chimney down-draught



chimney down-draught defined in 1909 year

chimney down-draught - Chimney Down-Draught;
chimney down-draught - It must first be explained that down-draught is of two characters. First, a down-blow in the chimney of a gusty character, which at one moment is sending smoke, and possibly flame, directly into the room; then the next moment the chimney is calm, and possibly working normally. Intermittent in its action, in an irregular way. Secondly, a down-draught of a very regular kind, one that acts for all the world as if the chimney wanted to show how permanent and steady its draught was; but, unfortunately, the draught takes a descending instead of an ascending direction.

The first of these manifestations is invariably noticeable at certain times when the wind is in particular quarters, blowing from what may be termed objectionable directions; but, of course, there is no one direction from which the wind blows more objectionable to chimneys than another, and in some of these troublesome cases it may be the west wind that causes the down-draught, and in others it is when the wind is from the east or some other quarter. Now, in practically all these cases the trouble is brought about by the chimney being of insufficient height, or of such a height that some adjacent building or object is higher, and deflects the wind so that it interferes with the ordinary up-draught of the chimney in question.

To insure any real reliance in the effective working of a chimney, it must not only be as high as its surroundings, but in nearly every case it should be higher by at least 6 ft. Take, for example, the shape of the roofs on most of our suburban properties - they are ridge-shaped. If a chimney terminates level with this ridge, or even a foot above it, there is the probability of its being troublesome, notwithstanding the chimney itself is several feet from the ridge in question. With this particular class of roof, it is not always the case that chimneys terminating less than 6 ft. above the roof have been a source of annoyance, as many other things govern the results; but many instances have come to the writer's notice, and been remedied by being heightened either with brickwork or a tall pot. In these cases the trouble has arisen chiefly when the wind has blown so that it passed over the roof before reaching the chimney, and as it passed the ridge, it had necessarily to pursue a downward direction more or less abrupt, and its force was thus directed into the open chimney-top, as Fig. 1. With an example of this kind it will be seen that any variation in the direction of the wind would bring about a varied degree of annoyance; and so, also, as the wind was strong or the reverse, the results would be greater or less accordingly.

A state of things which, it will be understood, brings about this down-draught, but which is more difficult to deal with, is when the chimney is part of an addition or wing to a house, and which is considerably lower than the main building. There are thousands of such cases amongst the large residences in the west of London, and a really great number of suburban houses are planned with what is termed a back addition. These additions are almost always lower than the general building, and the chimney tops often terminate from 20 ft. to 40 ft. below the roof of the main structure which is near beside them. With these the action is much more pronounced than with the ridge-roof example just given, for when the wind beats against a wall and has no ready or free means of escaping, it produces a very violent motion, swirling about, and, it would seem, blowing in ever so many directions at once: and where money is not a great object, there have been instances of people trying every description of cowl and wind-guard they could purchase to try and make such chimneys effective, but without avail, or with a very limited sort of success at the best.

In cases such as this a cure could be certainly effected by carrying and continuing the chimney up to above the main building, terminating it level with the other and effective chimneys that are up there, and which work the different fires in that part of the house without the trouble in question. This, however, is where the bother arises. The chimney to be cured may be 30 ft. or more from the wall, and supposing it were carried across to this point, it would, perhaps, have to go up a considerable distance, and although quite possible, it would be an expensive matter, and this leads people to trust to the statements that appear in the circulars of windguard-makers. If any readers want to make interesting little experiments respecting this subject, let them just get a short length of glass tube, and put very small pieces of cotton wool in it. The wool should be little, loose, fluffy pieces that will move with a little draught like smoke. It will be found in the very first place that a current of air across the top of the tube will induce a rapid up-draught in it without the assistance of fire or heated gases, and directly air passes sharply across the upper end, the cotton wool will fly up and out at a speed too quick for the eye to follow. By placing different objects around the tube, many peculiar results can be noted. Enough will be learnt to show that a chimney top should be clear above everything, and nothing ought to project above it anywhere, not even the chimney-pot next to it: they should be level. When chimneys are terminated in brickwork at the extreme top, a brick out of place, or even irregularly laid, may very possibly give pronounced ill results.

The other form of down-draught - viz., that which is more constant or steady than the last, proceeds from an entirely different cause, and many are the strange ideas workmen entertain in regard to it. To those who know the cause, the efforts to cure it on the part of those who are ignorant are always entertaining. This is a form of down-draught that no cowl and no raising of the chimney will cure; the latter, however, might possibly alter the effect.

