Nearly every home and building has heating needs, and the most common heating method in modern times has been a central furnace. A central furnace usually heats air or water by burning a fossil fuel inside an enclosed metal casing. The heat is then distributed through ductwork by forced air, or through pipes in the case of water. The most common fuel for central furnaces in the United States is natural gas, though coal, wood, and heating oil are also common furnace fuels.
Heating done by a central furnace, also called central heating, differs from local heating, mainly in that the heating occurs in one place that gives heat to the whole structure. This fact offers the advantage of only having to add fuel at one central location, in heating systems that require refueling. Also, while fireplaces and other types of one-room local heating are powerful heat sources for one room, they tend to draw cold air in from outside, making the house's outer rooms colder. A central furnace both draws from and forces air into the home, eliminating these types of problems.
There are records and evidences of the use of central heating which date back thousands of years. During this time, the design of the typical furnace has been simplified and somewhat standardized. A furnace must have a few essential components, organized in a way to maximize the furnace’s efficiency. In a typical central furnace, the actual heating takes place in the center of the unit, with air ducts at the top and the bottom.
At the top is the supply duct, through which air is forced into the ventilation system. Some units, especially those is dry climates, will have a humidifier on this part of the duct. The middle part of the furnace, where the combustion chamber is located, has two pipes attached to it. One brings in air for the combustion of the fuel; the other takes exhaust air away. Every central furnace must be exhausted outdoors, usually through the roof or side of a house, though modern furnaces produce only a little waste gas and heat.
The system of ventilation ducts in a house ends with the return duct that attaches to a central furnace at the bottom. Air is drawn from inside the house to be heated, sometimes after passing through a filter mounted on the return duct. Furnaces that are 90% efficient or more extract so much heat that water condenses inside the furnace housing. This water must be drained through a small hose which is also located at the bottom of the furnace, often opposite the return duct.