Modern buildings and houses are heated by stoves, steam, hot water, or furnaces. The choice of any particular method will depend upon special conditions and requirements. Heat is given from a stove by radiation (Fig. 172); that is, the stove becomes hot, due to the burning of coal, and the metallic parts radiate the heat. A stove is not an economical means of heating, because much of the hot air goes up the chimney and is wasted. Moreover, to heat a home it is necessary to have a stove in each room. A large furnace in the cellar overcomes these drawbacks, and is consequently used in most houses. From the furnace, hot air is distributed through ducts to the different rooms. Such a furnace draws in fresh outside air and passes it into a dome over and around the hot coal. As the air becomes hot, it expands, and thus makes its way to the several rooms.

Steam radiates its heat with ease, and also condenses very rapidly. Heat given off by a steam furnace is called steam heat and may be provided directly or indirectly. Direct heat is given off by radiators in the room to be warmed, while indirect heat is supplied by distributing throughout the building air that has been warmed by passing over radiators in the basement.

Wood and coal stoves, gas heaters, steam and hot-water radiators, coils of heated pipe, and electric resistance heaters are all examples of direct radiation. The air in a room is heated over and over again, and fresh air is admitted usually only by leakage around doors and windows or by the opening of one or the other.