Next in cost of installation and simplicity of operation is the hot-air furnace. In this method, the air is drawn over heated surfaces and then transmitted through pipes, while at a high temperature, to the rooms where heat is required. Furnaces are used largely for warming dwelling houses, also churches, halls and schoolhouses of small size. They are more costly than stoves, but have some advantages over that form of heating. They require less care, as several rooms may be warmed from a single furnace; and, being placed in the basement all dust from coal and ashes is kept from the rooms above.

In construction a furnace is a large stove with a combustion chamber of ample size over the fire; the whole being enclosed in a casing of sheet iron or brick. The bottom of the casing is provided with a cold-air inlet, and at the top are pipes which connect with registers placed in the various rooms to be heated. Cold fresh air is brought from out of doors through a pipe or duct called the "cold-air box;" this air enters the space between the casing and the furnace near the bottom and in passing over the hot surfaces of the fire pot and combustion chamber, becomes heated. It then rises through the warm-air pipes at the top of the casing and is discharged through the registers into the rooms above.

Fig. 1.

As the warm air is taken from the top of the furnace, cold air flows in through the cold-air box to take its place. The air for heating the rooms does not enter the combustion chamber.

Fig. 1 shows the general arrangement of a furnace with its connecting pipes. The cold-air inlet is seen at the bottom and the hot-air pipes at the top; these are all provided with dampers for shutting off or regulating the amount of air flowing through them. The feed or fire door is shown at the front and the ash door beneath it; a water pan is placed inside the casing and furnishes moisture to the warm air before passing into the rooms; water is either poured into the pan through an opening in the front, provided for this purpose, or is supplied automatically through a pipe.

The fire is regulated by means of a draft slide in the ash door and a cold-air or regulating damper placed in the smoke-pipe. Clean-outdoors are placed at different points in the casing for the removal of ashes and soot. Furnaces are made either of cast iron, or of wrought iron plates riveted together and provided with brick-lined fire pots.

One great advantage in this method of warming comes from the constant supply of fresh air which is required to bring the heat into the rooms. While this is greatly to be desired from a sanitary standpoint it requires a larger amount of fuel than would otherwise be necessary, for heat is required to warm the fresh air from out of doors up to the temperature of the rooms, in addition to that lost by leakage through walls and windows.

A more even temperature may be maintained in this way than by the use of stoves, owing to the greater depth and size of the fire, which causes it to be more easily controlled. When a building is placed in an exposed location, difficulty may be experienced at times in warming certain rooms, depending upon the direction of the wind; this may be overcome to a large extent by a proper location of the furnace and the exercise of suitable care in running the connecting pipes. This will be taken up later in the design of heating systems.

Fig. 2.