The warm-air furnace is a self-contained heating unit designed to supply warm air to the interior of a structure. Warm-air furnaces used in central heating systems usually employ a system of ducts to distribute the air to the various rooms and spaces within the structure.
These furnaces can be classified according to the method of air circulation into the following two basic categories: (1) gravity warm-air furnaces, and (2) forced-warm-air furnaces. A distinct advantage of the forced-warm-air furnace is that its blower can move the air in any direction. As a result, this type of furnace can be located anywhere in the structure. Furthermore, the ducts do not have to be located above the heating unit, as is the case with gravity warm-air furnaces. These advantages of the forced-warm-air furnace have contributed to its tremendous popularity over gravity — type furnaces. Both types are described in considerable detail in Chapter 10, “Furnace Fundamentals.”
Other criteria used for classifying warm-air furnaces include:
(1) the type of fuel used (e. g., gas, oil, coal, or electricity), (2) the method of air distribution (ducts or pipeless), and (3) the method of firing (automatic or manual). Specific chapters in this book deal individually with some of these criteria (e. g., Chapter 11, “Gas Furnaces,” or Chapter 12, “Oil Furnaces”).