Example.—A room 20 by 12 by 10 feet with glass exposure of 48 feet, ¼ of wall exposure (two sides exposed) 320 feet = 80, 1⁄55 of 2400 = 44.
48 + 80 + 44 = 172 × 0.25 = 43 feet.
If you add 2⁄55 the surface would be 54 feet.
If you add 3⁄55 the surface would be 65 feet.
PROPORTIONING THE SIZE OF MAINS
For any size system of steam or water heating the following rule will be found entirely satisfactory for mains 100 feet long; for each 100 feet additional use a size larger ratio.
Rule.—
r = (3.1416/d)R = a/r × 100.
r represents ratio of main in inches for each 100 feet of surface; d, diameter of pipe; R, quantity of radiation carried by size of pipe; a, area of pipe in inches.
From this the following table has been constructed:
| Diameter of pipe | Area of pipe | Ratio to each 100 feet of surface | Quantity of radiation, steam or water, on a given size pipe |
| 1½ | 1.767 | 2.10 | 84 |
| 2 | 3.141 | 1.57 | 200 |
| 2½ | 4.908 | 1.25 | 400 |
| 3 | 7.069 | 1.04 | 700 |
| 3½ | 9.621 | 0.90 | 1,062 |
| 4 | 12.566 | 0.78 | 1,590 |
| 4½ | 15.904 | 0.70 | 2,272 |
| 5 | 19.625 | 0.63 | 3,120 |
| 6 | 28.274 | 0.52 | 5,440 |
| 7 | 38.484 | 0.45 | 8,550 |
| 8 | 50.265 | 0.40 | 12,556 |
| 9 | 63.617 | 0.35 | 18,100 |
| 10 | 78.540 | 0.30 | 25,300 |