HVAC systems are any systems that deal with heating ventilation and air conditioning. These systems are sometimes called central heat or central air conditioning systems and use air ducts by which to move the air throughout the building. At one time considered a luxury, HVAC systems are now commonplace in new buildings, especially in industrialized nations.
Parts of an HVAC system include the heating or air conditioning unit, the ductwork, and vents, both intake and outtake. The intake vents return air to the the heating and cooling units through the ducts. The outtake vents distribute heated or cooled air throughout the rest of the home. A thermostat is also considered part of the system.
Most HVAC systems are powered by air conditioning units and furnace units. In some cases, one or the other may be the only one present. For example, at higher latitudes, it is common for older homes to have only a furnace. Air conditioning is not needed during many times of the year and was considered a needless luxury. While most homes now have complete HVAC systems, there still may be a few that don't.
HVAC systems may be powered a number of different ways. For the furnace, natural gas is, by far, the most popular option, followed by some type of heating oil and finally electricity. Often, the economics dictate what type of fuel will be used in a certain area. Most homeowners will obviously choose the cheapest, but most effective, option possible. Most furnaces are rated for their efficiency on a standard scale which can be compared to other models.
For the air conditioning part of HVAC systems, the most common power source is electricity. The cost of electricity can make cooling a home a very expensive proposition. Often, people may make a trade off, accepting a less efficient unit for heating or cooling depending on the part of the system that will be idle throughout a greater part of the year.
HVAC units are often rated by looking at a specific measurement. There are different measurements for different units. For example, the annual fuel utilization efficiency (AFUE) rating is used for furnaces. Anything below an 80 is considered a fairly inefficient unit. Anything rated 90 or higher is considered a super efficient unit. The seasonal energy efficient rating (SEER) is often used for air conditioners. The scale for a SEER is slightly different, but a higher rating is good. All new air conditioners in the United States must meet a standard rating of at least SEER 13.