It should go without saying that the air which passes from furnaces into living-rooms should always be taken from out of doors, and be conveyed in perfectly clean air-tight shafts to and around the base of the furnace. Preferably, the inlet of the shaft, or cold-air box, should be carried down and curved at a level (of its upper surface) with the bottom, and full width of the furnace. Thus applied, the air is equally distributed for warming and ascent through the hot-air pipes to the apartments to be warmed. On the outside the cold-air shaft should be turned up several feet from the surface of the ground, and its mouth protected from dust by an air-strainer. A simple but effectual way is to cover the mouth with wire cloth, and over this to lay a piece of loose cotton wadding. This may be kept in place with a weight made of a few crossings of heavy wire, and it should be changed every few months. And here, too, outside the house, should be placed the diaphragm for regulating the amount of cold-air supply, and not, as commonly, in the cellar.
As the best means of regulating the temperature and purity of the atmosphere from hot-air furnaces, it is necessary to provide sufficiently large channels for both the inlet of fresh air and its distribution through the hot-air pipes. The area of the smallest part of the inlet (or inlets, for it is sometimes better to have more than one) should be about ⅙ sq. ft. for every lb. of coal estimated to be burnt hourly in cold weather; and to prevent, in a measure, the inconvenience of one hot-air pipe drawing from another, the collective area of the hot-air pipes should not be more than ⅙ greater than the area of the cold-air inlet. These proportions will admit the hot air at a temperature of about 120° F. when at zero outside, and the velocity through the register will not exceed 5 ft. per second.
A large heating surface of the furnace is a well-recognised condition of both economy and efficiency. As a rule, there should be 10 sq. ft. of heating surface to every lb. of coal consumed per hour, when in active combustion; and the grate area should be about 1/50 of that of the heating surface. For the deficiency of heat, or the failure of some of the hot-air pipes of hot-air furnaces in certain winds and weathers in large houses or specially exposed rooms, the best addendum is an open fire-grate. With this provision in northerly rooms, to be used occasionally, hot-air furnaces may be made to produce all the advantages of steam heat in even the largest dwelling-houses.
44. Boyle’s Warm-air Stove.
Boyle’s system of warming fresh air is suitable where hot air, water, or steam pipes are not available. The arrangement (Fig. 44) consists of a copper or iron pipe a about 1½ in. diam. placed in an inlet tube b, preferably of the form of a bracket. This pipe is not vertical, as in the so-called Tobin’s shafts, but of zigzag shape, crossing and recrossing the tube from top to bottom, and so causing the incoming air to repeatedly impinge in its passage through the tube. At the bottom of the tube an air-tight chamber, so far as the interior of the tube is concerned, is fixed, in which a Bunsen gas-burner c is placed, the flame of which plays up into one of the lower ends of the pipe, the upper portion being about 5 ft. 9 in. from the floor. The other lower end of the pipe either dips into a condensation box d in the bottom of the tube or is continued into an existing flue or extraction shaft. If the pipe terminates in a box, the vapour is condensed there and carried off through the outside wall by means of a small pipe. At the bottom of the box is placed some loose charcoal, which needs renewing at intervals. This charcoal absorbs any products of combustion which have a tendency to rise. The heat thus passes through the entire length of the pipe, and warms the air as it travels through the tube to the room or hall as required.
Heating by gas is now growing in favour, and under favourable circumstances is to be recommended. There are two general methods adopted; firstly, by gas fires, which are asbestos or metal made incandescent by gas heat; these are made either portable, or by fitting a specially made burner to an existing fireplace, and filling the grate with Lumb asbestos (which is made for the purpose, and when heated has the appearance of glowing coals); and secondly, by gas stoves acting upon a similar principle to a hot-air coal stove. The former are now made in great variety; they chiefly take the form of an ornamental iron frame, in the centre of which is fitted a fire-brick thickly imbedded in front with asbestos fibre; the burner beneath comes immediately under the front of the fire-brick, and when the gas is ignited, the asbestos at once becomes incandescent, making it of cheerful and fire-like appearance, and the fire-brick in a few minutes becomes highly heated, radiating its warmth into the room. This description of stove and also the burner for existing fireplaces can be obtained at any ironmongers or gas-fitters.
In nearly all gas fires and stoves the gas is burnt with an admixture of air (atmospheric gas, 1 of gas and 2 of air), by means of an atmospheric burner; this is not only a source of economy, but atmospheric gas has the very great advantage of being smokeless; but for this, a gas fire would be an impossibility; it must, however, be borne in mind that although smokeless this gas gives off products of combustion (carbonic acid, watery vapour, &c.), which must be carried away by a flue or other means. The portable stoves are always provided with a nozzle for attaching a smoke-pipe. There is still a doubt as to which is most economical, coal or gas: we cannot do better than quote the words of a well-known gas-stove maker, Chas. Wilson, of Leeds. He says, speaking of heating by gas: “It is not cheaper than coal, taking fuel for fuel and continually used, unless, as in the case of offices where labour has to be employed to light fires, clean grates, &c.; but it is cheaper than coal if occasionally used, as in the case of bedrooms, or sitting-rooms used by visitors, or rooms used by children for music, &c.; for bedrooms it is especially adapted for use for an hour or two at night or in the morning or for giving an unvarying heat all night. It is preferable in the matter of cleanliness, and a true solution of the smoke-abatement problem” (probably a coal-stove manufacturer would speak as much in favour of fire-grates).
It should be seen when purchasing gas fires that they have silent burners, as some make an objectionable hissing noise when in use.
