Furnace Fundamentals
The American Society of Heating, Refrigerating, and Air- Conditioning Engineers (ASHRAE) defines a furnace as “a complete heating unit for transferring heat from fuel being burned to the air supplied to a heating system.” The Standard Handbook for Mechanical Engineers (Baumeister and Marks, seventh edition) provides a definition that differs only slightly from the one offered by the ASHRAE: “a self-enclosed, fuel-burning unit for heating air by transfer of combustion through metal directly to the air.” Contained within these closely similar definitions are the two basic operating principles of a furnace: (1) Some sort of fuel is used to produce combustion, and (2) the heat resulting from this combustion is transferred to the air within the structure. Note that air—not steam, water, or some other fluid—is used as the heat-conveying medium. This feature distinguishes warm-air heating systems from the other types; see Chapter 6, “Warm-Air Heating Systems.”
Most modern furnaces are used in warm-air heating systems in which the furnace is a centralized unit, and the heat produced in the furnace is forced or rises by means of gravity through a system of ducts or pipes to the various rooms in the structure. This is what one commonly refers to as a central heating system. In other words, the furnace is generally in a centralized location within the heating system in order to obtain the most economical and efficient distribution of heat (although this is not an absolute necessity when a forced-warm-air furnace is used).
Ductless or pipeless furnaces are also used in some heating applications but are limited in the size of the area that they can effectively heat. They are installed in the room or area to be heated but are provided with no means for distributing the heat beyond the immediately adjacent area. This is a far less efficient and economical method of heating than the central heating system, but it is found to be adequate for a room, an addition to an existing structure, or a small house or building.
Classifying Furnaces
There are several different ways in which furnaces can be classified. One of the more popular methods is based on the fuel used to fire the furnace. Using this method, the following four types of furnaces are recognized:
1. Gas-fired furnaces
2. Oil-fired furnaces
3. Coal, wood, and multi-fuel furnaces
4. Electric furnaces
The first three categories of furnaces use a fossil fuel to produce the combustion necessary for heat transfer. The last one, electric furnaces, uses electricity. Whether or not electricity can be justifiably called a fuel is not of great importance here, because in this particular instance it functions in the same manner as the three fossil fuels: It heats the air being distributed.
Furnaces can also be classified by the means with which the heated air is distributed to the room (or rooms) in the structure. This method of classifying furnaces establishes two broad categories: (1) gravity warm-air furnaces and (2) forced-warm-air furnaces. The gravity warm-air furnaces rely primarily upon the principle of gravity for circulating the heated air. Because warm air is lighter than cold air, it will rise and pass through ducts or directly into the rooms to be heated. After passing off its heat, the air, now cooler and heavier, descends through returns to the furnace, where it is reheated. Gravity-type furnaces represent the earliest designs used in warm-air heating systems. They were sometimes equipped with fans (integral or booster) to increase the rate of air flow. They have been largely replaced in popularity by forced-warm-air furnaces, which are equipped with integral fans.
Forced-warm-air furnaces are often divided into three principal classes, based primarily upon the location on the furnace of the warm-air discharge outlet and the return-air inlet. Furthermore, additional design considerations dependent upon the planned location of the furnace also enter into the classification of warm- air furnaces, resulting in three types, or classes, of warm-air furnaces:
1. Upflow furnaces
a. Upflow highboy furnaces
b. Upflow lowboy furnaces
2. Downflow furnaces
3. Horizontal furnaces