for heat distribution, and the British were the first to install steam central heating units in buildings in the early 1800s. But it wasn't until the 20th century that central air conditioning made its debut.
Central heating units are typically divided into three main categories: forced air systems, gravity systems and radiant systems. Forced air systems rely on an electric blower fan to force the heated air through a system of ducts to reach the desired location. Gravity systems rely on the thermodynamic properties of heated air to rise to distribute heat to rooms that are typically placed above the furnace. Radiant systems rely on circulating hot water, steam or electrical elements situated in the walls, under the floors or in the ceiling to distribute heat.
Refrigerated air and heat pump systems are the most common forced air central cooling systems. Refrigerated air systems run off electricity and use the basic principles of refrigeration to cool air surrounding the unit's coils. This air is then blown through the duct work to the appropriate room by an electric fan. Heat pumps have the ability to cool a building by extracting, or pumping, heat from within the home and expelling it outside. The process works in reverse when the need for heat arises. Heat pump systems also rely on electric blower fans to distribute the cooled or heated air throughout the building.
Geothermal central heat and air conditioner units are becoming more popular. These units operate on the same principle as the heat pump, but instead of relying on the outside air as a source of heating or cooling, geothermal units rely on the constant temperature of the earth. These units employ a system of tubes that are buried up to eight feet underground. The advantage of geothermal units is their ability to provide extremely cost-effective heating and cooling since they use no fossil fuels to power their heating and cooling mechanism.
All new central air conditioners are measured for efficiency and assigned a comparative rating called the Seasonal Energy Efficiency Rating (SEER) which was developed by the Air-Conditioning, Heating and Refrigeration Institute (AHRI). In 1992 the federal government began establishing SEER ratings for air conditioning units manufactured in the U.S. At that time, the minimum rating was 10. In 2006, the standard was raised to 13. By the end of 2008 central air conditioner manufacturers were producing units that achieved SEER ratings of up to 24 and the efficiency of those units are up to 75 percent more efficient than comparable units manufactured as recently as 2003.