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A furnace is a device for heating air or other materials. Typically the heat is produced from combustion of some fuel, although it may alternately be electric. Industrial furnaces are used in the extraction of metal from ore, smelting and firing clay to produce ceramics. The term is also used, primarily in North America, to describe household heating systems based on a central furnace.

A household furnace is major appliance that is permanently installed to provide heat to an interior space through intermediary fluid movement, which may be air, steam, or hot water. The most common fuel source for modern furnaces in the United States is natural gas, other common fuel sources include LPG[?] (liquid propane gas), fuel oil, coal or wood.

Furnaces always need to be vented to the outside. Traditionally, this was through a chimney with a large fraction of the energy of the fuel lost through the chimney. Modern high-efficiency furnaces can be 98% efficient and operate without a chimney. The small amount of waste gas and heat are mechanically ventillated through a small tube through the side of the house.

The heat is transferred from the furnace through an intermediary distribution system. If the distribution is through hot water (or other fluid) or through steam, then the furnace more commonly termed a boiler.

A condensing furnace (Larger Version)

Most modern furnace installations in the United States used forced-air heat, where ductwork carries air across the heating coils of the furnace, whence it is blown throughout the building. One major advantage of this type of system is that it enables easy installation of central air conditioning. If you avoid the common mistakes, condensing furnaces typically can deliver heating savings of 20%-35% assuming the old furnace was in the 60% Annual Fuel Utilization Efficiency (AFUE) range.

In such an air (convection) distribution system, a cold air return feeds the cooler incoming air into the heating chamber, where it passes into a plenum, or chamber, from which it goes into the ductwork to various parts of the building.

Air convection heating systems have been in use for over a century, but the older systems relied on a passive air circulation system where the greater density of cooler air caused it to sink into the furnace, and the lesser density of the warmed air caused it to rise in the ductwork, the two forces acting together to drive air circulation in a system termed "gravity-feed".

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