The fireplaces in a house frequently stand one immediately over the other, and each chimney flue1 from the lower rooms has in consequence to be carried to one side or the other to avoid the fireplaces above it.
Flues from the lower stories are therefore necessarily curved, but not those from the attics; a curve is, however, considered advisable in all flues, to prevent rain or sleet from beating vertically on to the fire, and to stop down-draughts of cold air. This curve should be sufficient to prevent daylight from being seen when looking up the flue.
The funnel, or opening above the fireplace, is gathered over (see page 29), so as to be contracted to the size of flue required.
Fireplaces generally require more depth than can be provided in the thickness of the wall; this necessitates a projection to contain the fireplace and flues called the chimney breast. Sometimes this projection is on the back of the wall, in which case it gives more space and a more convenient shape to the room. When on the outer wall of a house it may be made an ornamental feature.
Every fireplace should have a distinct flue to itself; if the flues of two fireplaces communicate, and only one fire he lighted, it will draw air from the other fireplace, and smoke ; moreover, its own smoke may enter any other room the fireplace of which is connected with the same flue.
The air heated by the fire is rarefied, rendered lighter, and ascends the flue, drawing the smoke with it, whilst cold air rushes into its place from below.
Hence the throat or lower opening of the flue should be small, so that no air may pass through it without first coming into contact with the fire and being thoroughly warmed.
The flue should not be larger than is necessary for conveying the smoke and heated air ; if too large, it will smoke in certain winds.
With regard to the proper size for flues there are great differences of opinion. The size should vary according to the circumstances of different cases ; but generally speaking a flue 9 inches square is sufficient to carry off the smoke from very small grates, a flue 14 inches x 9 inches for ordinary fireplaces, and a flue 14 inches x 14 inches for large kitchen ranges.
The smaller the flue, and the greater the height, the more rapid the draught and the less likely the chimney to smoke, provided that sufficient air is supplied and that the flue is large enough to carry off the smoke.
1 Sc. Vents.
The flue should change its direction by gradual curves and contain no sharp angles, otherwise soot accumulates and makes it smoke. The Building Act requires that if any angle is necessarily less than 130° an iron soot-door should be provided at the bend, so that the soot may be removed (see S, Fig. 61).
Much depends upon the height of a flue, the shortest, i.e. those from upper rooms, or in low buildings, being most liable to smoke.
The air should pass through or very near the fire.
Thus a high opening above the fire is bad, as it admits cold air, which gets up the flue without being heated, and cools the rising warm air.
The fireplace should, therefore, be not much higher than the grate.
All walls about chimneys should be well built, and so should the "withes" or partitions between flues, as cold air may penetrate badly-built walls from the outside, or from an unused flue cold air may get into one in use, thus cooling the heated air and causing the chimney to smoke.
If openings are left in the withes, the smoke from a flue in use may penetrate another, and from it enter a room in which the fire is not burning.
The width of the chimney breast for each room of a high building must be arrived at by drawing the plan of the fireplace of each room, including the flues from the fireplaces of the rooms below; they can be arranged in plan in such a form as may be most convenient for the chimney stack.
A very common practice is to build the fireplaces of adjacent rooms or houses back to back, in which case the arrangement on each side of the wall is exactly the same.
The plan of bringing a number of flues into a "stack" is economical, and tends to preserve an equal temperature in them.
Figs. 53, 54, are respectively longitudinal and cross-sections of the fireplaces and flues in the wall between two 5-storied buildings.
The dotted lines in Fig. 54 show the direction of the flues of the fireplaces on the other side of the wall.
The remaining figures on page 27 show the plan of the chimney breasts on the level of each floor.
The weight upon the chimney breasts should be spread over a greater area by introducing footings,1 as shown in Figs. 53, 54.
In some cases the same object is attained by turning an invert arch between the chimney breasts under the fireplace.
In order to economise the brickwork, and to leave as much interior space in the building as possible, the part of the chimney breast in each room is generally made of the minimum width that is absolutely necessary to contain the flues at that point.
