FIRST FLOOR -----------+-------------+------------+----------- Sizes of | Diameter of | Size of | Height of registers | pipes | rooms | ceilings in inches | in inches | in feet | in feet -----------+-------------+------------+----------- 12 by 15 | 12 | 18 by 20 | 11 10 by 14 | 10 | 15 by 15 | 10 9 by 12 | 9 | 14 by 15 | 9 8 by 12 | 9 | 13 by 13 | 9 -----------+-------------+------------+----------- SECOND FLOOR -----------+-------------+------------+----------- 10 by 14 | 10 | 18 by 20 | 10 9 by 12 | 9 | 16 by 16 | 9 8 by 12 | 8 | 13 by 13 | 8 8 by 10 | 7 | 12 by 12 | 8 -----------+-------------+------------+----------- The furnace is not only a means of heating the house but may be a means of ventilation as well; to this end it is desirable to arrange the air supply of the furnace to connect with the outside air. This arrangement assures a supply of oxygen even though no special means is arranged for discharging the vitiated air from the rooms. [Illustration: FIG. 47.--Interior construction of a combination hot-water and hot-air furnace.] =Combination Hot-air and Hot-water Heater.=--In the case of large houses heated by hot air it is sometimes better to use two or more furnaces than to attempt to carry the heat long distances in the customary pipes. Where heat is required in rooms located at a distance more than 30 feet, it is advisable to use a combination hot-air and hot-water heater, the distant rooms being heated by hot-water radiators. A furnace arranged for such a combination is shown in Fig. 47. This furnace contains, first, the essential features of a hot-air furnace; next, it includes a hot-water plant. The fire-box and air-heating surfaces are easily recognized. The arrows show the course of the air entering at the bottom of the furnace, which after being heated by passing over the heating surfaces, escapes at the openings marked _warm air_, to the distributing pipes. [Illustration: FIG. 48.--The hot-air furnace as it appears in the house.] Inside the air-heating surfaces are three hollow cast-iron pieces _W_, that form a part of the walls of the fire-box. These pieces, with their connecting pipes, form the water-heating part of the furnace, which supplies the hot water for the radiators. The pieces _W_, with the connecting pipes and radiators, form an independent heating plant, with a fire-box in common with the hot-air furnace.