To go back a little in history, central heating witha furnace is an idea that is centuries old. This idea was invented by the Romans and called a hypocaust. It was a form of under-floor heating using afire in one corner of a basement with the exhaust vented through flues in thewalls to chimneys. This form of heating could only be used in stone or brickhomes. It was also very dangerous because of the possibility of fire andsuffocation. Early furnaces burned wood. In the seventeenth century, coal began toreplace wood as a primary fuel. Coal was used until the early 1940s when gasbecame the primary fuel.In the 1970s, electric furnaces started to replace gasfurnaces because of the energy crisis. Wood and coalburning furnaces required constant feeding to maintain warmth in the home. Fromearly morning to late at night, usually three to five times a day, fuel neededto be put in the furnace. In addition, the waste from the ashes from the burntwood or coal had to be removed and disposed of.

Today, the gas furnace is still the most popular form of home heating equipment. Raw materials design of original fireplaces Today, stainless steel is used in the heat exchangers for corrosionresistance. Aluminized steel is used to construct the frame, blowers, andburners, and brass is used for valves, copper in the electrical wiring, and fiberglass to insulate the cabinet. Today's gas furnace consists of a heatexchanger, secondary heat exchanger (depending on efficiency rating), aircirculation blower, flue draft blower, gas control valve, burners, pilot lightor spark ignition, electronic control circuitry, and an external thermostat.The modern furnace is highly efficient—80-90%, allowing only 10-20% of theheated air to escape up the chimney. The manufacturing process today

　　The primary heat exchanger is formed from two separate pieces of 409 stainless steel sheet. Each half is formed into shape by a 400 ton hydraulic press.

　　The secondary heat exchanger is formed from 29-4°C stainless steel tubing and fins. The fins are welded to the tubing to form a radiator type configuration.

　　The primary heat exchanger is crimped to the secondary heat exchanger through a transition box. The flue draft blower is attached to the secondary heat exchanger.

　　The cabinet consists of two doors and the cabinet housing.

　　The cabinet is installed around the furnace assembly and secured with screws and nuts.

　　The doors are installed on the front of the cabinet assembly. The completed assembly is boxed and prepared for shipment.

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