Proper proportions of the fireplace opening and flue are among the most essential features of fireplace construction. The larger the opening, the larger the fire may be and the more air required or taken from the room for proper combustion, provided the flue is large enough to give sufficient draft. In the average home, the actual inside area of flue should never be less than one-tenth of the area of the fireplace opening. Each fireplace should have its own individual flue carried full size to the top of the chimney without connections of any kind from other sources. A low, broad opening is preferred to a narrow, high one, and splayed sides are best in any case, because better draft and radiation are thus secured. A low, broad opening will catch the smoke better and direct it up the chimney.
The following dimensions for fireplaces are considered good practice:
Fireplace Openings
| Width | Height | Depth |
| 2'-8" | 2'-4" | 17" to 21" |
| 3'-0" | 2'-4" to 6" | 21" |
| 4'-0" | 2'-8" | 21" to 25" |
Arched openings may be higher than indicated in the above dimensions, as their average height may be taken as the top line of the opening.
Construction
The ideal form of a fireplace would be a cone with all sides tapering to the apex for the escape of smoke. Therefore, to approach this ideal in practice, the sides of the fireplace are splayed and the back curved forward as shown in [Figs. 36 and 37]. This forward curve of the back throws the smoke from the fire forward to a throat 4 inches wide, extending the full width of the opening at the front and top of the fireplace, and at the same time forms a shelf above for the deflection of down drafts in the chimney. The rear wall of the fireplace should never be straight, having the throat at the rear, as down drafts would be sure to blow smoke and dust into the room.
Fig. 37. Cross Section of Fireplace