The chief difficulty in attaining a successful fireplace design does not lie in securing an abundant draft. In fact it is an easy matter to make a fireplace draw if the flue is large enough and the opening from the fire chamber into the flue unobstructed. There will never be any question of getting a roaring blaze the moment the fire is lighted.

This is, in a way, the type of fireplace that our Colonial ancestors built great cavernous openings and generous flues, with the result that the more wood was piled upon the blaze the more they blistered their toes and at the same time chilled their backs. For it is evident that when we secure such a strong, unobstructed current of hot air up the chimney, enough cool air to take its place must be drawn into the room through every opening and crevice. The result is a mighty draft that rushes past those unfortunate enough to be sitting about the fire and carries rapidly up the chimney almost all of the heat of combustion.

In the fireplace of our Colonial ancestors probably ninety per cent. of the heat was entirely lost, being carried up the chimney. However, cord wood was then to be had for the cutting.

We want a different sort of a fire in these days one that will burn with a steady, constant blaze or glow, conserving most of its heat, which the back and sides of the fire chamber will reflect out into the room.

Such a fireplace will not necessarily be a large one. It is amusing to hear how universally the demand goes up for large fireplaces “great big fellows that will burn full cord wood.” It is hard to see just why this is. It may be based on the assumption that if a small fireplace is desirable a large one is more so. This is a fallacy that the architect and fireplace builder find it hard to dispel. There is no objection whatever to a large fireplace in a summer camp or informal shack of that sort. In fact a small one would in such a place be ridiculous, but when we come to our year-round living-room or dining-room or den, where the walls of the room are tight and the whole atmosphere quieter and more restrained, a large fireplace would be distinctly a disturbing element. Such a room as this, unless very poorly built, would not permit the in-take of sufficient air for the draft of a big fireplace, whereas in our slab cabin or log bungalow the conditions are quite different.

For the ordinary room, therefore, a fair average size for the fireplace opening is three feet in width by two and a half feet high, with a depth half the width. From such a fireplace it is possible to get a maximum of heat with a minimum of draft.

There are two vital principles that should be observed in the design of any fireplace. One of these is the relation between the size of the opening into the room and the size of the flue itself. A cross-section of the flue which incidentally should be kept the same throughout its extent should be one-tenth of the area of the opening into the room. The second vital consideration is the introduction of what are known as a “smoke shelf” and a “smoke chamber.” The reason for constructing a fireplace with these two features will appear more readily by reference to the diagram. This is drawn to show that when a fire is kindled on the hearth the warm air current, which is generated immediately, begins to rise through the throat (the opening between the fire chamber and the smoke chamber) and at once induces a down-draft of cold air. If the back of the fireplace were on the same continuous plane with the rear side of the chimney flue, this downward current of cold air would strike directly upon the fire itself and force smoke out into the room. The smoke shelf is built just where it will prevent this action. The sectional diagram does not perhaps make quite clear the shape of this smoke chamber, but the accompanying perspective outline sketch will indicate the fact that the throat and the smoke chamber at the bottom must extend across the full width of the fire chamber. This width in the smoke chamber immediately diminishes in rising until it joins the flue at the flue’s own area.

The sectional diagram indicates a cast-iron damper built in the throat. This is not necessary, for it contributes nothing to the efficiency of the fire itself. Its one great advantage is that by furnishing the mason with an unalterable form, it forces him to build the throat properly rather than in one of the wrong ways that his own judgment might dictate. Such a cast-iron damper also forms a support for the flat arch of brick over the opening if bricks are used. If the damper is not built in, it is necessary to use an iron supporting bar to carry this flat arch. Then too, in case the damper is not used, there is lost the advantage of being able quite readily to close the throat entirely, which is highly desirable in the summertime and frequently in the winter when the fireplace is acting too strenuously as a ventilator. If the cast-iron throat is not used, therefore, it will be well to lay an iron plate on the smoke shelf in such a way that it could be drawn forward across the opening to close it.

There are other types of dampers, most of them patented and all of them aiming to provide an adjustable opening in the throat in some way. One or two of these have a knob or handle projecting through the brickwork of the arch, permitting the convenient adjustment of the damper from outside. As a general principle, however, it is well to choose the simplest possible device that will secure the desired result.

The terra cotta flue lining which is shown in the sectional diagram is not absolutely necessary, of course, as it is a rather modern introduction and unnumbered fireplaces have served their purpose without it. There is no question, however, regarding its worth, for it provides a flue with smooth, regular sides that will not clog nearly so readily as an ordinary brick flue. Besides that, it has the advantage of permitting a thinner wall for the chimney. It is dangerous to build a chimney with a single four-inch thickness of brick between the flue and whatever may adjoin the chimney. Of course no wood should be allowed to come within an inch or two of the brickwork in any event, but with a single thickness of brick, unlined, there is always the danger that the mortar will crumble from a joint and leave an opening through which it would be an easy matter for sparks or flame to do considerable damage. The introduction of a flue lining, however, into the chimney built in this way makes it entirely safe, provided the joints between sections of flue lining are carefully filled and made smooth with cement mortar.

The sectional diagram, it will be noticed, indicates a difference between the main back wall of the chimney, eight inches thick, and the brickwork laid inside the fire chamber to form the hearth and the back. The reason for this separation is that the rough brickwork of the chimney is always laid first as simply as possible, leaving the fire chamber with its sloping back and sides and the hearth to be filled in later with a better grade of brick or perhaps another kind. Frequently, also, tile will be combined with the brick finish as a hearth or facing.

A support for the hearth is usually obtained as indicated by bringing what is called a “row-lock” or “trimmer” arch between the foundation masonry of the chimney and a pair of floor joists set out at the proper distance, depending upon the desired width of the hearth. While this is the customary method, occasionally a support is secured in some other way, such as corbeling out from the masonry foundation, or by extending two short projections of this masonry from the bottom up at either end of the hearth and throwing an arch across between these. Upon a bed of cement the hearth bricks themselves are laid, usually flush with the floor, although occasionally enough higher to permit a beveled molding strip to cover the joint between brick and floor more closely. In some cases the hearth itself is raised the full thickness of a brick above the floor, as in one of the photographic illustrations shown.

The width of the hearth is ordinarily made about sixteen or eighteen inches beyond the face of the opening with the average size fireplace, twenty inches or even more with larger ones. This width should be increased, of course, if the opening is made considerably larger. The question of materials for the hearth and facing will be discussed in the next chapter.

The chimney itself should extend at least a foot or two above any nearby roof ridge and it should work without any cowl, whirligig or other device of that type on the top. There is no great objection to having the opening a horizontal one at the top of the chimney, although in that case if the flue is nearly straight throughout its course, some rain will find its way down to the hearth in a hard storm. In most cases there is enough bend in the flue to prevent this, and if not it may be avoided by covering the top of the chimney with a stone and having the openings vertical ones on all four sides just under this.

All of the brickwork throughout chimney and fireplace should be laid in first-class cement mortar which consists of one part Portland cement to three parts clean, sharp sand. Although lime mortar was used in all brickwork up to recent years, it is not durable, particularly in the vicinity of heat.