The cleanout doors are usually arranged at the back and sides of the furnace, and such extra doors as are found in front are generally intended either to facilitate the removal of clinkers, or to allow a pan of water to be introduced into the air-chamber. The clinker-door, which is placed just over the ash-door, communicates by a long horizontal opening with the lower portion of the fire-pot, just above the grate; and its purpose is to allow the "clinkers," or lumps of melted ashes, which are apt to collect just above the grate, to be fished for, so to speak, with a light poker, with a hook at the end, provided for such contingencies, and pulled to the edge of the grate, where there is room for them to drop into the ash-pit. The water-door is higher up, and communicates only with the air-chamber, on a shelf in which is placed a cast-iron pan, fitting the shape of the fire-pot, and intended to be kept full of water, so that the evaporation from this may supply moisture to the warm air issuing from the registers. A few years ago, evaporation of this kind was thought to be essential to the health of persons living in houses heated by furnaces in winter, and the regular filling of the water-pan in the furnace, either by means of a dipper, or by an automatic arrangement, comprising an exterior pan, with water-supply and ball-cock, connected by a pipe with the inner pan, was a part of the ordinary household duties; but many physiologists now believe that a dry, warm air, if pure, and moderately heated, as is generally the case where it is supplied through modern furnaces, is more favorable to health in winter than an atmosphere which is warm and moist, and that people who live in houses where no attempt is made to supply artificial moisture to the air in winter are less likely to take cold than those who have the pores of the skin, while they are in the house, kept open by an atmosphere intentionally moistened. Experience seems to have confirmed this view, and very few modern furnaces have any provision for evaporating water in the air-chamber. In the same way, the improved grates now in use remove clinkers much more quickly and effectually than is possible by the use of the hooked poker, and furnaces provided with such grates do not need a clinker-door, and are usually made without it.

Cleanouts.

Evapora-tion.

A little knowledge of the principles of construction of a furnace will greatly facilitate its management, which is not often a matter of serious difficulty. As the control of the fire, proper economy of fuel, and the prevention of the escape of gas or ashes into the house, depend upon a good draught in the chimney, every precaution mentioned in connection with the flues of kitchen stoves should be observed with even greater care in regard to the furnace flue. A furnace flue is less likely than a kitchen flue to have additional stoves or fireplaces opening into it, but these are sometimes found, and it is more liable than a kitchen flue to have its draught spoiled by not being carried down, entirely separate from all other flues or ash-pits, to the very bottom; or by the unobserved opening of ash-pit doors in the chimney, or cleanout doors in the furnace, by accident, or by rust; or by imperfect fitting of the smoke-pipe thimble into the brickwork of the chimney; or by any other accident or oversight through which cold air, even in small volume, is unintentionally admitted to the flue.

Supposing the flue to be tight, it must also be unobstructed. Most furnace flues are carried down to the cellar floor, so that a considerable space is left below the smoke-pipe connection, in which the fine ashes, carried into the chimney by the draught, can accumulate, but even this space will, in time, fill up, so that the ashes will rise above the mouth of the smoke-pipe, and choke the draught, unless they are from time to time removed through the iron door usually built in at the foot of the flue; and if, as is often the case in city houses, the space for ashes below the smoke-pipe connection is small, it is still more necessary to clear it out frequently.

Another common cause of sluggish or irregular draught in furnace flues is the improper arrangement of the smoke-pipe. This should be made as short and direct as possible, in order to avoid the cooling of the hot smoke and gases, and consequent loss of draught; and it should rise continuously from the furnace to the connection with the chimney. It is very common for the men who set furnaces, in order to give head-room for passing under the smoke-pipe, to make a portion of it nearly or quite level, or even to give it in some places a slight pitch downward. While a pipe laid in this way will draw when there is a hot fire in the furnace, the strong upward current so created in the flue being sufficient to draw the smoke even downward for a short distance, the draught suddenly stops as soon as the fire is allowed to burn low, and the fire either goes out, or is revived with difficulty. This trouble is easily cured by changing the course of the smoke-pipe, so as to ascend continuously, without regard to the head-room underneath it. Then the moderately heated gases from a low fire, instead of trying in vain to dive down a depression in the smoke-pipe, or to creep along on a level, will at all times rise naturally to the chimney, and the fire will respond quickly to the closing of the check-draught, or the opening of the lower slide; while, the draught being at all times inward, even a cracked fire-pot, or an open joint in the furnace, will not allow gas to escape into the air-chamber.

If the chimney draught is good, the smoke-pipe properly laid, and all leaks of cold air into the flue stopped, the only further requisite for the proper combustion of the coal in the fire-pot is a suitable admission of air under the grate; and this may be interfered with in several ways. The worst of these is by the accumulation of ashes in the ash-pit; and, if these are allowed to reach the under side of the grate, not only will the draught be checked, but, if an attempt is made to urge the fire, the grate itself is likely to be melted. Modern furnaces usually have small ash-pits, which must be cleared out at least once a day in cold weather, in order to prevent the dangerous accumulation which is likely to take place when the clearing can be postponed for a longer time, and is, in consequence, apt to be forgotten.