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Building a House - the Chimneys


"Why do chimneys smoke?"asked Paul of his cousin.

"You mean rather to ask me,"replied the latter,"why some chimneys smoke. Many causes contribute to make chimneys smoke, while there is only one condition which must be observed if they are not to smoke. We must therefore do our utmost to fulfil that condition, viz. a flue proportioned to the fireplace, and the supply of a quantity of air to the latter proportioned to the combus tion. If the flue is too narrow for the amount of smoke given off by the combustion, this smoke does not rise easily enough, its advance in ascending is checked by friction, and the discharge being insufficient for the pro duction, the smoke conies out into the room. We can stimulate the combustion, and consequently the ascent of the smoke by a current of external air directed towards the wood or coal. When the fire is well lighted it warms the column of air that fills the chimney, and the warmer this column is the lighter the air is, and the more it tends to rise.

"That is why in some ill-built chimneys a certain time is required before the smoke will take its proper course - that is to say, the column of air must be warmed. And until it is so, the smoke passes not into the flue, but into the room; then we open a window to supply the fire with air, which brightens it up so as to warm the flue and allow the smoke to take its proper course. For the same reason all new chimneys smoke. Flues carried up in masonry are damp and cold, and the air they contain is heavy; it takes some time to warm and lighten it.

"Instead of opening a window to stimulate the fire (which is a rather primitive method), we supply each grate with an air draught - that is, we give it a channel which conducts the external air to the combustible as soon as the least heat is developed, that, e.g. of a piece of paper lighted. Immediately this exterior air is called in to fill the vacuum produced by the commencement of combustion, and it stimulates the fire by bringing it oxygen. The livelier the fire the more rapid is the draught; and the more rapidly the air comes in, the more brightly does the wood or coal burn. The air-channel is to a grate what a pair of bellows are to a forge fire. But the air-channel, as well as the flue, must bear a due proportion to the fireplace. If the flue is too narrow, the smoke is obstructed, and comes out into the room; if it is too wide, it is not uniformly heated, and the external currents of air - the winds - exert a pressure at its upper extremity which neutralizes the effect of the draught, and the smoke is beaten down. If the air-channel is too small for the extent of the grate, it does not bring the quantity of air necessary for combustion; the fire languishes, it heats the flue imperfectly, and the lukewarm smoke does not ascend rapidly enough. If the air-channel is too large, or brings in too considerable a volume of air, the oxygen of which is not completely taken up, then a part of the cold air enters the flue and does not stimulate the draught; or, if there are changes in the temperature, the air-channel attracts the air from the chimney instead of bringing in air from the outside. The process is reversed, and the chimney smokes dreadfully." It was in the evening, after dinner, and when the family were seated around the hearth, that Eugene was pro pounding this theory."That appears to me simple enough,"said Madame de Gandelau;"but then why does the chimney in my room, which I have had altered several times, smoke on certain days?" "Because your room, Madame, is situated in the new wing of the house, the roof of which is lower than that of the older part. They could not carry the flue high enough to rise

above the ridges of the roof of the old building, for that isolated chimney would not have resisted the squalls. When the wind comes on your side it finds the obstacle presented by the loftier building, and rebounds : an eddy is formed, and whirling about on itself it becomes engulfed in your chimney-flue, or at least obstructs for a time the passage of the smoke. In such a case the flues should bifurcate; as the pressure of the wind is never exerted equally in both orifices, the air rushing into one makes the smoke issue violently through the other. I know of no other plan : I have already proposed it to you; but you have thought, not without reason, that these flues, which seem to raise two despairing arms towards heaven, would be very ugly; so you have resigned yourself to be smoked out of your room when a strong gust from the west is blowing." "Yet the chimney-doctor put a sheet-iron pipe with a revolving cowl - what he calls, I think, a gueule de Loup : he told me it would work admirably, but it was worse than before." "Certainly; when there are eddies and whirlwinds in consequence of some obstacle, as here. This cowl turns in all directions, and among its rapid gyrations sometimes presents its mouth to the gust, if only for an instant. This mouth then performs the office of a funnel, and the air, rushing into the pipe, sends the smoke in puffs into the very middle of the room." "Exactly so; you think, therefore, that those two ugly flues must be adopted?" "Certainly. There are cities near to mountains, all of whose houses, however lofty, are in the same condition. Geneva, for example, built between the Saleve and the Jura, is commanded, though at a great distance, by these mountains. The violent winds which sometimes prevail on the lake are imprisoned between these two chains, form eddies, rush backwards and forwards, and send violent gusts in every direction; so that the Genevese are obliged to put these double flues on their chimneys, which at a distance present the aspect of a forest of old-fashioned telegraphs." "I hope, then, that you will build the chimneys in the new house in such a way that they will not smoke. You know that Marie would be much vexed if they should." "We will do our best; and the local conditions are favourable to begin with; we are not commanded by other elevations, we have not any eddies of wind to fear; along the plateau on which we are building the breezes are regUlar. Moreover, we have only straight high roofs, and all the flues rise above the ridges. We shall build these flues with brick, and make them of ample size. Nothing obliges us to give them very oblique directions; they rise vertically, or nearly so. Lastly, we shall have a system of air-flues, arranged from the very foundations, in a cool aspect; for that must be attended to, since if the air-flues are open to the south - for example - the air they receive from without, even during the winter, is warmer than that of the room where the fire is lighted; and then the air flue draws the smoke which comes down into the room. At least the fire cannot be lighted; the wood is only charred, and does not burn.

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air, smoke, flue, fire and flues