Systems using air as the primary distribution fluid have a furnace as a heat-generating source, rather than the boiler used for water or steam. Warm air furnaces (Fig. 24-11) are usually located near the center of the building. The furnace is selected after the engineer determines the type of system and fuel source. Cool air returns from occupied spaces at around 16°C to 21°C (60°F-70°F) and passes through a filter, a fan or blower, and a heating chamber. When the air goes to the supply air ductwork, it is between 49°C and 60°C (120°F-140°F). The bonnet or plenum is a chamber at the top of the furnace from which the ducts emerge to conduct heated or conditioned air to inhabited spaces. The furnace may include a humidification system that evaporates moisture into the air as it passes through.

In a forced-air gas furnace, a thermostatically controlled valve feeds gas to a series of burner tubes, where it is lighted by an electric spark or pilot light flame. Air is warmed in a heat exchanger above the burners and circulated by the furnace blower. The exchanger must heat the air inside without allowing odorless, deadly carbon monoxide to get into the supply ducts, and should be checked for safety every few years. The burner and blower chambers have one or more access panels, and room must be left around the furnace for maintenance.

Figure 24-11 Warm air furnace and ducts.

Oil-fired forced-air furnaces are very efficient and durable, but more complicated than gas-fired furnaces. Oil is pumped from a storage tank into a combustion chamber, where it is atomized and ignited by a spark. The flame heats a heat exchanger that warms the air that is circulated through the system by a blower. If the burner fails to ignite, a safety switch opens when it senses that no heat is being produced. A second safety device is a photoelectric cell that detects when the chamber goes dark and shuts the system down. A safety note: both devices may have reset switches, which should never be pushed more than twice in succession, as excess fuel pumped into the combustion chamber could explode.

No combustion occurs in an electric forced-air furnace, so there is no flue through which heat can escape, resulting in very high efficiency. Electric furnaces are clean and simple and have fewer problems than combustion furnaces. However, even with high efficiency, the high cost of electricity may make them more expensive to operate.

In residential design, the burner is started and stopped by a thermostat, usually in or near the living room in a location where the temperature is unlikely to change rapidly, protected from drafts, direct sun, and the warmth of nearby warm air registers. When the thermostat indicates that heat is needed, the burner and blower start up. The blower continues after the burner stops, until the temperature in the furnace drops below a set point. A high limit switch shuts off the burner if the temperature is too high.

Conventional combustion techniques in furnaces are usually only around 80 percent efficient. Newer pulse-combustion and condensing combustion processes are designed to be up to 95 percent efficient, as they recover much of the heat that goes up the flue stack with other equipment. These newer furnaces have simple connections, requiring only a small vent and outside air pipes, and a condensate drain pipe. To receive Energy Star certification, a furnace must have an annual fuel usage efficiency of 90 percent. Furnaces can typically be expected to function for 15 to 20 years.

Gas and oil furnaces require combustion air and ventilation for exhausting combustion products to the outside. Gases rise up the flue from the furnace as a result of the chimney's heat and the temperature difference between the flue gases and the outside air. When either is increased, the force of the draft is increased. Flues extend past the top of the highest point of the building so combustion products are not drawn back into the building. Where the chimney is not high enough, a fan can help create the needed draft.

Energy can be recovered from exhausted air with a regenerative wheel, a rotating device of metal mesh that uses its large thermal capacity to transfer heat from one duct to another. Air-to-air heat exchangers with very large surfaces also save energy.

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