SMOKY CHIMNEYS
Reprinted from the Building Research Station Digests of the Department of Scientific and Industrial Research, England. by kind permission.
Introduction
Smoky chimneys are not infrequently a source of trouble and of reference to the Building Research Station, although methods of avoiding them have long been on record. While
it would be possible by further investigation usefully to amplify and to give greater precision to the knowledge avail- able, there already is, so far as open fireplaces are concerned, a sound basis for dealing with existing troublesome cases and for avoiding such trouble in new buildings.
The principles that govern the proper functioning of smoke Alues are quite simple, and only abnormal circumstances should prevent the efficient removal of smoke and other gaseous products of combustion from a fireplace. The heat of a fire warms the air in fireplace, causing the air to expand and so reducing its density. Cold air enters through ventilators, windows and doors, or even through the small gaps between doors and windows and their casings, and forces the lighter, warmed, air up the flue. The current thus caused carries with it, under normal conditions, the smoke from the fire.
Only obstruction or hindrance can prevent the natural tendency for smoke to rise in a flue, and for this reason a very large proportion of imperfectly designed and constructed fireplaces and flues function more or less satisfactorily most of the time and give trouble only when conditions are abnormal. Such trouble might have been prevented, in many cases, by more careful attention to details of construction and desigo.
Air Supply
The comfort of a warmed room depends largely upon the absence of draughts. These may be caused by ill-fitting doors and windows, and draught-tubing, felt strips, etc., are some- times employed to remedy matters. When no other ventila- tion is provided the almost air-tight condition of the room thus produced may make the fireplace smoke. When such a condition obtains a slight opening of a door or window is all that is required to produce an improved draught in the flue. In most cases a definite improvement, and often a complete cure, can be effected by correcting the design of the fire- place, e.g. by restricting the throat. This remedy, which reduces the intake of air to the flue, should be tried before any other, but, if this is unsuccessful, further ventilation must be provided.
Wind
Wind affects the action of a flue in several ways.
It may be directly helpful when, blowing across the flue outlet. it creates a suction in the flue. Other differences in air-pressure, which are created by the action of the fire and by wind, how- ever, are probably much more important than this suction. and, for this reason, the provision of special chimney-pots or cowls is not thought to serve any very useful purpose in most cases. Where a cowl or pot adds such height to a chimney that the outlet escapes an area of high pressure, a certain amount of benefit is obtained, but the mere raising of the chimneyhead might be equally beneficial.
ISOMETRIC SKETCH OF FIREPLACE
SCALE
CALE
I
THROAT A
FOR ISOMETRIC
SMOKE
ЗМОКЕ SHELF B
SHEL
THROAT A
SECTION THRO CENTRE LINE OF FIREPLACE
SECTIONAL PLAN THROUGH SMOKE CHAMBER SHOWING SMOKE SHELF AND THROAT AT X-X
FOR SECTIONS
2
FEET
45
Fig. 1 shows a section of an ordinary house with the chimney rising from an external wall, the direction of the prevailing wind being such that the chimney is between the roof and the wind, the roof slope being greater than 30°. The pressure of the wind on the roof is greatest ut A and lowest a! B, and the chimney is liable to smoke unless the action of the fire is sufficient to readjust the pressures or unless the chimney outlet is high enough to escape the area of high pressure. In the figure the wind pressures and suctions on the surfaces have been plotted as ordinates normal to the surfaces. The wind creates a suction on the lee side of the house and, if the window or door openings are at C, air may be drawn from the room and pressure reduced at the fireplace, thus tending to cause down-draught,
On roofs of a less slope than 30", suction and not pressure is produced by a wind, the suction becoming greater as the ridge is reached: for roofs approximating a 30 pitch, pressures or suction produced by wind on the roof may be neglected. The pressure on the walls of the weather side of a building. and the suction on the lee side, must, however, still be con- sidered.
Other buildings, trees, hills and the projections on roofs affect conditions of pressure, and these may vary considerably according to the size, height, shape and nearness of such objects.
Flues
Whatever the conditions may be at the flue outlet, much may be done to avoid or correct downdraught by careful design and construction of the flue and of the fireplace.
Examples of poor design and construction which should not be copied are apparent in flues containing quick bends and flat slopes, fire-places excessively large, flues of too small or too large a section, fire-places not solidly built or backed, improperly built or unparged flues with leaky masonry joints, and in other ways. These faults singly or in combination may seriously affect efficiency as regards smoke removal. Flues should be as straight as possible, especially near the tire-place, and their walls should be smooth and air-tight; Hues of circular section are the most efficient, though for most open fires a 9 in. by 9 in. square flue is satisfactory. Fireplaces
At the end of the eighteenth century scientifle principles were applied to the design of grates, and the methods adopted were successful in curing a very large number of smoky chimneys. The principles which were then enunciated appear to be sound and they have been put into practice in a consider- able number of cases brought to the notice of the Building Research Station, with the result that smoky flues which hac proved incurable previously were much improved. There is reason to believe that the design of the fireplace itself is the most important factor relating to the prevention of smoky chimneys,
The features of fireplace design which are considered desirable are as follows (the letters refer to Fig. 2):— (1) Correct design of the throat (A on Fig. 2). The throat should be 4 in. wide and 6 in. to 8 in. deep and should be perpendicularly over the fire. Smooth internal surfaces should be provided to all smoke passages and, in particular, the entrance to the throat under the front arch or lintel should be rounded (C on Fig. 2).
(2) A smoke-shelf should be provided (B on Fig. 2). This should be horizontal and level with the top of the throat which should be 6 in. to 6 in. above the top of the fireplace opening.
(3) The fireplace should have splayed sides on plan. The large open corners, which are formed if the back is as wide as the opening of the fireplace in front, can cause eddies which may bring the smoke into the room. (4) There should be sufficient depth from the face of the chimney breast to the back of the fireplace to prevent smoking caused, for example, by draught across the fire- place opening.
Conclusion
The effectiveness of these principles has been tested, in several hundreds of troublesome cases, by making alterations on the lines of Fig. 2.
The indications are that, provided the room is sufficiently well ventilated, even under adverse external conditions re- places in which all the features of design discussed above are included will not give rise to trouble unless the flue is badly constructed or too short. Flues less than 15 ft. long may require lengthening by extra brickwork or chimney pots.
In some cases of smoky chimneys brought to the notice of the Building Research Station it was found impossible to full all of the requirements except by incurring excessive expense. In most of these, much improvement was made by the provision of a "throat" and a small smoke-shelf, which are considered essential features of design. However, in aul instances where the recommendations have been adopted in their entirely a complete cure has resulted.