| Diameter of smoke pipe or round chimney flue. | Size of chimney flue. | Height of chimney flue above grate. | Diameter of smoke pipe or round chimney flue. | Size of chimney flue. | Height of chimney flue above grate. |
| Inches. | Inches. | Feet. | Inches. | Inches. | Feet. |
| 8 | 8 by 12 | 35 | 15 | 16 by 16 | 45 |
| 9 | 8 by 12 | 35 | 16 | 16 by 18 | 45 |
| 10 | 12 by 12 | 35 | 17 | 16 by 20 | 50 |
| 11 | 12 by 12 | 40 | 18 | 16 by 20 | 55 |
| 12 | 12 by 12 | 40 | 19 | 20 by 20 | 55 |
| 13 | 12 by 16 | 40 | 20 | 20 by 24 | 60 |
| 14 | 12 by 16 | 45 |
In [Table 1] the minimum height of the chimney above the grate is given as 35 feet. Higher chimneys are considered more satisfactory, and authorities claim that any flue under 40 feet in height will produce an erratic draft, good on some days but poor on others The force or direction of the wind may be the cause, or the amount of moisture in the air, or the quality of the fuel may be responsible. The higher the chimney the less will be the possibility of counter air currents and the stronger and more constant the draft. Soft coal and the sizes of hard coal known as pea and buckwheat are apt to cake and fill up the air spaces through the bed of the fire, with the result that an intense draft is required to give the fuel sufficient air.
The top of the chimney should extend at least 3 feet above flat roofs and 2 feet above the ridge of peak roofs (see figs. [2] and [3]), and it should not be on the side of the house adjacent to a large tree or a structure higher than itself (see [fig. 4]), for these may Cause eddies and force air down the chimney. A poor draft will most likely result when the wind is blowing in the direction indicated.
Fig. 3.—Extensions to the chimney required In order that it might draw properly.