1 Sc. Coddings.
Thus it will be seen that the chimney breasts on floors I K and G H are made narrower than those above them, because they contain fewer flues. The extra width required for the flues in the chimney breast on the other floors is gained by corbelling out as shown at t t. The projections in the brickwork are concealed under the floor and by the cornice of the ceiling below.
Sometimes one side of the chimney breast is made narrower than the other ; thus the side x (Fig. 59) might be made narrower than y, and w narrower than z (Fig. 57), for in each case the chimney breast on the left contains one flue less than that on the right. This causes an unsymmetrical appearance, but is often done even in superior buildings.
The whole of the external walls, both of chimney breasts and shafts, are generally made half a brick or only 4½- inches thick.
Again, even when this is done, the outside wall of the chimney shaft itself is often reduced to half a brick directly it has passed through the roof. It is better, however, to keep the external walls of the shaft 9 inches thick throughout (as dotted at S S in Fig. 53), for the reasons stated at p. 32.
It is frequently necessary, for the sake of appearance, to place the chimney in a symmetrical position, such as the centre of the roof. To this end, and also in order to avoid a multiplicity of chimney shafts, the flues have to be collected from opposite sides of the house into a central stack.
Fig. 61 shows an example of this. The flues from the rooms A, B, C, and E, converge towards a central stack, the space between the chimney breasts of the upper rooms being bridged by an arch W, over which the flues are carried; the brickwork forming the upper wall of the flue is racked back as shown, leaving only thickness sufficient for safety above the flues.
The chimney breast of the room C cannot be carried down to the foundation, as it would interfere with the folding-doors in the room below. It is therefore supported by courses corbelled out into the room from the wall, as shown in dotted lines.
It will be noticed that the chimney breast of the room A is nearer the outer wall than that of the room below ; in order to avoid widening the chimney breast below, the upper and outer chimney breast p is supported by courses corbelled over to one side as dotted. The corbelling is concealed by being carried out within the floor.
The projecting part of the upper chimney breast might be supported by turning an arch, as shown by the dotted line X, and this is a construction often adopted.
Fig. 53. Fig. 54. Scale, 1/10 inch = l foot.
Fig. 61. Scale, 1/10 inch = 1 foot.
This Figure is the section of an ordinary dwelling-house taken on this side of the flue from E, and is intended to show two or three different arrangements of flues. It also illustrates the remarks made in Part I. as to the increase of roofing required when the wall plates are placed in the middle of the thickness of the walls. The whole surface of the ground may, with advantage, be covered by a layer of concrete, as described at p. 4.
It will be seen that the flue from the room E is carried vertically up in the thickness of the outer wall as high nearly as the ceiling of room C, then over an arch covering the recess between the chimney breast and the outer wall.
The portion of flue in the thickness of the outer wall is rather apt to be cold and to check the draught, and the construction might in this case be avoided by carrying the flue across the corner of the party wall of room D, and up the left chimney breast s (which would have to be widened to receive it) of the room C, above.
The flues in this illustration are supposed to be formed with circular earthenware pipes1 of 9 inches diameter, shown in plan in Fig. 62.
The external walls are here shown only 4½ inches thick, because the thickness of the flue-pipe itself affords a great protection and renders it unnecessary to make the brickwork so thick as it should be round pargetted flues.
Chimney Shafts.2 - At the ceiling of the highest room the chimney breast is reduced in size to the chimney shaft of a width just sufficient to contain the flues. This shaft should be carried well above the roof, higher if possible than adjacent roofs or buildings, which are apt to cause eddies or down-draughts and make the chimneys smoke.
A few of the upper courses of high chimney shafts are generally made to project, and should be built in cement to serve as a protection from the weather.
The cap is frequently made ornamental by bricks, placed angle-wise, etc., in a similar manner to the brick cornices and coping referred to in Part I. Stone caps are also used for brick as well as for stone chimneys.