For economy of fuel we find recommended a furnace of the type known as indirect draft. In this type the heated gases are ordinarily obliged to pass downward to a radiating chamber at the base and thence upward again before escaping into the chimney, thus giving off a large proportion of their heat. Among other points to be considered in the selection of the furnace is the grate, which should be of a pattern that will allow of easy disposal of the ashes and clinkers without much disturbance of the fire. The construction of the fire pot is of great importance in the choice of the furnace, and here the choice will lie between a heavy cast iron pot, and a thin wrought iron or steel pot with a lining of fire bricks. The cast iron pot has the advantage of continuing to give off considerable heat when new coal is put on, while the brick linings allow the furnace to cool off more while the fire is dull. Other points to be desired are a large combustion chamber, as the space above the fire is called, and ample heating surfaces, so arranged that they will not become deadened by soot.

Cold Air Box

The furnace, which has been selected besides these points, on account of the smoothness of its castings and the evidently workmanlike construction of its joints and working parts, is made to set over a circular pit into which the cold air box runs. An important feature is the size of the cold air box which should be of a capacity nearly equal to the combined area of the piping. About one-sixth less is the close rule as the cold air will expand about that amount on becoming heated. Unless the supply from the cold air box is ample there will be danger that the long pipes will draw from the shorter pipes, or that the general flow will be weak and irregular. The opening of the cold air box is usually placed on the north or west side, as the coldest winds generally come from those directions, but a better way is to carry the cold air box completely across the cellar with a connection to furnace at right angles, so that the furnace may draw an equal supply without regard to the direction of the wind. By this means we shall avoid the very disagreeable feature which all who have heated by furnace in the usual way will have experienced, of finding that when the wind is high from the side where the cold air is taken, the chances are that the air will be driven so rapidly through that it does not get sufficiently heated, but comes in cold gusts from the registers. On the other hand, when the wind is strong from the side opposite the single cold air opening, the suction from the lee side of the house will often cause the air to flow down the registers and out of the cold air box, where there is danger of the already heated air becoming hot enough by reheating to set the cold air box on fire, if it is of wood. To guard against a possibility of this, it is best if possible to make all air ducts of galvanized iron, which is safer but unfortunately more expensive. In certain localities this metal construction will be required by the building laws, and is to be recommended in all cases.

Location

In locating the furnace care should be taken that the pipes are of about the same length, any differences should be made in favor of pipes running north or west. The smoke pipe should be run as directly as possible and should never come nearer than eight inches to the floor beams overhead. Care must be taken where the pipe enters the flue that the connections are tight, and that the pipe is not pushed in so far that it cuts off the area of the flue.

Supervision

Beyond seeing that the apparatus, the piping and the registers are ample in size and properly heated, the superintendent should watch carefully to see that there is no danger from fire. All pipes which are run in concealed places should be double, or at least well protected by bright tin or asbestos covering and the use of wire for lathing over them. The furnace should have over it, a circular shield of metal or else plastering on wire laths for an ample space on the ceiling. In running the pipes, sharp bends and sudden contraction or distortion of the pipes must be avoided, and free entrance of pipes to the register boxes must be provided in all cases. The practice of topping pipes or running one pipe to supply two registers, is productive of continual annoyance, as it is almost certain that one register will get the greater part of the heat at the expense of the other. The registers should always be set in borders of slate or soapstone, or in the iron rims which are provided and they should be free in action and tight when closed.