SMOKE consists of particles of carbon which are carried away by the updraught of hot air. In terms of combustion, smoke represents wasted fuel. But it is not possible, in ordinary open grates (or in many closed furnaces) to prevent some unburnt fuel escaping in this way. The amount of smoke should not, however, be excessive in proportion to the fire. It is not the function of a chimney to provide a convenient exit for smoke and gases from the fire. That is a domestic necessity which coincides with the more important purpose of creating a draught with the object of improving combustion. Since we hear so much about fuel economy it should be explained that the warmth emitted. From a fire is in proportion to the amount of fuel that can be efficiently consumed in a given time. Thus, in grates which work well and are furnished with a damper in the fret that closes in the space under the grate, we can open the damper after lighting the fire, and close it somewhat when the fire is well established. The use of the poker, within reason, opens more spaces for the air to penetrate, besides letting down ash or cinders that hinder the combustion of fuel. True economy lies in having a good fire when warmth is needed, and in keeping the fire low, or not lighting it, at other times. It is not economical to have a poor fire which gives off little else than smoke because it is kept too low to burn properly.
The chimney works on the principle of simple air convection. The column of air within the chimney, heated by the combustion of fuel in the hearth, possesses a natural tendency to rise, causing more air to be drawn over the fire. The fiercer the fire the greater the draught. On damp, humid days, I.e. when the air is heavy with moisture, a fire will tend to burn sluggishly until it is well alight. Therefore, on a damp, cold day, burn some newspaper in the grate before laying and lighting the fire. This will do away with the smoke trouble otherwise experienced on such occasions. For a similar reason, be more generous with the wood and paper, and get them well alight before putting on much coal. Many fires are stifled by too-hasty shovelling on of fuel. Aim at keeping a small, bright fire until it is well established, then add more fuel gradually, in small lumps at first. By this means the correct draught will be maintained to ensure good combustion. Never overload the fire with fuel. A mere piling on of coal will give room heat only in the latter stages of combustion after much smoke, I.e. wasted fuel.
To prevent the generation of excessive quantities of smoke, good modern grates have the fire-back so shaped that smoke, etc. from the front of the fire must pass through the region immediately above the glowing area of the fire, near the back, and much of it is consumed in the process. Smokeless fuels, such as anthracite (which are less gassy and not so volatile as ordinary coal) do, nevertheless, give off a certain amount of smoke, and also unconsumed gases. In a domestic boiler fire, for example, a small
area towards the back should be poked open so that red heat of the fire is exposed to the passage of smoke, thus burning the gases that would otherwise escape up the flue. This is a wise precaution after stoking up for the night. Sometimes the gas accumulates until it ignites with a sudden bang, accompanied by a cloud of dust from the fire into the kitchen and, perhaps, the blowing down of the front door of the furnace. If a glowing area is left uncovered, the gas will burn away (or most of it) as it is liberated from the fuel.
A smoking chimney is generally an indication that it requires sweeping. Some people are inclined to have the chimney swept at regular intervals regardless of the circumstances; but though this is an excellent rule, it should not exclude the summoning of the sweep at an earlier period if the chimney seems to call for it. The nature of the fuel may have changed, or weather conditions may have demanded more frequent fires. Be wary of devices sold as chimney cleaners. Only the sweep’s brush will properly remove accumulated soot; moreover, his operations will disclose if the chimney has become blocked by fallen bricks or mortar, and give a warning if attention is wanted in this respect.
A smoky flue is often due to an accumulation of soot in the space below the point at which the cast iron stove pipe enters the brick flue. Soot gathers here in a sloping heap and will eventually partially or entirely choke the end of the iron pipe. Open the access plate bolted to the bend of the pipe and scoop out the soot. At the same time, thrust a flue-brush down the pipe below and clear away any soot clinging to the pipe bore. Make sure, by shaking the damper vigorously, that this soot gets down into the furnace. Do not attempt to do this work with the fire burning. Inspect the cement pointing around the stove pipe where it goes into the brickwork; if it is cracked, chip it away and make a new joint with one of the fireproof cements sold for the purpose. If air enters the flue at this point, it may stop the fire from drawing properly, and is almost certain to cause a smoky flue. In order to function’ properly, air should be drawn in at the base of the furnace, to pass through the fuel bed and supply oxygen to the fire. Most domestic boiler flues are efficient, and leaks into the flue higher up may not have any noticeable effect, but in a few instances such leakage may prevent efficient working and cause much annoyance.
To avoid draught troubles it is inadvisable to take a boiler flue pipe into a disused flue of old construction, and it is bad practice to take the stove pipe into a flue already serving another fireplace. The domestic boiler should have an independent flue. If a boiler is installed in place of a cooking range, the flue opening that formerly served the range, ought to be diminished in area. If the flue pipe from the domestic boiler is merely taken up through the plate above, even though it be carried up some distance, the result will be less satisfactory. Owing to the enlarged, space, a low pressure region will be produced here and will reduce the draught. Too large a chimney may cause downdraught and excessive smoke.
Some chimneys smoke only when the wind is in certain quarters, and little can be done as a cure. A cowl will sometimes cure this trouble, and the local builder should be able to advise. Chimneys should be taken up some feet above the highest point of a building, or of adjoining buildings. The presence of tall trees near the house will sometimes affect the chimneys. Wind eddies set up by the shape of the roof and the position of adjoining or near-by buildings are often the cause of downdraught, generally noticeable only when the wind blows from certain quarters. Cowls or similar appliances designed to reduce downdraunht will improve matters, but expert advice is necessary. Tallboys, as they are called, mainly act by increasing the effective height of the chimney. Some cowls are mechanical in action, one member revolving in the wind. In time the moving parts get corroded and fail to work properly. Some specially shaped down-draught preventers act by utilizing wind action so as to induce an upward current of air in the chimney without the assistance of heat.
When an old-fashioned fireplace opening is converted to a modern grate, generally of smaller size, the larger opening should be properly reduced by brickwork. Skilled men should be employed to do such work, as there are many technical pitfalls which only experienced workmen can foresee.
Similarly, serious defects in the structure of the chimney flue should be left to the builder.