Of the methods of heating dwellings other than by stoves, that of the hot-air furnace is the most common. Of the various modes of furnace heating it is the least expensive in first cost and most rapid in effect. In the use of steam heat, the water in the boiler must be vaporized before its heat is available. With hot-water heating, the whole mass of water in the entire system must be raised considerably in temperature before its heat can affect the temperature of the rooms, and consequently in first effect it is very slow. In the use of the hot-air furnace the heat from the register begins to warm the rooms when the fire is started. Hot-air furnaces are made by manufacturing companies in a great variety of styles and forms to suit purposes of every kind. In practice the furnace is built in sizes, to heat a definite amount of cubical space. The maker designs a furnace to heat a certain number of cubic feet of space contained in a building. It must be sufficiently large to keep the temperature at 70°F. on the coldest nights of winter when the wind is blowing a gale. It is evident that with the variable factors entering the problem, the designer must be a person of experience in order that the furnace meet the requirements. The following table taken from a manufacturer’s catalogue shows the method of adapting the product of the maker to any size of dwelling. The volume of the house is calculated in cubic feet and from this result the size of furnace most nearly suited is selected from the table. ------------------------+--------+--------+--------+--------+-------- Furnace number | 1 | 2 | 3 | 4 | 5 ------------------------+--------+--------+--------+--------+-------- Weight without casing, | 984 | 1,111 | 1,340 | 1,531 | 1,934 lb. | | | | ------------------------+--------+--------+--------+--------+-------- Estimated capacities in | 8,000 | 12,000 | 20,000 | 35,000 | 60,000 cubic feet | to | to | to | to | to | 12,000 | 20,000 | 35,000 | 60,000 |100,000 ------------------------+--------+--------+--------+--------+-------- Capacity in number of | | | | | rooms of ordinary size | 3 to 5 | 5 to 7 | 7 to 9 |9 to 12 |12 to 15 in residence heating | | | | | ------------------------+--------+--------+--------+--------+-------- CONSTRUCTION [Illustration: FIG. 45.--Interior view of a hot-air furnace.]