The cause of this trouble has been christened "siphonage"; but this is scarcely a correct term. The real action is that of air descending one chimney to provide that which is needed to maintain the up-current in another. In other words, there are two chimneys, either in one room, or they may be in two adjacent rooms. Air is needed to make the up-draught in these but it cannot be obtained; consequently one chimney, the stronger we may assume, by its up-draught, causes the necessary inflow of air to come down the other. To some extent it is similar to the action explained above, when it was tried to work one stove with two chimneys.

Everyone knows that the up-draught in a chimney is nothing more nor less than a current of air, and a moment's thought is sufficient to show that this unceasing up-flow represents a considerable volume. Now, where does this air proceed from? From the room in which the stove is, of course; but where does it proceed from to get into the room? The chimney does not exhaust the room of air. In fact, the chimney would not act unless art-entered the room just as fast as the chimney withdrew it. It will be found that the primary entrance for air into a house is the outer doors, front and back, and through the crevices of these (when they are not open) comes all the air for respiration, combustion, and draughts for chimneys. This is supposing some special system or mode of inlet ventilation is not provided, and it quite usually is not. The crevices round windows provide a little, but it is a very small quantity, as these are usually well-fitting, especially in good houses.

Having explained this much, an explanation of an instance where this trouble occurred will make the rest easily understood. A complaint came from a house in the suburbs of London that a room had a fireplace in it that could not be used owing to the persistent down-draught in the chimney, and the room itself and the one adjoining were rendered almost unbearable by the very strong odour of soot. Upon examination it was found that these two rooms that were adjacent were only divided by folding doors, and the doors were kept open. There was a fireplace and chimney in each, but while one was effective (there was a fire burning in it at the time) the other was quite the reverse, and it caused the odour complained of. The air seemed to issue from this chimney into the room instead of being the other way about. Further examination showed that, with the view of j making the rooms very cosy and free from draughts, the crevices all round the doors, and also the windows, had been neatly and well fitted with draught tubing, and the rooms were practically air-tight when the doors and windows were closed. These were kept closed as a matter of course in the severe weather when fires were needed. Now where was the air needed for the chimney draught to come from? The first thought would be that both chimneys would fail to act, there would and could not be any up-draught in them. This, however, can rarely be the case, as the power of the chimneys would not be so exactly balanced. Furthermore, whichever had the fire lighted first would be rendered more effective than the other, and once there is that which goes to make the chimney act in the way it should, the air will rush in from somewhere, and in this instance it came down the other chimney, for there was no other way. The occupants had tried to stop it by closing the folding doors; but these did not fit at all closely; if they had, then there would have been every chance of both chimneys being failures. Of course, the down-draught in this particular chimney was instantly stopped when a door or window was opened to let a supply of air in, but to have a door or window constantly open could not be thought of, so the trouble was overcome with an inlet ventilator placed in each room. The cause of the trouble was merely want of air. It had happened in this very instance that a local tradesman had been called in, and he volunteered the information that his cures of down-draught were very numerous, as he had a patent pot of his own that was irresistible. Of course, he put one of these pots on, and equally of course it did no good whatever. He then attributed the trouble to a supposed defective state of the chimney, and from first to last he never went into the room where the trouble was noticeable - he had no need, he said. It is a too common practice for people to think that application must be had to the top of every troublesome chimney; certainly with quite the majority of instances of down-draught the top is the part to be attacked; but otherwise it is as often the bottom where the cure will be effected.

In case it should have been gathered from what was just said, that short or low chimneys are not effective, it must be clearly explained that their non-effectiveness when it occurs is entirely due to their surroundings. A short chimney will work well if there is nothing higher adjacent to it, and, in fact, when working stoves and cooking ranges in open fields, military camps, etc., it is astonishing how effective a very short chimney will be found. As this is being written there is a gas-fire near by, and this has a pipe-chimney to it, which is only 4 ft. in its extreme height. Although this is so short, the passage of hot gases up it make quite a loud and hoarse humming noise; in fact, it is too effective, for it lessens the effectiveness of the stove a little. A cooking range at Aldershot in a. low building has 8 ft. of pipe to act as a chimney, and remarkable though it may seem, this induces sufficient draught to draw the flame and smoke from the fire round the oven, to all appearances as well as a tall brick chimney would do. At any rate, the range works most successfully, and shows no need for improvement. (F. Dye in 'The Building News.')

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