Rules for proportioning of registers and conducting flues to suit rooms of various sizes are entirely empirical. The sizes of registers and flues found satisfactory in practice is generally a guide for the designer. The following table is taken from a manufacturer’s catalogue and gives a list of sizes that have proven satisfactory under a great variety of conditions and may be taken as good practice:

First Floor
Sizes of
registers
in inches		 Diameter of pipes
in inches
 Size of rooms
in feet 		Height of ceilings
in feet
12 by 15 12 18 by 20 11
10 by 14 10 15 by 1510
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.

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.