In another type of furnace, the extension of the heating surface is obtained by means of a system of pipes, vertical, horizontal, circular, conical, or cylindrical, through which the smoke and heated gases are drawn by the draught of the chimney. The whole system is enclosed in a large casing, and it is presumed that the air passing through the casing will either come in contact with one or more of the pipes, or will be mixed with air that has been in contact with them, so that the temperature of the air from the registers will be nearly uniform. In furnaces of this class, as the smoke from a freshly kindled fire could not be drawn by a cold chimney through all the convolutions of the system of pipes, a short cut is always provided, by which it can, at first, pass directly into the chimney; and later, when the chimney has become warm, and the draught is well established, the current can, by closing a damper, be made to pass through the rest of the system of pipes before it reaches the chimney. The short-cut, or "direct-draught" smoke-pipe is usually near the top of the furnace, and the damper in this is opened while a fresh fire is being kindled, or when the chimney has become cold and it is desirable to warm it to set up a proper draught. So long as the direct-draught damper is open, while the fire will burn briskly, little warm air will pass through the registers; but, by closing the direct draught, after the fire is well established, the smoke is sent through the "indirect draught," which usually issues from the furnace near the bottom, bringing all the heating surfaces into action, and greatly increasing the amount of warm air delivered through the registers.

Although the distinction between the two methods of increasing the amount of air heated by a furnace has a certain importance, most furnaces partake more or less of both types, those with deep flanges having also some convolution of the smoke-pipe, by which a little extra heating surface can be gained, while those which depend upon a complication of smoke-flues usually have the fire-pot flanged, or corrugated, or covered with spikes, to increase its surface; so that, in practice, nearly every furnace has its direct and indirect draught.

In addition to these provisions, the control of a furnace fire is usually made very perfect by contriving, not only slides in the upper and lower doors, but a "check-draught," either in the "indirect-draught " pipe, or in the main smoke-pipe. This check-draught, in most cases, acts solely by admitting more or less cold air to the smoke-pipe or chimney-flue, but it is sometimes fitted with a damper, so that, when the current is checked by admitting cold air, the smoke-pipe is at the same time partially closed, producing a kind of double effect. In either case, the check-draught is made nicely adjustable, so that the entrance of cold air can be accurately controlled.

Many devices have been introduced for making this control automatic, so that the furnace will open its own check-draught when the fire burns too briskly, and close it when the temperature falls. The cheaper inventions for this purpose, which act by the difference of expansion of two metal rods or tubes, set in the furnace casing, are, in most cases, of little use, their action being variable and uncertain; but the more expensive ones, which act by compressed air, or by electricity, are very efficient.

Check-draught.

Automatic control.

Most furnaces, in addition to their feeding and ash doors, and their direct and indirect smoke-pipes and check-draughts, show various complications, the purpose of which is not always very evident. Where the capacity of the heating portion of the furnace is comparatively small, as in those consisting merely of a drum over the fire-pot, the opening of the upper door will often chill the air above the fire so much as to check the draught, and, in consequence, the combustion of the coal, causing a quantity of poisonous carbonic oxide gas to be generated, which escapes through the door into the cellar, and is thence drawn into the crevices of the cold-air box, and sent through the registers into the rooms. In order to capture this gas as it issues from the door, and draw it off into the chimney, a narrow hood is often placed over the upper door of furnaces of this type, communicating with the smoke-pipe; and, in order that this may not act as a check-draught, a cover is fitted to .it, and attached to the door, in such a way that when this is shut, the hood is closed, the opening of the door uncovering also the hood. As the leakage of cold air into the hood, even when it is closed by its cover, interferes appreciably with the draught of the furnace, it is doubtful whether it offers, on the whole, any advantage; and the same may be said of the openings which, in some furnaces, are made from the ash-pit into the smoke-pipe, or into some internal flue, in order to carry off the cloud of fine ashes produced by shaking the grate. Although such openings are usually provided with a cover, which can be opened or shut from the outside, the cover is never perfectly tight, and, when clogged with ashes or cinders, as it is very apt to be, cannot be shut at all, so that the opening then acts as a powerful check-draught, interfering materially with the proper action of the furnace. In general, all openings into the smoke-pipes or flues of furnaces, except the direct and indirect draught connections, and the usual check-draught on the latter, are objectionable, interfering with the regularity and strength of the draught in the chimney, which affords the best safeguard against the escape of gas or fine ashes into the cellar.

Additional devices.

There are, however, in every furnace certain openings, necessary for cleaning the heating surfaces, which vary in position, according to the design and construction of the furnace. These openings extend to the outside of the casing, and always have small iron doors, fitted as nearly airtight as possible; and, unless they are forced open by rust, as is often the case, they do not materially affect the draught.