=57. The Stove Part.= The stove part of the hot-air furnace consists of a fire pot supported above a place where the ashes may fall and a chimney to carry off smoke. The draft below the grate in the fire pot lets in air which is essential to the proper burning of the fuel. In this respect, it is similar to a cook stove. A draft above the fire when opened a little lets in air which aids in the complete combustion of the gases given off by the fuel. Burning these gases adds to the amount of heat secured from the fuel. Opening the draft wider checks the burning of the fire. There should be a damper in the smoke pipe. When this is closed, it checks the draft up the chimney. This is needed because some chimneys often draw up air too fast to make the fire burn well. When checking the fire, close the draft below, open the one above the fire box, and close the one in the pipe. To make the fire burn fast, open the draft below, close the one above the fire box, and open the one in the pipe. Remember that a fire will not burn well if there is too much draft. Adjust the drafts until the fire burns with a clear, bright flame without giving off smoke. After a fire is built, the manner of adding fuel makes a difference in the efficiency of the furnace. When using coal, add it in rather small amounts, spreading it in a layer over the entire fire. Do not make this layer so thick that the fire smokes. The fuel will not burn with a clear flame if the fire is being smothered. Much fuel is wasted by ignorant and careless management of furnaces. [Illustration: FIG. 27. Circulation of warm air.] =58. The Cold-Air Shaft.= It is through a cold-air shaft that the cooler air comes into the furnace. Some furnaces have this built so that it draws the cooling air from the rooms above down into the furnace to be heated again. This is an economical arrangement. Some others draw fresh air from out of doors into the furnace, letting the cold air from the rooms above drain into the cellar and out of doors. This is more expensive, as the air to be heated is usually colder, but it has the advantage of helping ventilate the rooms by bringing a constant supply of fresh air. The cold-air shaft leading from out of doors should have the outer end covered with wire mesh, and a cloth which should be washed or renewed often. Never sweep dirt down a register or cold-air shaft. It comes back into the room in time. Dust the registers occasionally. In older heating systems, there was but one large cold-air shaft to drain the cold air from the rooms above. In more modern houses, a cold-air shaft is placed in every room that may be shut off from the others. This does away with the old difficulty of heating a closed room, for it is as important that the colder air gets out as that the warm air gets in.