Central Heating Systems
Although central heating is considered a modern convenience, it actually dates back to ancient Greece. It is believed that in 350 B.C. the Great Temple of Ephesus was warmed by heated air that was circulated through flues laid in the floor. Today, with electricity to drive blowers, forced-air systems heat nearly 35 million American homes.
A forced-air heating system draws the room air through ductwork to a furnace, where the air is filtered and heated. The warmed air is then blown back into the rooms through other ductwork. With older gravity furnaces, the heated air is delivered by natural convection, not by a blower; the warmed air simply rises through the ducts to heat the rooms above. Most manufacturers make several sizes of each model. These furnaces come in "upflow," "downflow" and "horizontal" models designed to accommodate basement, attic or limited-space installation.
A forced-air system can be combined with an air-conditioning unit, a humidifier, and an air filter. The system's ductwork is usually metal wrapped in insulation to help keep in heat. In some cases, flexible ductwork is preferred. Note: homeowners should not disturb asbestos-wrapped ductwork since the asbestos fibers are dangerous to breathe.
-- Tips courtesy of HomeAdvisor.com