13. How may one determine when it is economical to use a fireless cooker?
PART II
Heating Devices
CHAPTER VII
Warm-Air Furnaces
56. Principle Upon Which a Furnace Works. The success of warm-air heating depends on a natural circulation of air thruout all the rooms which are to be heated. The air is the vehicle of transmission of the heat from the fire to the rooms to be warmed.
A warm-air furnace is simply a large stove encased in a sheet-metal jacket (Figs. 26 and 27). The jacket is usually insulated with asbestos, since the stove is set in the basement where radiation of heat is not desired. The air entering the casing is warmed by the stove. As the air is warmed, it expands and becomes lighter, so rises to the top of the furnace; from here it is conducted to the rooms above. The warm air which has passed upward must be replaced by cooler air entering at the bottom of the jacket. In the rooms above, there must be outlets for the cold air, already in them, so that it may be replaced by the incoming warm air. Cold-air shafts from the floor leading downward serve as outlets. Sometimes they return the cooled air to the base of the furnace jacket.
