From this point the consideration of dimensions goes up the chimney. In its standard ordinance for chimney construction, the National Board of Fire Underwriters calls for fireplace flues with a draft area of one-twelfth of that of the fireplace opening and determines this area as a circle or ellipse that will fit within the tile used to line the flue. As it is difficult to obtain flue linings of exactly the desired area, it is better to select a size slightly larger, rather than one smaller, and so make sure of sufficient capacity under all weather conditions.
Between the lintel of the fireplace and the point where the flue commences come the three structural features so stressed by Count Rumford. They are the throat, smoke shelf, and smoke chamber. As its name implies, the throat is the opening through which smoke, hot gases, and some flames pass on their way upward. Experts hold that its correct construction contributes more to the efficiency of a fireplace than any other feature, save proper flue design. The area of the throat opening should not be less than that of the flue and its length must be equal to the width of the fireplace. It should be located eight inches above the lintel. Under present practice, a cast-iron throat with a damper which can be opened and closed to regulate the up-chimney flow is standard. Also, when the fireplace is not in use, this damper can be closed and so prevent loss of other heat.
The smoke shelf comes immediately above the throat and is formed by recessing the brickwork of the back the full width of the chimney for at least four inches. With very large fireplaces, it may be as much as twelve inches. The object of this feature is to stop any accidental draft within the flue from going farther and blowing smoke out into the room. The area in between this and the flue itself is called the smoke chamber. Here the walls are drawn in with a gradual upward taper to the point where the flue lining begins. The chamber so formed can and does hold accumulated smoke temporarily when a gust of wind across the chimney top cuts off the draft for a moment.
In building chimneys, the old masons varied their structural ways and materials according to the part of the country in which they worked. New England workmen were partial to a central chimney, the core around which the house was built, and their usual material was stone. Occasionally brick was used but this material was more in favor with old houses of the middle states and the South. Here, instead of the central stack, a chimney was built in each of the two end walls. The climate was milder and the style of architecture, with central hall and stairway, made such practice desirable.