Next to the kitchen stove, the furnace presents the most troublesome problems to the housekeeper; and, in fact, most housekeepers abandon at once problems connected with the furnace, imagining that they can be dealt with only by experts. In reality, however, a furnace is a comparatively simple contrivance; and, except that it is bigger, is no more difficult to manage than a stove.

Very many ladies suppose that the hot-air registers in a furnace-heated house communicate with the fire-box, just as the smoke-pipe does, and fortify their opinion by observing that, when a new fire is kindled, smoke comes up the registers, and, when the furnace is shaken, fine ashes often make their appearance through the same openings. This impression of the construction of a furnace is very erroneous. Briefly described, every furnace is simply a large stove, surrounded by a casing, exactly like the jacketed stoves described in a preceding chapter, the fire being contained in the interior portion, while the air which is to be warmed and distributed through the house passes up through the space between the inner portion, or furnace proper, and the galvanized iron or brick casing, but without communicating with the inner, or stove portion, in any way. It is true that smoke and fine ashes do often come up through the registers; but, unless the inner portion of the furnace is cracked or disjointed in some way, they do not pass from it into the warm-air pipes directly, but are drawn from the cellar, into which they escape when the furnace doors are opened, through the leaks which generally exist in the conduits supplying fresh air. Under the old-fashioned practice, by which the fresh-air supply to the furnace was taken entirely from the cellar, not only smoke and ashes, but coal-dust, mould-spores, and aromas of various descriptions were warmed up, and poured, all winter long, through the registers into the rooms. Now, however, nearly all furnaces are supplied with fresh air from out of doors through a "cold-air box," or conduit, of wood or galvanized iron, or, occasionally, of earthenware pipe, which leads from a cellar window, or other opening, either to the side of the furnace, or to a pit beneath it, so that the fresh air which comes through it can rise between the inner portion of the furnace and the casing to the registers above. If the cold-air conduit were always tight, no dust or smoke could get into it from the cellar, but it is usually furnished with a door, opening into the cellar, for cleaning, or for taking air from the cellar in extremely cold weather, when the power of the furnace is inadequate to the task of raising the fresh-air supply from the exterior temperature to that required for the rooms; and, as this door is never quite air-tight, smoke and fine dust are drawn through the crevices, and, entering there the current of fresh air, reach the registers. Many cold-air boxes, also, are made of wood, the sides consisting simply of matched boards; and the dry air of the cellar soon causes these boards to shrink, opening innumerable small crevices, through which dust, smoke, and gas can reach the rooms.

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Cold-air box.

Although the principle of construction is the same for all hot-air furnaces, there is great variety in its application. The simplest furnace consists of an iron "fire-pot," usually, in this type, lined with fire-brick, to prevent the iron from cracking with the heat of the fire, and surmounted by a "drum," or cylinder with a closed top, usually of wrought iron. The fire-pot has a grate of some sort at the bottom, and under this is the ash-pit; and a door is fitted in the drum, above the fire-pot, for feeding the furnace with coal, and another in the ash-pit, under the grate, for removing ashes; while a galvanized iron smoke-pipe carries the smoke and heated gases from the upper part of the drum to the chimney. The fire-pot, drum, and ash-pit are enclosed by the casing, which is simply a galvanized iron cylinder, with a closed top, of a diameter some inches greater than that of the drum; and the feeding and ash doors are extended so as to reach to the outside of the casing, the smoke-pipe, of course, passing through the latter on its way to the chimney. From the top of the casing proceed the tin pipes which lead the air admitted from the cold-air box, and warmed by its passage past the fire-pot and the drum, to the registers in the rooms. With a slide in the feeding-door and ash-door, to regulate the supply of air above and below the grate, and a damper in the smoke-pipe, to give still further control, the apparatus is, in all essentials, complete; and many such furnaces are sold, and give satisfaction under suitable circumstances.

A furnace of this kind cannot, however, be advantageously used to supply a large number of registers. It is a principle of the art of heating that air can be warmed only by actual contact with a heated surface; and, as the fire-pot and drum of such a furnace present a comparatively small surface, the air which can be made to come into actual contact with them on its passage between them and the casing is very limited in amount, and an attempt to enlarge the diameter of the casing, so as to allow more air to pass through it, simply results in admitting cold air to some of the registers, without increasing the amount of warm air. For this reason, all improved furnaces present devices, more or less successful, for enlarging the surface heated by the fire, so that more air can be brought in contact with the hot iron, and, in consequence, a greater amount of warm air can be delivered through the registers, without admitting to them air which has escaped actual contact with the heating surfaces, and is therefore cold.

A simple method of increasing the surface heated by the fire is found in the "gill stoves," often used in England for heating churches, in which deep, vertical ribs are formed on the outside of the stove. As the heat from the fire extends very rapidly by conduction into these ribs, the surface available for warming the air which passes over the stove is multiplied by them many fold. As applied to furnaces, the principle of the gill stove is modified, the solid ribs of the stove being usually replaced by deep corrugations, into which the smoke and hot gases from the fire penetrate, increasing the effect; and, in a very good form of furnace, these vertical corrugations are themselves undulated, so that the air passing between them is thrown more effectually into contact with the heated iron.