In warm air heating, furnace is a device for the useful application of heat produced by the combustion of fuel or by the conversion of electrical energy. A great variety of furnaces results from the many uses to which their heat produced is put.
For space heating, the warm-air furnace is either a cast iron or steel heater enclosed in a metal casing. The so-called gravity type furnace is located on a level below the space to be heated. At the base of the furnace is a large duct through which cooled air is delivered from one or more cold air faces on the floor of the space to be heated. The cooled air, because it is heavier, flows to floor level and through the cold air return ducts to the area between the heater and the casing of the furnace. The exterior of the heater is designed with flanges and deep corrugations to increase the area of heat-radiating fire pot over which the air current passes. As the air passes through the furnace, it is heated through contact with the heated surfaces and then, being lighter, rises through warm-air pipes to the space to be heated.
In the case of the forced type warm air furnace a power-driven fan pulls the cooled air from the space to be heated and blows the warmed air in. As a result an installation of this type may be made on the same level as the space to be heated.
Coal, either bituminous or anthracite, coke, oil or gas may be used as fuel for furnaces. Specially designed furnaces are available for the most efficient use of gas and oil, as well as conversion burners which will utilize these fuels in installations designed for coal. Furnaces are frequently equipped with humidifiers to moisten the air. Modern forced air furnaces may also be used to circulate the cooler air of the basement area in which they may be located throughout the structure for summer cooling. By means of an electrical thermostat operated manually or by
means of a clock, the furnace can be operated from a remote point so that it requires no attention other than regular firing. Where oil or gas is used, supplying of fuel to the furnace is automatic; hence, no attention is required to maintain any desired temperature over extended periods.
